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

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Message-ID: <CAEf4Bza3RH6p=KJu8cm2jb4QwKCHc5ZUskE9cvWTBXyXFUKHuA@mail.gmail.com>
Date: Mon, 12 Aug 2024 14:02:51 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alan Maguire <alan.maguire@...cle.com>
Cc: Sam James <sam@...too.org>, 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>, "Jose E . Marchesi" <jose.marchesi@...cle.com>, 
	Andrew Pinski <quic_apinski@...cinc.com>, 
	Kacper Słomiński <kacper.slominski72@...il.com>, 
	Arsen Arsenović <arsen@...too.org>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] libbpf: workaround -Wmaybe-uninitialized false positive

On Mon, Aug 12, 2024 at 6:57 AM Alan Maguire <alan.maguire@...cle.com> wrote:
>
> 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 -

yes, I don't like that each caller has to remember to initialize the
struct that is clearly initialized by elf_open() itself, so see below.

pw-bot: cr

> I suppose another option would perhaps be to
>
> - have elf_open() to init int fd = -1, Elf *elf = NULL.

I'd do just

elf_fd->elf = NULL;
elf_fd->fd = -1;

and do nothing else. This should be enough for compiler to not trigger this.

> - 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
>

Let's not touch anything else, this should be enough.


> 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@...too.org>
>
> Reviewed-by: Alan Maguire <alan.maguire@...cle.com>
>
> > ---
> > 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);