[PATCH v2] libbpf: workaround -Wmaybe-uninitialized false positive

From: Sam James
Date: Fri Aug 09 2024 - 13:27:18 EST


In `elf_close`, we get this with GCC 15 -O3 (at least):
```
In function ‘elf_close’,
inlined from ‘elf_close’ at elf.c:53:6,
inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
elf.c:57:9: warning: ‘elf_fd.elf’ may be used uninitialized [-Wmaybe-uninitialized]
57 | elf_end(elf_fd->elf);
| ^~~~~~~~~~~~~~~~~~~~
elf.c: In function ‘elf_find_func_offset_from_file’:
elf.c:377:23: note: ‘elf_fd.elf’ was declared here
377 | struct elf_fd elf_fd;
| ^~~~~~
In function ‘elf_close’,
inlined from ‘elf_close’ at elf.c:53:6,
inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
elf.c:58:9: warning: ‘elf_fd.fd’ may be used uninitialized [-Wmaybe-uninitialized]
58 | close(elf_fd->fd);
| ^~~~~~~~~~~~~~~~~
elf.c: In function ‘elf_find_func_offset_from_file’:
elf.c:377:23: note: ‘elf_fd.fd’ was declared here
377 | struct elf_fd elf_fd;
| ^~~~~~
```

In reality, our use is fine, it's just that GCC doesn't model errno
here (see linked GCC bug). Suppress -Wmaybe-uninitialized accordingly.

Link: https://gcc.gnu.org/PR114952
Signed-off-by: Sam James <sam@xxxxxxxxxx>
---
v2: Fix Clang build.

Range-diff against v1:
1: 3ebbe7a4e93a ! 1: 8f5c3b173e4c libbpf: workaround -Wmaybe-uninitialized false positive
@@ tools/lib/bpf/elf.c: long elf_find_func_offset(Elf *elf, const char *binary_path
return ret;
}

++#if !defined(__clang__)
+#pragma GCC diagnostic push
+/* https://gcc.gnu.org/PR114952 */
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
++#endif
/* Find offset of function name in ELF object specified by path. "name" matches
* symbol name or name@@LIB for library functions.
*/
@@ tools/lib/bpf/elf.c: long elf_find_func_offset_from_file(const char *binary_path
elf_close(&elf_fd);
return ret;
}
++#if !defined(__clang__)
+#pragma GCC diagnostic pop
++#endif

struct symbol {
const char *name;

tools/lib/bpf/elf.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
index c92e02394159..7058425ca85b 100644
--- a/tools/lib/bpf/elf.c
+++ b/tools/lib/bpf/elf.c
@@ -369,6 +369,11 @@ long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
return ret;
}

+#if !defined(__clang__)
+#pragma GCC diagnostic push
+/* https://gcc.gnu.org/PR114952 */
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
/* Find offset of function name in ELF object specified by path. "name" matches
* symbol name or name@@LIB for library functions.
*/
@@ -384,6 +389,9 @@ long elf_find_func_offset_from_file(const char *binary_path, const char *name)
elf_close(&elf_fd);
return ret;
}
+#if !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif

struct symbol {
const char *name;
--
2.45.2