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

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Message-ID: <8f5c3b173e4cb216322ae19ade2766940c6fbebb.1723224401.git.sam@gentoo.org>
Date: Fri,  9 Aug 2024 18:26:41 +0100
From: Sam James <sam@...too.org>
To: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>
Cc: "Jose E . Marchesi" <jose.marchesi@...cle.com>,
	Andrew Pinski <quic_apinski@...cinc.com>,
	Kacper Słomiński <kacper.slominski72@...il.com>,
	Arsen Arsenović <arsen@...too.org>,
	Sam James <sam@...too.org>,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: [PATCH v2] libbpf: workaround -Wmaybe-uninitialized false positive

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@...too.org>
---
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