blobmsg: use flexible-array member in blobmsg_name() - #50
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blobmsg_name() returned (const char *)(hdr + 1), i.e. a pointer to
the byte immediately after struct blobmsg_hdr. GCC's stringop
analysis treats that region as size 0, so any caller that passes the
result to strcmp()/strlen()/etc. under -Wall -Wstringop-overread
triggers a false-positive diagnostic. For example, building uhttpd
(which uses -Wall -Werror) on aarch64 glibc with GCC 12.3.0 fails
with:
client.c:302:22: error: 'strcmp' reading 1 or more bytes from a
region of size 0 [-Werror=stringop-overread]
302 | if (!strcmp(blobmsg_name(cur), "URL"))
Return hdr->name instead. The flexible array member has unknown
size in GCC's object-size model, so no warning is emitted. The
generated pointer is identical.
Signed-off-by: Michael Pfeifroth <micpf@westermo.com>
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Applied, thanks. |
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Problem
`blobmsg_name()` currently returns `(const char *)(hdr + 1)`, a pointer to the byte immediately after `struct blobmsg_hdr`. GCC's `__builtin_object_size` treats that region as size 0, so any caller passing the result to a string function (`strcmp`, `strlen`, `strchr`, …) under `-Wall -Wstringop-overread` gets a false-positive diagnostic.
Concrete example: building `uhttpd` (which uses `-Wall -Werror` in its CMakeLists.txt) on aarch64 glibc with GCC 12.3.0 fails:
```
client.c:302:22: error: 'strcmp' reading 1 or more bytes from a region of size 0
[-Werror=stringop-overread]
302 | if (!strcmp(blobmsg_name(cur), "URL"))
```
Fix
Return `hdr->name` (the flexible-array member) instead of `(hdr + 1)`. GCC treats flexible arrays as having unknown size, so the false positive disappears. The resulting pointer value is identical - purely a source-level change.
No behavioral change, no ABI change.