Re: [PATCH v3] libbpf: workaround -Wmaybe-uninitialized false positive

From: Alan Maguire
Date: Mon Aug 12 2024 - 09:57:55 EST


On 12/08/2024 11:37, Sam James wrote:
> 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.elf to -1.
>
> I've done this in two other functions as well given it could easily
> occur there too (same access/use pattern).
>

hmm, looking at this again - given that there are multiple consumers -
I suppose another option would perhaps be to

- have elf_open() to init int fd = -1, Elf *elf = NULL.
- have error paths in elf_open() "goto out"; at out: we set elf_fd->fd,
elf_fd->elf to fd, elf
- have elf_close() exit it elf_fd < 0 (since 0 is a valid fd), as it
will for the error cases

Might all be bit excessive, and might not even fix the false positive
issue here, so

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

Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>

> ---
> v3: Initialize to -1 instead of using a pragma.
>
> Range-diff against v2:
> 1: 8f5c3b173e4cb < -: ------------- libbpf: workaround -Wmaybe-uninitialized false positive
> -: ------------- > 1: 12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive
>
> tools/lib/bpf/elf.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> index c92e02394159e..00ea3f867bbc8 100644
> --- a/tools/lib/bpf/elf.c
> +++ b/tools/lib/bpf/elf.c
> @@ -374,7 +374,7 @@ long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
> */
> long elf_find_func_offset_from_file(const char *binary_path, const char *name)
> {
> - struct elf_fd elf_fd;
> + struct elf_fd elf_fd = { .fd = -1 };
> long ret = -ENOENT;
>
> ret = elf_open(binary_path, &elf_fd);
> @@ -412,7 +412,7 @@ int elf_resolve_syms_offsets(const char *binary_path, int cnt,
> int err = 0, i, cnt_done = 0;
> unsigned long *offsets;
> struct symbol *symbols;
> - struct elf_fd elf_fd;
> + struct elf_fd elf_fd = { .fd = -1 };
>
> err = elf_open(binary_path, &elf_fd);
> if (err)
> @@ -507,7 +507,7 @@ int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
> int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
> unsigned long *offsets = NULL;
> size_t cap = 0, cnt = 0;
> - struct elf_fd elf_fd;
> + struct elf_fd elf_fd = { .fd = -1 };
> int err = 0, i;
>
> err = elf_open(binary_path, &elf_fd);