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Add fbounds-safety annotations for VP8LBitWriter.

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Reasoning:

The struct `VP8LBitWriter` in `src/utils/bit_writer_utils.h` uses
`buf`, `cur`, and `end` pointers to manage a dynamic buffer. Pointer
arithmetic on `buf` and `cur` caused bounds safety errors.

Initial attempts using `WEBP_ENDED_BY(end)` for both `buf` and `cur`
failed due to compiler limitations (one field cannot be the upper
bound for multiple other fields). Using `WEBP_INDEXABLE` for `buf` and
`cur` resolved the arithmetic errors but caused ABI incompatibility
issues, as `VP8LBitWriter` is used in multiple compilation units with
different settings.

The final approach annotates `buf` with `WEBP_ENDED_BY(end)` and `cur`
with `WEBP_UNSAFE_INDEXABLE`. This resolves the arithmetic errors for
both pointers without changing the ABI. `WEBP_UNSAFE_INDEXABLE` is
used for `cur` as a workaround for the `WEBP_ENDED_BY` limitation and
ABI constraints.

Additionally, the `VP8LBitWriterResize` function in
`src/utils/bit_writer_utils.c` was modified:
- The assignments to `bw->buf`, `bw->end`, and `bw->cur` (lines
  231-233) were reordered and updated to use the local
  `allocated_buf` variable on the right-hand side. This satisfies the
  consecutive assignment requirement imposed by `WEBP_ENDED_BY(end)`
  on `bw->buf`.
- The local variable `allocated_buf` (line 207) was annotated with
  `WEBP_BIDI_INDEXABLE`.
- The allocation for `allocated_buf` (line 222) now uses
  `WEBP_UNSAFE_FORGE_BIDI_INDEXABLE` to create a safe, bounded
  pointer from the result of `WebPSafeMalloc`, fixing potential type
  mismatches when `suppress_fbounds_errors=yes`.

Bug: 432511821
Change-Id: I603a6a7d3ff3bf2a8edf9749a9898ea227c32982
This commit is contained in:
Arman Hasanzadeh
2025-08-14 08:36:16 -07:00
parent 2246828be3
commit 30b2c593c9
2 changed files with 10 additions and 9 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
src/utils/bit_writer_utils.c
View File

@@ -204,7 +204,7 @@ void VP8BitWriterWipeOut(VP8BitWriter* const bw) {
// Returns 1 on success.
static int VP8LBitWriterResize(VP8LBitWriter* const bw, size_t extra_size) {
uint8_t* allocated_buf;
uint8_t* WEBP_BIDI_INDEXABLE allocated_buf;
size_t allocated_size;
const size_t max_bytes = bw->end - bw->buf;
const size_t current_size = bw->cur - bw->buf;
@@ -219,7 +219,8 @@ static int VP8LBitWriterResize(VP8LBitWriter* const bw, size_t extra_size) {
if (allocated_size < size_required) allocated_size = size_required;
// make allocated size multiple of 1k
allocated_size = (((allocated_size >> 10) + 1) << 10);
allocated_buf = (uint8_t*)WebPSafeMalloc(1ULL, allocated_size);
allocated_buf = (uint8_t*)WEBP_UNSAFE_FORGE_BIDI_INDEXABLE(
void*, WebPSafeMalloc(1ULL, allocated_size), allocated_size);
if (allocated_buf == NULL) {
bw->error = 1;
return 0;
@@ -229,8 +230,8 @@ static int VP8LBitWriterResize(VP8LBitWriter* const bw, size_t extra_size) {
}
WebPSafeFree(bw->buf);
bw->buf = allocated_buf;
bw->cur = bw->buf + current_size;
bw->end = bw->buf + allocated_size;
bw->end = allocated_buf + allocated_size;
bw->cur = allocated_buf + current_size;
return 1;
}

10
src/utils/bit_writer_utils.h
View File

@@ -92,11 +92,11 @@ typedef uint16_t vp8l_wtype_t;
#endif
typedef struct {
vp8l_atype_t bits; // bit accumulator
int used; // number of bits used in accumulator
uint8_t* buf; // start of buffer
uint8_t* cur; // current write position
uint8_t* end; // end of buffer
vp8l_atype_t bits; // bit accumulator
int used; // number of bits used in accumulator
uint8_t* WEBP_ENDED_BY(end) buf; // start of buffer
uint8_t* WEBP_UNSAFE_INDEXABLE cur; // current write position
uint8_t* end; // end of buffer
// After all bits are written (VP8LBitWriterFinish()), the caller must observe
// the state of 'error'. A value of 1 indicates that a memory allocation