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Fixed seek undefined behavior on signed integer overflow

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In the previous implementation of lfs_file_seek, we calculated the new
offset using signed arithmetic before checking for possible
overflow/underflow conditions. This results in undefined behavior in C.

Fortunately for us, littlefs is now limited to 31-bit file sizes for API
reasons, so we don't have to be too clever here. Doing the arithmetic
with unsigned integers and just checking if we're in a valid range
afterwards should work.

Found by m-kostrzewa and lucic71
This commit is contained in:
Christopher Haster 2024-09-24 13:47:28 -05:00
parent f1c430e779
commit abaec45652
1 changed files with 5 additions and 11 deletions
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16
lfs.c
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@ -3664,22 +3664,16 @@ static lfs_ssize_t lfs_file_write_(lfs_t *lfs, lfs_file_t *file,
static lfs_soff_t lfs_file_seek_(lfs_t *lfs, lfs_file_t *file,
lfs_soff_t off, int whence) {
// find new pos
//
// fortunately for us, littlefs is limited to 31-bit file sizes, so we
// don't have to worry too much about integer overflow
lfs_off_t npos = file->pos;
if (whence == LFS_SEEK_SET) {
npos = off;
} else if (whence == LFS_SEEK_CUR) {
if ((lfs_soff_t)file->pos + off < 0) {
return LFS_ERR_INVAL;
} else {
npos = file->pos + off;
}
npos = file->pos + (lfs_off_t)off;
} else if (whence == LFS_SEEK_END) {
lfs_soff_t res = lfs_file_size_(lfs, file) + off;
if (res < 0) {
return LFS_ERR_INVAL;
} else {
npos = res;
}
npos = (lfs_off_t)lfs_file_size_(lfs, file) + (lfs_off_t)off;
}
if (npos > lfs->file_max) {