Parse netlink message

kernel/src/context.rs

@@ -166,24 +166,31 @@ impl<'a> CurrentUserSpace<'a> {
 }
 
 /// A trait providing the ability to read a C string from the user space.
+pub trait ReadCString {
+    /// Reads a C string from `self`.
     ///
-/// The user space should be of the current process. The implemented method
-/// should read the bytes iteratively in the reader ([`VmReader`]) until
+    /// This method should read the bytes iteratively in `self` until
     /// encountering the end of the reader or reading a `\0` (which is also
     /// included in the final C String).
-pub trait ReadCString {
     fn read_cstring(&mut self) -> Result<CString>;
+
+    /// Reads a C string from `self` with a maximum length of `max_len`.
+    ///
+    /// This method functions similarly to [`ReadCString::read_cstring`],
+    /// but imposes an additional limit on the length of the C string.
+    fn read_cstring_with_max_len(&mut self, max_len: usize) -> Result<CString>;
 }
 
 impl ReadCString for VmReader<'_, Fallible> {
-    /// Reads a C string from the user space.
-    ///
-    /// This implementation is inspired by
-    /// the `do_strncpy_from_user` function in Linux kernel.
-    /// The original Linux implementation can be found at:
-    /// <https://elixir.bootlin.com/linux/v6.0.9/source/lib/strncpy_from_user.c#L28>
     fn read_cstring(&mut self) -> Result<CString> {
-        let max_len = self.remain();
+        self.read_cstring_with_max_len(self.remain())
+    }
+
+    fn read_cstring_with_max_len(&mut self, max_len: usize) -> Result<CString> {
+        // This implementation is inspired by
+        // the `do_strncpy_from_user` function in Linux kernel.
+        // The original Linux implementation can be found at:
+        // <https://elixir.bootlin.com/linux/v6.0.9/source/lib/strncpy_from_user.c#L28>
         let mut buffer: Vec<u8> = Vec::with_capacity(max_len);
 
         if read_until_nul_byte(self, &mut buffer, max_len)? {
@@ -199,7 +206,10 @@ impl ReadCString for VmReader<'_, Fallible> {
 
 impl ReadCString for VmReaderArray<'_> {
     fn read_cstring(&mut self) -> Result<CString> {
-        let max_len = self.sum_lens();
+        self.read_cstring_with_max_len(self.sum_lens())
+    }
+
+    fn read_cstring_with_max_len(&mut self, max_len: usize) -> Result<CString> {
         let mut buffer: Vec<u8> = Vec::with_capacity(max_len);
 
         for reader in self.readers_mut() {

kernel/src/lib.rs

@@ -28,6 +28,7 @@
 #![feature(step_trait)]
 #![feature(trait_alias)]
 #![feature(trait_upcasting)]
+#![feature(associated_type_defaults)]
 #![register_tool(component_access_control)]
 
 use kcmdline::KCmdlineArg;

kernel/src/net/socket/netlink/message/attr/mod.rs

@@ -0,0 +1,134 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! Netlink attributes.
+//!
+//! Netlink attributes provide additional information for each [`segment`].
+//! Each netlink attribute consists of two components:
+//! 1. Header: The attribute header is of type [`CNlAttrHeader`],
+//!    which specifies the type and length of the attribute. The attribute
+//!    type belongs to different classes, which rely on the segment type.
+//! 2. Payload: The attribute's payload, which can vary in type.
+//!    Currently, payload types include primitive types, C string, and binary.
+//!    The payload can also include one or multiple other attributes,
+//!    known as nested attributes.
+//!
+//! Similar to [`super::segment::NlSegment`], attributes have alignment requirements;
+//! both the header and payload must be aligned to [`super::NLMSG_ALIGN`]
+//! when being transferred to and from user space.
+//!
+//! The layout of a netlink attribute is depicted as follows:
+//!
+//! ┌────────┬─────────┬─────────┬─────────┐
+//! │ Header │ Padding │ Payload │ Padding │
+//! └────────┴─────────┴─────────┴─────────┘
+//!
+//! [`segment`]: super::segment
+
+use align_ext::AlignExt;
+
+use super::NLMSG_ALIGN;
+use crate::{
+    prelude::*,
+    util::{MultiRead, MultiWrite},
+};
+
+pub mod noattr;
+
+/// Netlink attribute header.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L229>.
+//
+// The layout of the `type_` field is structured as follows:
+// ┌────────┬───────────────┬──────────┐
+// │ Nested │ Net Byteorder │ Payload  │
+// └────────┴───────────────┴──────────┘
+//   bit 15      bit 14       bits 13-0
+#[derive(Debug, Clone, Copy, Pod)]
+#[repr(C)]
+pub struct CAttrHeader {
+    len: u16,
+    type_: u16,
+}
+
+impl CAttrHeader {
+    pub fn type_(&self) -> u16 {
+        self.type_ & ATTRIBUTE_TYPE_MASK
+    }
+}
+
+const IS_NESTED_MASK: u16 = 1u16 << 15;
+const IS_NET_BYTEORDER_MASK: u16 = 1u16 << 14;
+const ATTRIBUTE_TYPE_MASK: u16 = !(IS_NESTED_MASK | IS_NET_BYTEORDER_MASK);
+
+/// Netlink Attribute.
+pub trait Attribute: Debug + Send + Sync {
+    /// Returns the type of the attribute.
+    fn type_(&self) -> u16;
+
+    /// Returns the byte representation of the payload.
+    fn payload_as_bytes(&self) -> &[u8];
+
+    /// Returns the payload length (excluding padding).
+    fn payload_len(&self) -> usize {
+        self.payload_as_bytes().len()
+    }
+
+    /// Returns the total length of the attribute (header + payload, excluding padding).
+    fn total_len(&self) -> usize {
+        core::mem::size_of::<CAttrHeader>() + self.payload_len()
+    }
+
+    /// Returns the total length of the attribute (header + payload, including padding).
+    fn total_len_with_padding(&self) -> usize {
+        self.total_len().align_up(NLMSG_ALIGN)
+    }
+
+    /// Returns the length of the padding bytes.
+    fn padding_len(&self) -> usize {
+        self.total_len_with_padding() - self.total_len()
+    }
+
+    /// Reads the attribute from the `reader`.
+    fn read_from(reader: &mut dyn MultiRead) -> Result<Self>
+    where
+        Self: Sized;
+
+    /// Reads all attributes from the reader.
+    ///
+    /// The cumulative length of the read attributes must not exceed total_len.
+    fn read_all_from(reader: &mut dyn MultiRead, mut total_len: usize) -> Result<Vec<Self>>
+    where
+        Self: Sized,
+    {
+        let mut res = Vec::new();
+
+        while total_len > 0 {
+            let attr = Self::read_from(reader)?;
+            total_len -= attr.total_len();
+
+            let padding_len = attr.padding_len().min(total_len);
+            reader.skip(padding_len);
+            total_len -= padding_len;
+
+            res.push(attr);
+        }
+
+        Ok(res)
+    }
+
+    /// Writes the attribute to the `writer`.
+    fn write_to(&self, writer: &mut dyn MultiWrite) -> Result<()> {
+        let header = CAttrHeader {
+            type_: self.type_(),
+            len: self.total_len() as u16,
+        };
+
+        writer.write_val(&header)?;
+        writer.write(&mut VmReader::from(self.payload_as_bytes()))?;
+
+        let padding_len = self.padding_len();
+        writer.skip(padding_len);
+
+        Ok(())
+    }
+}

kernel/src/net/socket/netlink/message/attr/noattr.rs

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use super::Attribute;
+use crate::{prelude::*, util::MultiRead};
+
+/// A special type indicates that a segment cannot have attributes.
+#[derive(Debug)]
+pub enum NoAttr {}
+
+impl Attribute for NoAttr {
+    fn type_(&self) -> u16 {
+        match *self {}
+    }
+
+    fn payload_as_bytes(&self) -> &[u8] {
+        match *self {}
+    }
+
+    fn read_from(_reader: &mut dyn MultiRead) -> Result<Self>
+    where
+        Self: Sized,
+    {
+        return_errno_with_message!(Errno::EINVAL, "`NoAttr` cannot be read");
+    }
+
+    fn read_all_from(_reader: &mut dyn MultiRead, _total_len: usize) -> Result<Vec<Self>>
+    where
+        Self: Sized,
+    {
+        Ok(Vec::new())
+    }
+}

kernel/src/net/socket/netlink/message/mod.rs

@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! Netlink message types for all netlink protocols.
+//!
+//! This module defines how to interpret messages sent from user space and how to write
+//! kernel messages back to user space.
+
+mod attr;
+mod segment;
+
+pub(super) use attr::{noattr::NoAttr, Attribute, CAttrHeader};
+pub(super) use segment::{
+    ack::{DoneSegment, ErrorSegment},
+    common::SegmentCommon,
+    header::{CMsgSegHdr, GetRequestFlags, SegHdrCommonFlags},
+    CSegmentType, SegmentBody,
+};
+
+use crate::{
+    prelude::*,
+    util::{MultiRead, MultiWrite},
+};
+
+/// A netlink message.
+///
+/// A netlink message can be transmitted to and from user space using a single send/receive syscall.
+/// It consists of one or more [`ProtocolSegment`]s.
+#[derive(Debug)]
+pub(super) struct Message<T: ProtocolSegment> {
+    segments: Vec<T>,
+}
+
+impl<T: ProtocolSegment> Message<T> {
+    pub(super) const fn new(segments: Vec<T>) -> Self {
+        Self { segments }
+    }
+
+    pub(super) fn segments(&self) -> &[T] {
+        &self.segments
+    }
+
+    pub(super) fn segments_mut(&mut self) -> &mut [T] {
+        &mut self.segments
+    }
+
+    pub(super) fn read_from(reader: &mut dyn MultiRead) -> Result<Self> {
+        // FIXME: Does a request contain only one segment? We need to investigate further.
+        let segments = {
+            let segment = T::read_from(reader)?;
+            vec![segment]
+        };
+
+        Ok(Self { segments })
+    }
+
+    pub(super) fn write_to(&self, writer: &mut dyn MultiWrite) -> Result<()> {
+        for segment in self.segments.iter() {
+            segment.write_to(writer)?;
+        }
+
+        Ok(())
+    }
+
+    pub(super) fn total_len(&self) -> usize {
+        self.segments
+            .iter()
+            .map(|segment| segment.header().len as usize)
+            .sum()
+    }
+}
+
+pub(super) trait ProtocolSegment: Sized {
+    fn header(&self) -> &CMsgSegHdr;
+    fn header_mut(&mut self) -> &mut CMsgSegHdr;
+    fn read_from(reader: &mut dyn MultiRead) -> Result<Self>;
+    fn write_to(&self, writer: &mut dyn MultiWrite) -> Result<()>;
+}
+
+pub(super) const NLMSG_ALIGN: usize = 4;

kernel/src/net/socket/netlink/message/segment/ack.rs

@@ -0,0 +1,90 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! This module defines segments that only appear in acknowledgment messages.
+//!
+//! An acknowledgment segment appears as the final segment in a response message from the kernel.
+//! Netlink utilizes two classes of acknowledgment segments:
+//! 1. Done Segment: Indicates the conclusion of a message comprised of multiple segments.
+//! 2. Error Segment: Indicates that an error occurred while the kernel processed the user space request.
+//!
+
+use super::{
+    common::SegmentCommon,
+    header::{CMsgSegHdr, SegHdrCommonFlags},
+    CSegmentType, SegmentBody,
+};
+use crate::{net::socket::netlink::message::NoAttr, prelude::*};
+
+pub type DoneSegment = SegmentCommon<DoneSegmentBody, NoAttr>;
+
+#[repr(C)]
+#[derive(Debug, Clone, Copy, Pod)]
+pub struct DoneSegmentBody {
+    error_code: i32,
+}
+
+impl SegmentBody for DoneSegmentBody {
+    type CType = DoneSegmentBody;
+}
+
+impl DoneSegment {
+    pub fn new_from_request(request_header: &CMsgSegHdr, error: Option<Error>) -> Self {
+        let header = CMsgSegHdr {
+            len: 0,
+            type_: CSegmentType::DONE as _,
+            flags: SegHdrCommonFlags::empty().bits(),
+            seq: request_header.seq,
+            pid: request_header.pid,
+        };
+
+        let body = {
+            let error_code = error_to_error_code(error);
+            DoneSegmentBody { error_code }
+        };
+
+        Self::new(header, body, Vec::new())
+    }
+}
+
+pub type ErrorSegment = SegmentCommon<ErrorSegmentBody, NoAttr>;
+
+#[derive(Debug, Pod, Clone, Copy)]
+#[repr(C)]
+pub struct ErrorSegmentBody {
+    error_code: i32,
+    request_header: CMsgSegHdr,
+}
+
+impl SegmentBody for ErrorSegmentBody {
+    type CType = ErrorSegmentBody;
+}
+
+impl ErrorSegment {
+    pub fn new_from_request(request_header: &CMsgSegHdr, error: Option<Error>) -> Self {
+        let header = CMsgSegHdr {
+            len: 0,
+            type_: CSegmentType::ERROR as _,
+            flags: SegHdrCommonFlags::empty().bits(),
+            seq: request_header.seq,
+            pid: request_header.pid,
+        };
+
+        let body = {
+            let error_code = error_to_error_code(error);
+            ErrorSegmentBody {
+                error_code,
+                request_header: *request_header,
+            }
+        };
+
+        Self::new(header, body, Vec::new())
+    }
+}
+
+const fn error_to_error_code(error: Option<Error>) -> i32 {
+    if let Some(error) = error {
+        -(error.error() as i32)
+    } else {
+        0
+    }
+}

kernel/src/net/socket/netlink/message/segment/common.rs

@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use align_ext::AlignExt;
+
+use super::{header::CMsgSegHdr, SegmentBody};
+use crate::{
+    net::socket::netlink::message::{attr::Attribute, NLMSG_ALIGN},
+    prelude::*,
+    util::{MultiRead, MultiWrite},
+};
+
+#[derive(Debug)]
+pub struct SegmentCommon<Body, Attr> {
+    header: CMsgSegHdr,
+    body: Body,
+    attrs: Vec<Attr>,
+}
+
+impl<Body, Attr> SegmentCommon<Body, Attr> {
+    pub const HEADER_LEN: usize = size_of::<CMsgSegHdr>();
+
+    pub fn header(&self) -> &CMsgSegHdr {
+        &self.header
+    }
+
+    pub fn header_mut(&mut self) -> &mut CMsgSegHdr {
+        &mut self.header
+    }
+
+    pub fn body(&self) -> &Body {
+        &self.body
+    }
+
+    pub fn attrs(&self) -> &Vec<Attr> {
+        &self.attrs
+    }
+}
+
+impl<Body: SegmentBody, Attr: Attribute> SegmentCommon<Body, Attr> {
+    pub const BODY_LEN: usize = size_of::<Body::CType>();
+
+    pub fn new(header: CMsgSegHdr, body: Body, attrs: Vec<Attr>) -> Self {
+        let mut res = Self {
+            header,
+            body,
+            attrs,
+        };
+        res.header.len = res.total_len() as u32;
+        res
+    }
+
+    pub fn read_from(header: CMsgSegHdr, reader: &mut dyn MultiRead) -> Result<Self>
+    where
+        Error: From<<Body::CType as TryInto<Body>>::Error>,
+    {
+        let (body, remain_len) = Body::read_from(&header, reader).unwrap();
+
+        let attrs = Attr::read_all_from(reader, remain_len)?;
+
+        Ok(Self {
+            header,
+            body,
+            attrs,
+        })
+    }
+
+    pub fn write_to(&self, writer: &mut dyn MultiWrite) -> Result<()> {
+        // FIXME: If the message can be truncated, we should avoid returning an error.
+        // Furthermore, we need to check the Linux behavior to determine whether to return an error
+        // if the writer is not large enough to accommodate the final padding bytes.
+        if writer.sum_lens() < (self.header.len as usize).align_up(NLMSG_ALIGN) {
+            return_errno_with_message!(Errno::EFAULT, "the writer length is too small");
+        }
+
+        writer.write_val(&self.header)?;
+        self.body.write_to(writer)?;
+        for attr in self.attrs.iter() {
+            attr.write_to(writer)?;
+        }
+
+        Ok(())
+    }
+
+    pub fn total_len(&self) -> usize {
+        Self::HEADER_LEN + Self::BODY_LEN + self.attrs_len()
+    }
+}
+
+impl<Body, Attr: Attribute> SegmentCommon<Body, Attr> {
+    pub fn attrs_len(&self) -> usize {
+        self.attrs
+            .iter()
+            .map(|attr| attr.total_len_with_padding())
+            .sum()
+    }
+}

kernel/src/net/socket/netlink/message/segment/header.rs

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! General netlink message types for all netlink protocols.
+
+use crate::prelude::*;
+
+/// `nlmsghdr` in Linux.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L52>.
+#[repr(C)]
+#[derive(Debug, Clone, Copy, Pod)]
+pub struct CMsgSegHdr {
+    /// Length of the message, including the header
+    pub len: u32,
+    /// Type of message content
+    pub type_: u16,
+    /// Additional flags
+    pub flags: u16,
+    /// Sequence number
+    pub seq: u32,
+    /// Sending process port ID
+    pub pid: u32,
+}
+
+bitflags! {
+    /// Common flags used in [`CMsgSegmentHdr`].
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L62>.
+    pub struct SegHdrCommonFlags: u16 {
+        /// Indicates a request message
+        const REQUEST = 0x01;
+        /// Multipart message, terminated by NLMSG_DONE
+        const MULTI = 0x02;
+        /// Reply with an acknowledgment, with zero or an error code
+        const ACK = 0x04;
+        /// Echo this request
+        const ECHO = 0x08;
+        /// Dump was inconsistent due to sequence change
+        const DUMP_INTR = 0x10;
+        /// Dump was filtered as requested
+        const DUMP_FILTERED = 0x20;
+    }
+}
+
+bitflags! {
+    /// Modifiers for GET requests.
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L70>.
+    pub struct GetRequestFlags: u16 {
+        /// Specify the tree root
+        const ROOT = 0x100;
+        /// Return all matching results
+        const MATCH = 0x200;
+        /// Atomic get request
+        const ATOMIC = 0x400;
+        /// Combination flag for root and match
+        const DUMP = Self::ROOT.bits | Self::MATCH.bits;
+    }
+}
+
+bitflags! {
+    /// Modifiers for NEW requests.
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L76>.
+    pub struct NewRequestFlags: u16 {
+        /// Override existing entries
+        const REPLACE = 0x100;
+        /// Do not modify if it exists
+        const EXCL = 0x200;
+        /// Create if it does not exist
+        const CREATE = 0x400;
+        /// Add to the end of the list
+        const APPEND = 0x800;
+    }
+}
+
+bitflags! {
+    /// Modifiers for DELETE requests.
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L82>.
+    pub struct DeleteRequestFlags: u16 {
+        /// Do not delete recursively
+        const NONREC = 0x100;
+        /// Delete multiple objects
+        const BULK = 0x200;
+    }
+}
+
+bitflags! {
+    /// Flags for ACK messages.
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/netlink.h#L86>.
+    pub struct AckFlags: u16 {
+        const CAPPED = 0x100;
+        const ACK_TLVS = 0x100;
+    }
+}

kernel/src/net/socket/netlink/message/segment/mod.rs

@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use align_ext::AlignExt;
+use header::CMsgSegHdr;
+
+use super::NLMSG_ALIGN;
+use crate::{
+    prelude::*,
+    util::{MultiRead, MultiWrite},
+};
+
+pub mod ack;
+pub mod common;
+pub mod header;
+
+pub trait SegmentBody: Sized + Clone + Copy {
+    // The actual message body should be `Self::CType`,
+    // but older versions of Linux use a legacy type (usually `CRtGenMsg` here).
+    // Additionally, some software, like iproute2, also uses this legacy type.
+    // Therefore, we need to handle both cases.
+    // Reference: <https://elixir.bootlin.com/linux/v6.13/source/net/core/rtnetlink.c#L2393>.
+    // FIXME: Verify whether the legacy type includes any types other than `CRtGenMsg`.
+    type CLegacyType: Pod = Self::CType;
+    type CType: Pod + From<Self::CLegacyType> + TryInto<Self> + From<Self>;
+
+    /// Reads the segment body from the `reader`.
+    ///
+    /// This method returns the body and the remaining length to be read from the `reader`.
+    fn read_from(header: &CMsgSegHdr, reader: &mut dyn MultiRead) -> Result<(Self, usize)>
+    where
+        Error: From<<Self::CType as TryInto<Self>>::Error>,
+    {
+        let mut remaining_len = (header.len as usize)
+            .checked_sub(size_of_val(header))
+            .ok_or_else(|| Error::with_message(Errno::EINVAL, "the message length is too small"))?;
+
+        // Align `remaining_len` up to `NLMSG_ALIGN`.
+        let reader_len = reader.sum_lens();
+        if reader_len < remaining_len {
+            return_errno_with_message!(Errno::EINVAL, "the reader length is too small");
+        }
+        remaining_len = remaining_len.align_up(NLMSG_ALIGN).min(reader_len);
+
+        // Read the body.
+        let (c_type, padding_len) = if remaining_len >= size_of::<Self::CType>() {
+            let c_type = reader.read_val::<Self::CType>()?;
+            remaining_len -= size_of_val(&c_type);
+
+            (c_type, Self::padding_len())
+        } else if remaining_len >= size_of::<Self::CLegacyType>() {
+            let legacy = reader.read_val::<Self::CLegacyType>()?;
+            remaining_len -= size_of_val(&legacy);
+
+            (Self::CType::from(legacy), Self::lecacy_padding_len())
+        } else {
+            return_errno_with_message!(Errno::EINVAL, "the message length is too small");
+        };
+
+        // Skip the padding bytes.
+        let padding_len = padding_len.min(remaining_len);
+        reader.skip(padding_len);
+        remaining_len -= padding_len;
+
+        let body = c_type.try_into()?;
+        Ok((body, remaining_len))
+    }
+
+    fn write_to(&self, writer: &mut dyn MultiWrite) -> Result<()> {
+        // Write the body.
+        let c_body = Self::CType::from(*self);
+        writer.write_val(&c_body)?;
+
+        // Skip the padding bytes.
+        let padding_len = Self::padding_len();
+        writer.skip(padding_len);
+        Ok(())
+    }
+
+    fn padding_len() -> usize {
+        let payload_len = size_of::<Self::CType>();
+        payload_len.align_up(NLMSG_ALIGN) - payload_len
+    }
+
+    fn lecacy_padding_len() -> usize {
+        let payload_len = size_of::<Self::CLegacyType>();
+        payload_len.align_up(NLMSG_ALIGN) - payload_len
+    }
+}
+
+#[repr(u16)]
+#[derive(Debug, Clone, Copy, TryFromInt, PartialEq, Eq, PartialOrd, Ord)]
+pub enum CSegmentType {
+    // Standard netlink message types
+    NOOP = 1,
+    ERROR = 2,
+    DONE = 3,
+    OVERRUN = 4,
+
+    // protocol-level types
+    NEWLINK = 16,
+    DELLINK = 17,
+    GETLINK = 18,
+    SETLINK = 19,
+
+    NEWADDR = 20,
+    DELADDR = 21,
+    GETADDR = 22,
+
+    NEWROUTE = 24,
+    DELROUTE = 25,
+    GETROUTE = 26,
+    // TODO: The list is not exhaustive.
+}

kernel/src/net/socket/netlink/route/message/attr/addr.rs

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use super::IFNAME_SIZE;
+use crate::{
+    net::socket::netlink::message::{Attribute, CAttrHeader},
+    prelude::*,
+    util::MultiRead,
+};
+
+/// Address-related attributes.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/if_addr.h#L26>.
+#[derive(Debug, Clone, Copy, TryFromInt)]
+#[repr(u16)]
+#[expect(non_camel_case_types)]
+enum AddrAttrClass {
+    UNSPEC = 0,
+    ADDRESS = 1,
+    LOCAL = 2,
+    LABEL = 3,
+    BROADCAST = 4,
+    ANYCAST = 5,
+    CACHEINFO = 6,
+    MULTICAST = 7,
+    FLAGS = 8,
+    RT_PRIORITY = 9,
+    TARGET_NETNSID = 10,
+}
+
+#[derive(Debug)]
+pub enum AddrAttr {
+    Address([u8; 4]),
+    Local([u8; 4]),
+    Label(CString),
+}
+
+impl AddrAttr {
+    fn class(&self) -> AddrAttrClass {
+        match self {
+            AddrAttr::Address(_) => AddrAttrClass::ADDRESS,
+            AddrAttr::Local(_) => AddrAttrClass::LOCAL,
+            AddrAttr::Label(_) => AddrAttrClass::LABEL,
+        }
+    }
+}
+
+impl Attribute for AddrAttr {
+    fn type_(&self) -> u16 {
+        self.class() as u16
+    }
+
+    fn payload_as_bytes(&self) -> &[u8] {
+        match self {
+            AddrAttr::Address(address) => address,
+            AddrAttr::Local(local) => local,
+            AddrAttr::Label(label) => label.as_bytes_with_nul(),
+        }
+    }
+
+    fn read_from(reader: &mut dyn MultiRead) -> Result<Self>
+    where
+        Self: Sized,
+    {
+        let header = reader.read_val::<CAttrHeader>()?;
+        // TODO: Currently, `IS_NET_BYTEORDER_MASK` and `IS_NESTED_MASK` are ignored.
+        let res = match AddrAttrClass::try_from(header.type_())? {
+            AddrAttrClass::ADDRESS => Self::Address(reader.read_val()?),
+            AddrAttrClass::LOCAL => Self::Local(reader.read_val()?),
+            AddrAttrClass::LABEL => Self::Label(reader.read_cstring_with_max_len(IFNAME_SIZE)?),
+            class => {
+                // FIXME: Netlink should ignore all unknown attributes.
+                // See the reference in `LinkAttr::read_from`.
+                warn!("address attribute `{:?}` is not supported", class);
+                return_errno_with_message!(Errno::EINVAL, "unsupported address attribute");
+            }
+        };
+
+        Ok(res)
+    }
+}

kernel/src/net/socket/netlink/route/message/attr/link.rs

@@ -0,0 +1,161 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use super::IFNAME_SIZE;
+use crate::{
+    net::socket::netlink::message::{Attribute, CAttrHeader},
+    prelude::*,
+    util::MultiRead,
+};
+
+/// Link-level attributes.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/if_link.h#L297>.
+#[derive(Debug, Clone, Copy, TryFromInt)]
+#[repr(u16)]
+#[allow(non_camel_case_types)]
+enum LinkAttrClass {
+    UNSPEC = 0,
+    ADDRESS = 1,
+    BROADCAST = 2,
+    IFNAME = 3,
+    MTU = 4,
+    LINK = 5,
+    QDISC = 6,
+    STATS = 7,
+    COST = 8,
+    PRIORITY = 9,
+    MASTER = 10,
+    /// Wireless Extension event
+    WIRELESS = 11,
+    /// Protocol specific information for a link
+    PROTINFO = 12,
+    TXQLEN = 13,
+    MAP = 14,
+    WEIGHT = 15,
+    OPERSTATE = 16,
+    LINKMODE = 17,
+    LINKINFO = 18,
+    NET_NS_PID = 19,
+    IFALIAS = 20,
+    /// Number of VFs if device is SR-IOV PF
+    NUM_VF = 21,
+    VFINFO_LIST = 22,
+    STATS64 = 23,
+    VF_PORTS = 24,
+    PORT_SELF = 25,
+    AF_SPEC = 26,
+    /// Group the device belongs to
+    GROUP = 27,
+    NET_NS_FD = 28,
+    /// Extended info mask, VFs, etc.
+    EXT_MASK = 29,
+    /// Promiscuity count: > 0 means acts PROMISC
+    PROMISCUITY = 30,
+    NUM_TX_QUEUES = 31,
+    NUM_RX_QUEUES = 32,
+    CARRIER = 33,
+    PHYS_PORT_ID = 34,
+    CARRIER_CHANGES = 35,
+    PHYS_SWITCH_ID = 36,
+    LINK_NETNSID = 37,
+    PHYS_PORT_NAME = 38,
+    PROTO_DOWN = 39,
+    GSO_MAX_SEGS = 40,
+    GSO_MAX_SIZE = 41,
+    PAD = 42,
+    XDP = 43,
+    EVENT = 44,
+    NEW_NETNSID = 45,
+    IF_NETNSID = 46,
+    CARRIER_UP_COUNT = 47,
+    CARRIER_DOWN_COUNT = 48,
+    NEW_IFINDEX = 49,
+    MIN_MTU = 50,
+    MAX_MTU = 51,
+    PROP_LIST = 52,
+    /// Alternative ifname
+    ALT_IFNAME = 53,
+    PERM_ADDRESS = 54,
+    PROTO_DOWN_REASON = 55,
+    PARENT_DEV_NAME = 56,
+    PARENT_DEV_BUS_NAME = 57,
+}
+
+#[derive(Debug)]
+pub enum LinkAttr {
+    Name(CString),
+    Mtu(u32),
+    TxqLen(u32),
+    LinkMode(u8),
+    ExtMask(RtExtFilter),
+}
+
+impl LinkAttr {
+    fn class(&self) -> LinkAttrClass {
+        match self {
+            LinkAttr::Name(_) => LinkAttrClass::IFNAME,
+            LinkAttr::Mtu(_) => LinkAttrClass::MTU,
+            LinkAttr::TxqLen(_) => LinkAttrClass::TXQLEN,
+            LinkAttr::LinkMode(_) => LinkAttrClass::LINKMODE,
+            LinkAttr::ExtMask(_) => LinkAttrClass::EXT_MASK,
+        }
+    }
+}
+
+impl Attribute for LinkAttr {
+    fn type_(&self) -> u16 {
+        self.class() as u16
+    }
+
+    fn payload_as_bytes(&self) -> &[u8] {
+        match self {
+            LinkAttr::Name(name) => name.as_bytes_with_nul(),
+            LinkAttr::Mtu(mtu) => mtu.as_bytes(),
+            LinkAttr::TxqLen(txq_len) => txq_len.as_bytes(),
+            LinkAttr::LinkMode(link_mode) => link_mode.as_bytes(),
+            LinkAttr::ExtMask(ext_filter) => ext_filter.as_bytes(),
+        }
+    }
+
+    fn read_from(reader: &mut dyn MultiRead) -> Result<Self>
+    where
+        Self: Sized,
+    {
+        let header = reader.read_val::<CAttrHeader>()?;
+        // TODO: Currently, `IS_NET_BYTEORDER_MASK` and `IS_NESTED_MASK` are ignored.
+        let res = match LinkAttrClass::try_from(header.type_())? {
+            LinkAttrClass::IFNAME => Self::Name(reader.read_cstring_with_max_len(IFNAME_SIZE)?),
+            LinkAttrClass::MTU => Self::Mtu(reader.read_val()?),
+            LinkAttrClass::TXQLEN => Self::TxqLen(reader.read_val()?),
+            LinkAttrClass::LINKMODE => Self::LinkMode(reader.read_val()?),
+            LinkAttrClass::EXT_MASK => Self::ExtMask(reader.read_val()?),
+            class => {
+                // FIXME: Netlink should ignore all unknown attributes.
+                // But how to decide the payload type if the class is unknown?
+                // Reference: https://docs.kernel.org/userspace-api/netlink/intro.html#unknown-attributes
+                warn!("link attribute `{:?}` is not supported", class);
+                return_errno_with_message!(Errno::EINVAL, "unsupported link attribute");
+            }
+        };
+
+        Ok(res)
+    }
+}
+
+bitflags! {
+    /// New extended info filters for [`NlLinkAttr::ExtMask`].
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/rtnetlink.h#L819>.
+    #[repr(C)]
+    #[derive(Pod)]
+    pub struct RtExtFilter: u32 {
+        const VF = 1 << 0;
+        const BRVLAN = 1 << 1;
+        const BRVLAN_COMPRESSED = 1 << 2;
+        const SKIP_STATS = 1 << 3;
+        const MRP = 1 << 4;
+        const CFM_CONFIG = 1 << 5;
+        const CFM_STATUS = 1 << 6;
+        const MST = 1 << 7;
+    }
+}

kernel/src/net/socket/netlink/route/message/attr/mod.rs

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: MPL-2.0
+
+pub mod addr;
+pub mod link;
+
+/// The size limit for interface names.
+const IFNAME_SIZE: usize = 16;

kernel/src/net/socket/netlink/route/message/mod.rs

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! Netlink message types for the netlink route protocol.
+//!
+//! This module defines how to interpret messages sent from user space and how to write
+//! kernel messages back to user space.
+
+mod attr;
+mod segment;
+
+pub(super) use attr::{addr::AddrAttr, link::LinkAttr};
+pub(super) use segment::{
+    addr::{AddrMessageFlags, AddrSegment, AddrSegmentBody, RtScope},
+    link::{LinkSegment, LinkSegmentBody},
+    RtnlSegment,
+};
+
+use crate::net::socket::netlink::message::Message;
+
+/// A netlink route message.
+pub(super) type RtnlMessage = Message<RtnlSegment>;

kernel/src/net/socket/netlink/route/message/segment/addr.rs

@@ -0,0 +1,115 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use core::num::NonZeroU32;
+
+use super::legacy::CRtGenMsg;
+use crate::{
+    net::socket::netlink::{
+        message::{SegmentBody, SegmentCommon},
+        route::message::attr::addr::AddrAttr,
+    },
+    prelude::*,
+};
+
+pub type AddrSegment = SegmentCommon<AddrSegmentBody, AddrAttr>;
+
+impl SegmentBody for AddrSegmentBody {
+    type CLegacyType = CRtGenMsg;
+    type CType = CIfaddrMsg;
+}
+
+/// `ifaddrmsg` in Linux.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/if_addr.h#L8>.
+#[repr(C)]
+#[derive(Debug, Clone, Copy, Pod)]
+pub struct CIfaddrMsg {
+    pub family: u8,
+    /// The prefix length
+    pub prefix_len: u8,
+    /// Flags
+    pub flags: u8,
+    /// Address scope
+    pub scope: u8,
+    /// Link index
+    pub index: u32,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct AddrSegmentBody {
+    pub family: i32,
+    pub prefix_len: u8,
+    pub flags: AddrMessageFlags,
+    pub scope: RtScope,
+    pub index: Option<NonZeroU32>,
+}
+
+impl TryFrom<CIfaddrMsg> for AddrSegmentBody {
+    type Error = Error;
+
+    fn try_from(value: CIfaddrMsg) -> Result<Self> {
+        // TODO: If the attribute IFA_FLAGS exists, the flags in header should be ignored.
+        let flags = AddrMessageFlags::from_bits_truncate(value.flags as u32);
+        let scope = RtScope::try_from(value.scope)?;
+        let index = NonZeroU32::new(value.index);
+
+        Ok(Self {
+            family: value.family as i32,
+            prefix_len: value.prefix_len,
+            flags,
+            scope,
+            index,
+        })
+    }
+}
+
+impl From<AddrSegmentBody> for CIfaddrMsg {
+    fn from(value: AddrSegmentBody) -> Self {
+        let index = if let Some(index) = value.index {
+            index.get()
+        } else {
+            0
+        };
+        CIfaddrMsg {
+            family: value.family as u8,
+            prefix_len: value.prefix_len,
+            flags: value.flags.bits() as u8,
+            scope: value.scope as _,
+            index,
+        }
+    }
+}
+
+bitflags! {
+    /// Flags in [`CIfaddrMsg`].
+    ///
+    /// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/if_addr.h#L45>.
+    pub struct AddrMessageFlags: u32 {
+        const SECONDARY      = 0x01;
+        const NODAD          = 0x02;
+        const OPTIMISTIC     = 0x04;
+        const DADFAILED      = 0x08;
+        const HOMEADDRESS    = 0x10;
+        const DEPRECATED	 = 0x20;
+        const TENTATIVE		 = 0x40;
+        const PERMANENT		 = 0x80;
+        const MANAGETEMPADDR = 0x100;
+        const NOPREFIXROUTE	 = 0x200;
+        const MCAUTOJOIN	 = 0x400;
+        const STABLE_PRIVACY = 0x800;
+    }
+}
+
+/// `rt_scope_t` in Linux.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/rtnetlink.h#L320>.
+#[repr(u8)]
+#[derive(Debug, Clone, Copy, TryFromInt)]
+pub enum RtScope {
+    UNIVERSE = 0,
+    // User defined values
+    SITE = 200,
+    LINK = 253,
+    HOST = 254,
+    NOWHERE = 255,
+}

kernel/src/net/socket/netlink/route/message/segment/legacy.rs

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use super::{addr::CIfaddrMsg, link::CIfinfoMsg};
+use crate::prelude::*;
+
+/// `rtgenmsg` in Linux.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/rtnetlink.h#L548>.
+#[derive(Debug, Clone, Copy, Pod)]
+#[repr(C)]
+pub struct CRtGenMsg {
+    pub family: u8,
+}
+
+impl From<CRtGenMsg> for CIfinfoMsg {
+    fn from(value: CRtGenMsg) -> Self {
+        Self {
+            family: value.family,
+            _pad: 0,
+            type_: 0,
+            index: 0,
+            flags: 0,
+            change: 0,
+        }
+    }
+}
+
+impl From<CRtGenMsg> for CIfaddrMsg {
+    fn from(value: CRtGenMsg) -> Self {
+        Self {
+            family: value.family,
+            prefix_len: 0,
+            flags: 0,
+            scope: 0,
+            index: 0,
+        }
+    }
+}

kernel/src/net/socket/netlink/route/message/segment/link.rs

@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use core::num::NonZeroU32;
+
+use aster_bigtcp::iface::{InterfaceFlags, InterfaceType};
+
+use super::legacy::CRtGenMsg;
+use crate::{
+    net::socket::netlink::{
+        message::{SegmentBody, SegmentCommon},
+        route::message::attr::link::LinkAttr,
+    },
+    prelude::*,
+    util::net::CSocketAddrFamily,
+};
+
+pub type LinkSegment = SegmentCommon<LinkSegmentBody, LinkAttr>;
+
+impl SegmentBody for LinkSegmentBody {
+    type CLegacyType = CRtGenMsg;
+    type CType = CIfinfoMsg;
+}
+
+/// `ifinfomsg` in Linux.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/rtnetlink.h#L561>.
+#[repr(C)]
+#[derive(Debug, Clone, Copy, Pod)]
+pub struct CIfinfoMsg {
+    /// AF_UNSPEC
+    pub family: u8,
+    /// Padding byte
+    pub _pad: u8,
+    /// Device type
+    pub type_: u16,
+    /// Interface index
+    pub index: u32,
+    /// Device flags
+    pub flags: u32,
+    /// Change mask
+    pub change: u32,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct LinkSegmentBody {
+    pub family: CSocketAddrFamily,
+    pub type_: InterfaceType,
+    pub index: Option<NonZeroU32>,
+    pub flags: InterfaceFlags,
+}
+
+impl TryFrom<CIfinfoMsg> for LinkSegmentBody {
+    type Error = Error;
+
+    fn try_from(value: CIfinfoMsg) -> Result<Self> {
+        let family = CSocketAddrFamily::try_from(value.family as i32)?;
+        let type_ = InterfaceType::try_from(value.type_)?;
+        let index = NonZeroU32::new(value.index);
+        let flags = InterfaceFlags::from_bits_truncate(value.flags);
+
+        Ok(Self {
+            family,
+            type_,
+            index,
+            flags,
+        })
+    }
+}
+
+impl From<LinkSegmentBody> for CIfinfoMsg {
+    fn from(value: LinkSegmentBody) -> Self {
+        CIfinfoMsg {
+            family: value.family as _,
+            _pad: 0,
+            type_: value.type_ as _,
+            index: value.index.map(NonZeroU32::get).unwrap_or(0),
+            flags: value.flags.bits(),
+            change: 0,
+        }
+    }
+}

kernel/src/net/socket/netlink/route/message/segment/mod.rs

@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! This module defines the message segment,
+//! which is the basic unit of a netlink message.
+//!
+//! Typically, a segment will consist of three parts:
+//!
+//! 1. Header: The headers of all segments are of type [`CMessageSegmentHeader`],
+//! which indicate the type and total length of the segment.
+//!
+//! 2. Body: The body is the main component of a segment.
+//! Each segment will have one and only one body.
+//! The body type is defined by the `type_` field of the header.
+//!
+//! 3. Attributes: Attributes are optional.
+//! A segment can have zero or multiple attributes.
+//! Attributes belong to different classes,
+//! with the class defined by the `type_` field of the header.
+//! The total number of attributes is controlled by the `len` field of the header.
+//!
+//! Note that all headers, bodies, and attributes require
+//! their starting address in memory to be aligned to [`super::NLMSG_ALIGN`]
+//! when copying to and from user space.
+//! Therefore, necessary padding must be added to ensure alignment.
+//!
+//! The layout of a segment in memory is shown below:
+//!
+//! ┌────────┬─────────┬──────┬─────────┬──────┬──────┬──────┐
+//! │ Header │ Padding │ Body │ Padding │ Attr │ Attr │ Attr │
+//! └────────┴─────────┴──────┴─────────┴──────┴──────┴──────┘
+
+pub mod addr;
+mod legacy;
+pub mod link;
+pub mod route;
+
+use addr::AddrSegment;
+use link::LinkSegment;
+
+use crate::{
+    net::socket::netlink::message::{
+        CMsgSegHdr, CSegmentType, DoneSegment, ErrorSegment, ProtocolSegment,
+    },
+    prelude::*,
+    util::{MultiRead, MultiWrite},
+};
+
+/// The netlink route segment, which is the basic unit of a netlink route message.
+#[derive(Debug)]
+pub enum RtnlSegment {
+    NewLink(LinkSegment),
+    GetLink(LinkSegment),
+    NewAddr(AddrSegment),
+    GetAddr(AddrSegment),
+    Done(DoneSegment),
+    Error(ErrorSegment),
+}
+
+impl ProtocolSegment for RtnlSegment {
+    fn header(&self) -> &CMsgSegHdr {
+        match self {
+            RtnlSegment::NewLink(link_segment) | RtnlSegment::GetLink(link_segment) => {
+                link_segment.header()
+            }
+            RtnlSegment::NewAddr(addr_segment) | RtnlSegment::GetAddr(addr_segment) => {
+                addr_segment.header()
+            }
+            RtnlSegment::Done(done_segment) => done_segment.header(),
+            RtnlSegment::Error(error_segment) => error_segment.header(),
+        }
+    }
+
+    fn header_mut(&mut self) -> &mut CMsgSegHdr {
+        match self {
+            RtnlSegment::NewLink(link_segment) | RtnlSegment::GetLink(link_segment) => {
+                link_segment.header_mut()
+            }
+            RtnlSegment::NewAddr(addr_segment) | RtnlSegment::GetAddr(addr_segment) => {
+                addr_segment.header_mut()
+            }
+            RtnlSegment::Done(done_segment) => done_segment.header_mut(),
+            RtnlSegment::Error(error_segment) => error_segment.header_mut(),
+        }
+    }
+
+    fn read_from(reader: &mut dyn MultiRead) -> Result<Self> {
+        let header = reader.read_val::<CMsgSegHdr>()?;
+
+        let segment = match CSegmentType::try_from(header.type_)? {
+            CSegmentType::GETLINK => RtnlSegment::GetLink(LinkSegment::read_from(header, reader)?),
+            CSegmentType::GETADDR => RtnlSegment::GetAddr(AddrSegment::read_from(header, reader)?),
+            _ => return_errno_with_message!(Errno::EINVAL, "unsupported segment type"),
+        };
+
+        Ok(segment)
+    }
+
+    fn write_to(&self, writer: &mut dyn MultiWrite) -> Result<()> {
+        match self {
+            RtnlSegment::NewLink(link_segment) => link_segment.write_to(writer)?,
+            RtnlSegment::NewAddr(addr_segment) => addr_segment.write_to(writer)?,
+            RtnlSegment::Done(done_segment) => done_segment.write_to(writer)?,
+            RtnlSegment::Error(error_segment) => error_segment.write_to(writer)?,
+            RtnlSegment::GetAddr(_) | RtnlSegment::GetLink(_) => {
+                unreachable!("kernel should not write get requests to user space");
+            }
+        }
+        Ok(())
+    }
+}

kernel/src/net/socket/netlink/route/message/segment/route.rs

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use crate::prelude::*;
+
+/// `rtmsg` in Linux.
+///
+/// Reference: <https://elixir.bootlin.com/linux/v6.13/source/include/uapi/linux/rtnetlink.h#L237>.
+#[repr(C)]
+#[derive(Debug, Clone, Copy, Pod)]
+#[expect(unused)]
+pub(super) struct CRtMsg {
+    family: u8,
+    dst_len: u8,
+    src_len: u8,
+    tos: u8,
+    table: u8,
+    protocol: u8,
+    scope: u8,
+    type_: u8,
+    flags: u32,
+}

kernel/src/util/iovec.rs

@@ -154,6 +154,13 @@ pub trait MultiRead: ReadCString {
     fn is_empty(&self) -> bool {
         self.sum_lens() == 0
     }
+
+    /// Skips the first `nbytes` bytes of data.
+    ///
+    /// # Panics
+    ///
+    /// If `nbytes` is greater that [`MultiRead::sum_lens`], this method will panic.
+    fn skip(&mut self, nbytes: usize);
 }
 
 /// Trait defining the write behavior for a collection of [`VmWriter`]s.
@@ -177,6 +184,13 @@ pub trait MultiWrite {
     fn is_empty(&self) -> bool {
         self.sum_lens() == 0
     }
+
+    /// Skips the first `nbytes` bytes of space.
+    ///
+    /// # Panics
+    ///
+    /// If `nbytes` is greater that [`MultiWrite::sum_lens`], this method will panic.
+    fn skip(&mut self, nbytes: usize);
 }
 
 impl MultiRead for VmReaderArray<'_> {
@@ -196,6 +210,23 @@ impl MultiRead for VmReaderArray<'_> {
     fn sum_lens(&self) -> usize {
         self.0.iter().map(|vm_reader| vm_reader.remain()).sum()
     }
+
+    fn skip(&mut self, mut nbytes: usize) {
+        for reader in &mut self.0 {
+            let bytes_to_skip = reader.remain().min(nbytes);
+            reader.skip(bytes_to_skip);
+            nbytes -= bytes_to_skip;
+
+            if nbytes == 0 {
+                return;
+            }
+        }
+
+        panic!(
+            "the readers are exhausted but there are {} bytes remaining to skip",
+            nbytes
+        );
+    }
 }
 
 impl MultiRead for VmReader<'_> {
@@ -206,6 +237,18 @@ impl MultiRead for VmReader<'_> {
     fn sum_lens(&self) -> usize {
         self.remain()
     }
+
+    fn skip(&mut self, nbytes: usize) {
+        VmReader::skip(self, nbytes);
+    }
+}
+
+impl dyn MultiRead + '_ {
+    pub fn read_val<T: Pod>(&mut self) -> Result<T> {
+        let mut val = T::new_zeroed();
+        self.read(&mut VmWriter::from(val.as_bytes_mut()))?;
+        Ok(val)
+    }
 }
 
 impl MultiWrite for VmWriterArray<'_> {
@@ -225,6 +268,23 @@ impl MultiWrite for VmWriterArray<'_> {
     fn sum_lens(&self) -> usize {
         self.0.iter().map(|vm_writer| vm_writer.avail()).sum()
     }
+
+    fn skip(&mut self, mut nbytes: usize) {
+        for writer in &mut self.0 {
+            let bytes_to_skip = writer.avail().min(nbytes);
+            writer.skip(bytes_to_skip);
+            nbytes -= bytes_to_skip;
+
+            if nbytes == 0 {
+                return;
+            }
+        }
+
+        panic!(
+            "the writers are exhausted but there are {} bytes remaining to skip",
+            nbytes
+        );
+    }
 }
 
 impl MultiWrite for VmWriter<'_> {
@@ -235,4 +295,15 @@ impl MultiWrite for VmWriter<'_> {
     fn sum_lens(&self) -> usize {
         self.avail()
     }
+
+    fn skip(&mut self, nbytes: usize) {
+        VmWriter::skip(self, nbytes);
+    }
+}
+
+impl dyn MultiWrite + '_ {
+    pub fn write_val<T: Pod>(&mut self, val: &T) -> Result<()> {
+        self.write(&mut VmReader::from(val.as_bytes()))?;
+        Ok(())
+    }
 }