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Use REF_COUNT_MAX to avoid overflowing

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This commit is contained in:
Ruihan Li
2024-12-23 23:09:19 +08:00
committed by Tate, Hongliang Tian
parent f03ffd0d10
commit 5aa73d3bcf
2 changed files with 33 additions and 5 deletions
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26
ostd/src/mm/page/meta.rs
View File

@ -53,6 +53,7 @@ use crate::{
kspace::LINEAR_MAPPING_BASE_VADDR, paddr_to_vaddr, page_size, page_table::boot_pt,
CachePolicy, Paddr, PageFlags, PageProperty, PrivilegedPageFlags, Vaddr, PAGE_SIZE,
},
panic::abort,
};
/// The maximum number of bytes of the metadata of a page.
@ -81,7 +82,10 @@ pub(in crate::mm) struct MetaSlot {
/// * `REF_COUNT_UNUSED`: The page is not in use.
/// * `0`: The page is being constructed ([`Page::from_unused`])
/// or destructured ([`drop_last_in_place`]).
/// * `1..u32::MAX`: The page is in use.
/// * `1..REF_COUNT_MAX`: The page is in use.
/// * `REF_COUNT_MAX..REF_COUNT_UNUSED`: Illegal values to
/// prevent the reference count from overflowing. Otherwise,
/// overflowing the reference count will cause soundness issue.
///
/// [`Page::from_unused`]: super::Page::from_unused
pub(super) ref_count: AtomicU32,
@ -90,6 +94,7 @@ pub(in crate::mm) struct MetaSlot {
}
pub(super) const REF_COUNT_UNUSED: u32 = u32::MAX;
const REF_COUNT_MAX: u32 = i32::MAX as u32;
type PageMetaVtablePtr = core::ptr::DynMetadata<dyn PageMeta>;
@ -132,6 +137,25 @@ macro_rules! impl_page_meta {
pub use impl_page_meta;
impl MetaSlot {
/// Increases the page reference count by one.
///
/// # Safety
///
/// The caller must have already held a reference to the page.
pub(super) unsafe fn inc_ref_count(&self) {
let last_ref_cnt = self.ref_count.fetch_add(1, Ordering::Relaxed);
debug_assert!(last_ref_cnt != 0 && last_ref_cnt != REF_COUNT_UNUSED);
if last_ref_cnt >= REF_COUNT_MAX {
// This follows the same principle as the `Arc::clone` implementation to prevent the
// reference count from overflowing. See also
// <https://doc.rust-lang.org/std/sync/struct.Arc.html#method.clone>.
abort();
}
}
}
/// An internal routine in dropping implementations.
///
/// # Safety

12
ostd/src/mm/page/mod.rs
View File

@ -189,7 +189,9 @@ impl<M: PageMeta> Page<M> {
impl<M: PageMeta> Clone for Page<M> {
fn clone(&self) -> Self {
self.slot().ref_count.fetch_add(1, Ordering::Relaxed);
// SAFETY: We have already held a reference to the page.
unsafe { self.slot().inc_ref_count() };
Self {
ptr: self.ptr,
_marker: PhantomData,
@ -331,7 +333,9 @@ impl From<Frame> for DynPage {
impl Clone for DynPage {
fn clone(&self) -> Self {
self.slot().ref_count.fetch_add(1, Ordering::Relaxed);
// SAFETY: We have already held a reference to the page.
unsafe { self.slot().inc_ref_count() };
Self { ptr: self.ptr }
}
}
@ -370,6 +374,6 @@ pub(in crate::mm) unsafe fn inc_page_ref_count(paddr: Paddr) {
// an immutable reference to it is always safe.
let slot = unsafe { &*(vaddr as *const MetaSlot) };
let old = slot.ref_count.fetch_add(1, Ordering::Relaxed);
debug_assert!(old > 0);
// SAFETY: We have already held a reference to the page.
unsafe { slot.inc_ref_count() };
}