linux-kernel - Re: [PATCH] perf kcore_copy: Do not check /proc/modules is unchanged

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Date:   Wed, 14 Sep 2022 14:12:29 +0100
From:   Daniel Dao <dqminh@...udflare.com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf kcore_copy: Do not check /proc/modules is unchanged

On Wed, Sep 14, 2022 at 1:24 PM Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> /proc/kallsyms and /proc/modules are compared before and after the copy
> in order to ensure no changes during the copy. However /proc/modules
> also might change due to reference counts changing even though that
> does not make any difference. Any modules loaded or unloaded should be
> visible in changes to kallsyms, so it is not necessary to check
> /proc/modules also anyway.
>
> Remove the comparison checking that /proc/modules is unchanged.
>
> Reported-by: Daniel Dao <dqminh@...udflare.com>
> Fixes: fc1b691d7651 ("perf buildid-cache: Add ability to add kcore to the cache")
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> ---
>  tools/perf/util/symbol-elf.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index 75bec32d4f57..647b7dff8ef3 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -2102,8 +2102,8 @@ static int kcore_copy__compare_file(const char *from_dir, const char *to_dir,
>   * unusual.  One significant peculiarity is that the mapping (start -> pgoff)
>   * is not the same for the kernel map and the modules map.  That happens because
>   * the data is copied adjacently whereas the original kcore has gaps.  Finally,
> - * kallsyms and modules files are compared with their copies to check that
> - * modules have not been loaded or unloaded while the copies were taking place.
> + * kallsyms file is compared with its copy to check that modules have not been
> + * loaded or unloaded while the copies were taking place.
>   *
>   * Return: %0 on success, %-1 on failure.
>   */
> @@ -2166,9 +2166,6 @@ int kcore_copy(const char *from_dir, const char *to_dir)
>                         goto out_extract_close;
>         }
>
> -       if (kcore_copy__compare_file(from_dir, to_dir, "modules"))
> -               goto out_extract_close;
> -
>         if (kcore_copy__compare_file(from_dir, to_dir, "kallsyms"))
>                 goto out_extract_close;
>
> --
> 2.25.1
>
Thanks Adrian,

Tested-by: Daniel Dao <dqminh@...udflare.com>