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

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Message-ID: <878qx0gqlo.fsf@gentoo.org>
Date: Tue, 13 Aug 2024 20:38:59 +0100
From: Sam James <sam@...too.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alan Maguire <alan.maguire@...cle.com>,  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

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> 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.

OK.

>
>> - 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);

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