From: Nick Terrell <terrelln@fb.com>

Zstd used an array of length 1 to mean a flexible array for C89
compatibility. Switch to a C99 flexible array to fix the UBSAN warning.

Tested locally by booting the kernel and writing to and reading from a
BtrFS filesystem with zstd compression enabled. I was unable to reproduce
the issue before the fix, however it is a trivial change.

Reported-by: syzbot+1f2eb3e8cd123ffce499@syzkaller.appspotmail.com
Reported-by: Eric Biggers <ebiggers@kernel.org>
Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nick Terrell <terrelln@meta.com>
---
 lib/zstd/common/fse_decompress.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/zstd/common/fse_decompress.c b/lib/zstd/common/fse_decompress.c
index a0d06095be83..8dcb8ca39767 100644
--- a/lib/zstd/common/fse_decompress.c
+++ b/lib/zstd/common/fse_decompress.c
@@ -312,7 +312,7 @@ size_t FSE_decompress_wksp(void* dst, size_t dstCapacity, const void* cSrc, size
 
 typedef struct {
     short ncount[FSE_MAX_SYMBOL_VALUE + 1];
-    FSE_DTable dtable[1]; /* Dynamically sized */
+    FSE_DTable dtable[]; /* Dynamically sized */
 } FSE_DecompressWksp;
 
 
-- 
2.42.0

On Thu, Oct 12, 2023 at 02:34:28PM -0700, Nick Terrell wrote:
> From: Nick Terrell <terrelln@fb.com>
> 
> Zstd used an array of length 1 to mean a flexible array for C89
> compatibility. Switch to a C99 flexible array to fix the UBSAN warning.
> 
> Tested locally by booting the kernel and writing to and reading from a
> BtrFS filesystem with zstd compression enabled. I was unable to reproduce
> the issue before the fix, however it is a trivial change.
> 
> Reported-by: syzbot+1f2eb3e8cd123ffce499@syzkaller.appspotmail.com
> Reported-by: Eric Biggers <ebiggers@kernel.org>
> Reported-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Nick Terrell <terrelln@meta.com>

Thanks! This 1-element conversion doesn't appear to need any sizeof()
related changes, so AFAICT this header change is sufficient.

Reviewed-by: Kees Cook <keescook@chromium.org>


-- 
Kees Cook