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

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 26, 2023 at 04:12:23PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.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@kernel.org>
> Signed-off-by: Arnd Bergmann <arnd@arndb.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