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Rework std::sys::windows::alloc

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Add documentation to the system functions and `SAFETY` comments.
Refactored helper functions, fixing the correctness of `get_header`.
This commit is contained in:
Christiaan Dirkx 2021-03-12 21:32:44 +01:00
parent db492ecd5b
commit 0dbed6161a
3 changed files with 174 additions and 33 deletions
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191
library/std/src/sys/windows/alloc.rs
View file

@ -1,51 +1,179 @@
#![deny(unsafe_op_in_unsafe_fn)]
use crate::alloc::{GlobalAlloc, Layout, System};
use crate::ptr;
use crate::sys::c;
use crate::sys_common::alloc::{realloc_fallback, MIN_ALIGN};
#[cfg(test)]
mod tests;
// Heap memory management on Windows is done by using the system Heap API (heapapi.h)
// See https://docs.microsoft.com/windows/win32/api/heapapi/
// Flag to indicate that the memory returned by `HeapAlloc` should be zeroed.
const HEAP_ZERO_MEMORY: c::DWORD = 0x00000008;
extern "system" {
// Get a handle to the default heap of the current process, or null if the operation fails.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-getprocessheap
fn GetProcessHeap() -> c::HANDLE;
// Allocate a block of `dwBytes` bytes of memory from a given heap `hHeap`.
// The allocated memory may be uninitialized, or zeroed if `dwFlags` is
// set to `HEAP_ZERO_MEMORY`.
//
// Returns a pointer to the newly-allocated memory or null if the operation fails.
// The returned pointer will be aligned to at least `MIN_ALIGN`.
//
// SAFETY:
// - `hHeap` must be a non-null handle returned by `GetProcessHeap`.
// - `dwFlags` must be set to either zero or `HEAP_ZERO_MEMORY`.
//
// Note that `dwBytes` is allowed to be zero, contrary to some other allocators.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heapalloc
fn HeapAlloc(hHeap: c::HANDLE, dwFlags: c::DWORD, dwBytes: c::SIZE_T) -> c::LPVOID;
// Reallocate a block of memory behind a given pointer `lpMem` from a given heap `hHeap`,
// to a block of at least `dwBytes` bytes, either shrinking the block in place,
// or allocating at a new location, copying memory, and freeing the original location.
//
// Returns a pointer to the reallocated memory or null if the operation fails.
// The returned pointer will be aligned to at least `MIN_ALIGN`.
// If the operation fails the given block will never have been freed.
//
// SAFETY:
// - `hHeap` must be a non-null handle returned by `GetProcessHeap`.
// - `dwFlags` must be set to zero.
// - `lpMem` must be a non-null pointer to an allocated block returned by `HeapAlloc` or
// `HeapReAlloc`, that has not already been freed.
// If the block was successfully reallocated at a new location, pointers pointing to
// the freed memory, such as `lpMem`, must not be dereferenced ever again.
//
// Note that `dwBytes` is allowed to be zero, contrary to some other allocators.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heaprealloc
fn HeapReAlloc(
hHeap: c::HANDLE,
dwFlags: c::DWORD,
lpMem: c::LPVOID,
dwBytes: c::SIZE_T,
) -> c::LPVOID;
// Free a block of memory behind a given pointer `lpMem` from a given heap `hHeap`.
// Returns a nonzero value if the operation is successful, and zero if the operation fails.
//
// SAFETY:
// - `dwFlags` must be set to zero.
// - `lpMem` must be a pointer to an allocated block returned by `HeapAlloc` or `HeapReAlloc`,
// that has not already been freed.
// If the block was successfully freed, pointers pointing to the freed memory, such as `lpMem`,
// must not be dereferenced ever again.
//
// Note that both `hHeap` is allowed to be any value, and `lpMem` is allowed to be null,
// both of which will not cause the operation to fail.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heapfree
fn HeapFree(hHeap: c::HANDLE, dwFlags: c::DWORD, lpMem: c::LPVOID) -> c::BOOL;
}
// Header containing a pointer to the start of an allocated block.
// SAFETY: size and alignment must be <= `MIN_ALIGN`.
#[repr(C)]
struct Header(*mut u8);
unsafe fn get_header<'a>(ptr: *mut u8) -> &'a mut Header {
&mut *(ptr as *mut Header).offset(-1)
}
unsafe fn align_ptr(ptr: *mut u8, align: usize) -> *mut u8 {
let aligned = ptr.add(align - (ptr as usize & (align - 1)));
*get_header(aligned) = Header(ptr);
aligned
}
// Allocates a block of optionally zeroed memory for a given `layout`.
// Returns a pointer satisfying the guarantees of `System` about allocated pointers.
#[inline]
unsafe fn allocate_with_flags(layout: Layout, flags: c::DWORD) -> *mut u8 {
if layout.align() <= MIN_ALIGN {
return c::HeapAlloc(c::GetProcessHeap(), flags, layout.size()) as *mut u8;
unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
let heap = unsafe { GetProcessHeap() };
if heap.is_null() {
// Allocation has failed, could not get the current process heap.
return ptr::null_mut();
}
let size = layout.size() + layout.align();
let ptr = c::HeapAlloc(c::GetProcessHeap(), flags, size);
if ptr.is_null() { ptr as *mut u8 } else { align_ptr(ptr as *mut u8, layout.align()) }
// Allocated memory will be either zeroed or uninitialized.
let flags = if zeroed { HEAP_ZERO_MEMORY } else { 0 };
if layout.align() <= MIN_ALIGN {
// SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`.
// The returned pointer points to the start of an allocated block.
unsafe { HeapAlloc(heap, flags, layout.size()) as *mut u8 }
} else {
// Allocate extra padding in order to be able to satisfy the alignment.
let total = layout.align() + layout.size();
// SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`.
let ptr = unsafe { HeapAlloc(heap, flags, total) as *mut u8 };
if ptr.is_null() {
// Allocation has failed.
return ptr::null_mut();
}
// Create a correctly aligned pointer offset from the start of the allocated block,
// and write a header before it.
let offset = layout.align() - (ptr as usize & (layout.align() - 1));
// SAFETY: `MIN_ALIGN` <= `offset` <= `layout.align()` and the size of the allocated
// block is `layout.align() + layout.size()`. `aligned` will thus be a correctly aligned
// pointer inside the allocated block with at least `layout.size()` bytes after it and at
// least `MIN_ALIGN` bytes of padding before it.
let aligned = unsafe { ptr.add(offset) };
// SAFETY: Because the size and alignment of a header is <= `MIN_ALIGN` and `aligned`
// is aligned to at least `MIN_ALIGN` and has at least `MIN_ALIGN` bytes of padding before
// it, it is safe to write a header directly before it.
unsafe { ptr::write((aligned as *mut Header).offset(-1), Header(ptr)) };
// SAFETY: The returned pointer does not point to the to the start of an allocated block,
// but there is a header readable directly before it containing the location of the start
// of the block.
aligned
}
}
// All pointers returned by this allocator have, in addition to the guarantees of `GlobalAlloc`, the
// following properties:
//
// If the pointer was allocated or reallocated with a `layout` specifying an alignment <= `MIN_ALIGN`
// the pointer will be aligned to at least `MIN_ALIGN` and point to the start of the allocated block.
//
// If the pointer was allocated or reallocated with a `layout` specifying an alignment > `MIN_ALIGN`
// the pointer will be aligned to the specified alignment and not point to the start of the allocated block.
// Instead there will be a header readable directly before the returned pointer, containing the actual
// location of the start of the block.
#[stable(feature = "alloc_system_type", since = "1.28.0")]
unsafe impl GlobalAlloc for System {
#[inline]
unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
allocate_with_flags(layout, 0)
// SAFETY: pointers returned by `allocate` satisfy the guarantees of `System`
unsafe { allocate(layout, false) }
}
#[inline]
unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
allocate_with_flags(layout, c::HEAP_ZERO_MEMORY)
// SAFETY: pointers returned by `allocate` satisfy the guarantees of `System`
unsafe { allocate(layout, true) }
}
#[inline]
unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
if layout.align() <= MIN_ALIGN {
let err = c::HeapFree(c::GetProcessHeap(), 0, ptr as c::LPVOID);
debug_assert!(err != 0, "Failed to free heap memory: {}", c::GetLastError());
} else {
let header = get_header(ptr);
let err = c::HeapFree(c::GetProcessHeap(), 0, header.0 as c::LPVOID);
let block = {
if layout.align() <= MIN_ALIGN {
ptr
} else {
// The location of the start of the block is stored in the padding before `ptr`.
// SAFETY: Because of the contract of `System`, `ptr` is guaranteed to be non-null
// and have a header readable directly before it.
unsafe { ptr::read((ptr as *mut Header).offset(-1)).0 }
}
};
// SAFETY: `block` is a pointer to the start of an allocated block.
unsafe {
let err = HeapFree(GetProcessHeap(), 0, block as c::LPVOID);
debug_assert!(err != 0, "Failed to free heap memory: {}", c::GetLastError());
}
}
@ -53,9 +181,20 @@ unsafe impl GlobalAlloc for System {
#[inline]
unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
if layout.align() <= MIN_ALIGN {
c::HeapReAlloc(c::GetProcessHeap(), 0, ptr as c::LPVOID, new_size) as *mut u8
let heap = unsafe { GetProcessHeap() };
if heap.is_null() {
// Reallocation has failed, could not get the current process heap.
return ptr::null_mut();
}
// SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`,
// `ptr` is a pointer to the start of an allocated block.
// The returned pointer points to the start of an allocated block.
unsafe { HeapReAlloc(heap, 0, ptr as c::LPVOID, new_size) as *mut u8 }
} else {
realloc_fallback(self, ptr, layout, new_size)
// SAFETY: `realloc_fallback` is implemented using `dealloc` and `alloc`, which will
// correctly handle `ptr` and return a pointer satisfying the guarantees of `System`
unsafe { realloc_fallback(self, ptr, layout, new_size) }
}
}
}

9
library/std/src/sys/windows/alloc/tests.rs Normal file
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@ -0,0 +1,9 @@
use super::{Header, MIN_ALIGN};
use crate::mem;
#[test]
fn alloc_header() {
// Header must fit in the padding before an aligned pointer
assert!(mem::size_of::<Header>() <= MIN_ALIGN);
assert!(mem::align_of::<Header>() <= MIN_ALIGN);
}

7
library/std/src/sys/windows/c.rs
View file

@ -285,8 +285,6 @@ pub const FD_SETSIZE: usize = 64;
pub const STACK_SIZE_PARAM_IS_A_RESERVATION: DWORD = 0x00010000;
pub const HEAP_ZERO_MEMORY: DWORD = 0x00000008;
pub const STATUS_SUCCESS: NTSTATUS = 0x00000000;
#[repr(C)]
@ -1017,11 +1015,6 @@ extern "system" {
timeout: *const timeval,
) -> c_int;
pub fn GetProcessHeap() -> HANDLE;
pub fn HeapAlloc(hHeap: HANDLE, dwFlags: DWORD, dwBytes: SIZE_T) -> LPVOID;
pub fn HeapReAlloc(hHeap: HANDLE, dwFlags: DWORD, lpMem: LPVOID, dwBytes: SIZE_T) -> LPVOID;
pub fn HeapFree(hHeap: HANDLE, dwFlags: DWORD, lpMem: LPVOID) -> BOOL;
// >= Vista / Server 2008
// https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-createsymboliclinkw
pub fn CreateSymbolicLinkW(