linux-hardening - Re: [PATCH v10 01/15] modpost: fix removing numeric suffixes

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Message-ID: <YnDaO5ThkiYkRXbO@alley>
Date:   Tue, 3 May 2022 09:31:07 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     linux-hardening@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Bruce Schlobohm <bruce.schlobohm@...el.com>,
        Jessica Yu <jeyu@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        Jonathan Corbet <corbet@....net>,
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        llvm@...ts.linux.dev
Subject: Re: [PATCH v10 01/15] modpost: fix removing numeric suffixes

On Wed 2022-02-09 19:57:38, Alexander Lobakin wrote:
> `-z unique-symbol` linker flag which is planned to use with FG-KASLR
> to simplify livepatching (hopefully globally later on) triggers the
> following:
> 
> ERROR: modpost: "param_set_uint.0" [vmlinux] is a static EXPORT_SYMBOL
> 
> The reason is that for now the condition from remove_dot():
> 
> if (m && (s[n + m] == '.' || s[n + m] == 0))
> 
> which was designed to test if it's a dot or a '\0' after the suffix
> is never satisfied.
> This is due to that `s[n + m]` always points to the last digit of a
> numeric suffix, not on the symbol next to it (from a custom debug
> print added to modpost):
> 
> param_set_uint.0, s[n + m] is '0', s[n + m + 1] is '\0'

Yup, the + 1  is for the '.' between the symbol name and the number.
In the order of apperance it would be: n + 1 + m

> So it's off-by-one and was like that since 2014.
> Fix this for the sake of upcoming features, but don't bother
> stable-backporting, as it's well hidden -- apart from that LD flag,
> can be triggered only by GCC LTO which never landed upstream.
> 
> Fixes: fcd38ed0ff26 ("scripts: modpost: fix compilation warning")
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr