[PATCH v3] bpf: Annotate bpf_obj_memcpy with data_race

[Quanye Yang via B4 Relay]

From: Quanye Yang <quanyeyang@xxxxxxxxx>

syzbot reported KCSAN write-write races when two tasks concurrently
update the same map value. Both accesses reach the ordinary memcpy()
paths in bpf_obj_memcpy() through copy_map_value().

Unlocked in-place updates of published map values are intentionally not
serialized and may produce torn values. Callers requiring consistency
must provide synchronization appropriate for the map type.
bpf_long_memcpy() already annotates the same behavior for long-aligned
copies.

Annotate the ordinary memcpy() sites in bpf_obj_memcpy() with
data_race(), matching bpf_long_memcpy(). This documents the existing
concurrency semantics and suppresses KCSAN reports for these intentional
races without changing synchronization or map update behavior.

Reported-by: syzbot+44044637ef892e79ca2b@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=44044637ef892e79ca2b
Signed-off-by: Quanye Yang <quanyeyang@xxxxxxxxx>
---
The annotations remain in the common bpf_obj_memcpy() helper, matching
bpf_long_memcpy(). This keeps the existing copy helper interfaces
unchanged. A narrower annotation would require propagating the
concurrency context through copy_map_value() or introducing separate
copy helpers.

The following checkpatch warnings are expected:

- DATA_RACE is reported for the three annotations because checkpatch
  only recognizes an immediately adjacent comment.
- MISSING_FIXES_TAG is reported because the commit references syzkaller.
  No Fixes tag is included because this documents long-standing
  intentional lockless semantics rather than a regression introduced
  by a particular commit.
---
Changes in v3:
- Restore the original bpf_obj_memcpy() comment as suggested by Andrii.
- Use the properly cased full name for authorship and Signed-off-by.
- Link to v2: https://patch.msgid.link/20260820-bpf-kcsan-obj-memcpy-v2-1-672517a3145f@xxxxxxxxx

Changes in v2:
- Drop the BPF_F_LOCK recommendation because it is unavailable for
  per-CPU maps.
- Scope the concurrency description to unlocked in-place updates of
  published map values.
- Fold the redundant commit message paragraphs.
- Link to v1:
  https://patch.msgid.link/20260820-bpf-kcsan-obj-memcpy-v1-1-372c59462268@xxxxxxxxx

To: Alexei Starovoitov <ast@xxxxxxxxxx>
To: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
To: Andrii Nakryiko <andrii@xxxxxxxxxx>
To: Eduard Zingerman <eddyz87@xxxxxxxxx>
To: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
To: Martin KaFai Lau <martin.lau@xxxxxxxxx>
To: Song Liu <song@xxxxxxxxxx>
To: Yonghong Song <yonghong.song@xxxxxxxxx>
To: Jiri Olsa <jolsa@xxxxxxxxxx>
To: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
To: John Fastabend <john.fastabend@xxxxxxxxx>
To: Ihor Solodrai <ihor.solodrai@xxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 include/linux/bpf.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index b7dbf3d9b5c0..6248ff2f506d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -572,7 +572,7 @@ static inline void bpf_obj_memcpy(struct btf_record *rec,
 		if (long_memcpy)
 			bpf_long_memcpy(dst, src, size);
 		else
-			memcpy(dst, src, size);
+			data_race(memcpy(dst, src, size));
 		return;
 	}
 
@@ -580,10 +580,10 @@ static inline void bpf_obj_memcpy(struct btf_record *rec,
 		u32 next_off = rec->fields[i].offset;
 		u32 sz = next_off - curr_off;
 
-		memcpy(dst + curr_off, src + curr_off, sz);
+		data_race(memcpy(dst + curr_off, src + curr_off, sz));
 		curr_off += rec->fields[i].size + sz;
 	}
-	memcpy(dst + curr_off, src + curr_off, size - curr_off);
+	data_race(memcpy(dst + curr_off, src + curr_off, size - curr_off));
 }
 
 static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)

---
base-commit: 75b0a6db4300e4c2c9e97a0848deaa7acfb42fb7
change-id: 20260819-bpf-kcsan-obj-memcpy-67042b1fce7d

Best regards,
--  
Quanye Yang <quanyeyang@xxxxxxxxx>