linux-kernel - Re: [PATCH] [v5] kallsyms: rework symbol lookup return codes

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Date: Tue, 28 May 2024 15:03:40 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	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@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Kumar Kartikeya Dwivedi <memxor@...il.com>, 
	Puranjay Mohan <puranjay12@...il.com>, Jakub Kicinski <kuba@...nel.org>, 
	Kees Cook <keescook@...omium.org>, Zhen Lei <thunder.leizhen@...wei.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, linux-modules@...r.kernel.org
Subject: Re: [PATCH] [v5] kallsyms: rework symbol lookup return codes

Hi Arnd,

On Thu, Apr 4, 2024 at 4:52 PM Arnd Bergmann <arnd@...nel.org> wrote:
> From: Arnd Bergmann <arnd@...db.de>
>
> Building with W=1 in some configurations produces a false positive
> warning for kallsyms:
>
> kernel/kallsyms.c: In function '__sprint_symbol.isra':
> kernel/kallsyms.c:503:17: error: 'strcpy' source argument is the same as destination [-Werror=restrict]
>   503 |                 strcpy(buffer, name);
>       |                 ^~~~~~~~~~~~~~~~~~~~
>
> This originally showed up while building with -O3, but later started
> happening in other configurations as well, depending on inlining
> decisions. The underlying issue is that the local 'name' variable is
> always initialized to the be the same as 'buffer' in the called functions
> that fill the buffer, which gcc notices while inlining, though it could
> see that the address check always skips the copy.
>
> The calling conventions here are rather unusual, as all of the internal
> lookup functions (bpf_address_lookup, ftrace_mod_address_lookup,
> ftrace_func_address_lookup, module_address_lookup and
> kallsyms_lookup_buildid) already use the provided buffer and either return
> the address of that buffer to indicate success, or NULL for failure,
> but the callers are written to also expect an arbitrary other buffer
> to be returned.
>
> Rework the calling conventions to return the length of the filled buffer
> instead of its address, which is simpler and easier to follow as well
> as avoiding the warning. Leave only the kallsyms_lookup() calling conventions
> unchanged, since that is called from 16 different functions and
> adapting this would be a much bigger change.
>
> Link: https://lore.kernel.org/all/20200107214042.855757-1-arnd@arndb.de/
> Link: https://lore.kernel.org/lkml/20240326130647.7bfb1d92@gandalf.local.home/
> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
> Acked-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> v5: fix ftrace_mod_address_lookup return value,
>     rebased on top of 2e114248e086 ("bpf: Replace deprecated strncpy with strscpy")
> v4: fix string length
> v3: use strscpy() instead of strlcpy()
> v2: complete rewrite after the first patch was rejected (in 2020). This
>     is now one of only two warnings that are in the way of enabling
>     -Wextra/-Wrestrict by default.

Aha, commit 06bb7fc0feee32d9 ("kbuild: turn on -Wrestrict by default")
still made v6.10-rc1, without this one...

> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Thanks, this fixes

    kernel/kallsyms.c: In function ‘__sprint_symbol.constprop’:
    kernel/kallsyms.c:492:17: warning: ‘strcpy’ source argument is the
same as destination [-Werror=restrict]
      492 |                 strcpy(buffer, name);
          |                 ^~~~~~~~~~~~~~~~~~~~

I am seeing with shmobile_defconfig and gcc version 11.4.0 (Ubuntu
11.4.0-1ubuntu1~22.04).

Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds