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

[Geert Uytterhoeven]

Hi Arnd,

On Thu, Apr 4, 2024 at 4:52 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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@xxxxxxxx/
> Link: https://lore.kernel.org/lkml/20240326130647.7bfb1d92@xxxxxxxxxxxxxxxxxx/
> Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> 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@xxxxxxxx>

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@xxxxxxxxx>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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