linux-kernel - [PATCH v4] libbpf: workaround -Wmaybe-uninitialized false positive

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Message-ID: <14ec488a1cac02794c2fa2b83ae0cef1bce2cb36.1723578546.git.sam@gentoo.org>
Date: Tue, 13 Aug 2024 20:49:06 +0100
From: Sam James <sam@...too.org>
To: Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	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>
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
Subject: [PATCH v4] 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
by initializing elf_fd.fd to -1 and elf_fd.elf to NULL.

I've done this in two other functions as well given it could easily
occur there too (same access/use pattern).

Link: https://gcc.gnu.org/PR114952
Signed-off-by: Sam James <sam@...too.org>
---
v4: Initialize elf and fd members in elf_open to avoid meddling with callers.

 tools/lib/bpf/elf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
index c92e02394159e..b5ab1cb13e5ea 100644
--- a/tools/lib/bpf/elf.c
+++ b/tools/lib/bpf/elf.c
@@ -28,6 +28,9 @@ int elf_open(const char *binary_path, struct elf_fd *elf_fd)
 	int fd, ret;
 	Elf *elf;
 
+	elf_fd->elf = NULL;
+	elf_fd->fd = -1;
+
 	if (elf_version(EV_CURRENT) == EV_NONE) {
 		pr_warn("elf: failed to init libelf for %s\n", binary_path);
 		return -LIBBPF_ERRNO__LIBELF;
-- 
2.45.2