linux-kernel - Re: [PATCH] [v3] kallsyms: rework symbol lookup return codes

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Date:   Wed, 26 Jul 2023 15:43:17 -0700
From:   Kees Cook <keescook@...omium.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>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        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>,
        Petr Mladek <pmladek@...e.com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-modules@...r.kernel.org
Subject: Re: [PATCH] [v3] kallsyms: rework symbol lookup return codes

On Wed, Jul 26, 2023 at 04:12:23PM +0200, Arnd Bergmann 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/
> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> v3: use strscpy() instead of strlcpy()

Thank you! :) (Though see my notes below...)

> [...]
> @@ -344,13 +345,12 @@ const char *module_address_lookup(unsigned long addr,
>  #endif
>  		}
>  
> -		ret = find_kallsyms_symbol(mod, addr, size, offset);
> -	}
> -	/* Make a copy in here where it's safe */
> -	if (ret) {
> -		strncpy(namebuf, ret, KSYM_NAME_LEN - 1);

This -1 was to keep the buffer NUL-terminated.

> -		ret = namebuf;
> +		sym = find_kallsyms_symbol(mod, addr, size, offset);
> +
> +		if (sym)
> +			ret = strscpy(namebuf, sym, KSYM_NAME_LEN - 1);

This strscpy should use KSYM_NAME_LEN without the "- 1".

>  	}
> +
>  	preempt_enable();
>  

-Kees

-- 
Kees Cook