Re: [PATCH] sprint_symbol: Replace strcpy with memmove to handle potential overlap

[Kees Cook]

On Tue, Jun 04, 2024 at 12:42:28PM +0800, liujinlong wrote:
> In the function __sprint_symbol, replace strcpy with memmove to ensure
> correct behavior even if the source and destination buffers overlap.
> This change prevents potential undefined behavior flagged by recent
> compilers as [-Werror=restrict].
> 
> Reported-by: k2ci <kernel-bot@xxxxxxxxxx>
> Signed-off-by: liujinlong <liujinlong@xxxxxxxxxx>
> ---
>  kernel/kallsyms.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 22ea19a36e6e..3c3a77fcd020 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -489,7 +489,7 @@ static int __sprint_symbol(char *buffer, unsigned long address,
>  		return sprintf(buffer, "0x%lx", address - symbol_offset);
>  
>  	if (name != buffer)
> -		strcpy(buffer, name);
> +		memmove(buffer, name, strlen(name) + 1);
>  	len = strlen(buffer);
>  	offset -= symbol_offset;

The warning is[1]:

>   CC      kernel/kallsyms.o
> In file included from ./include/linux/string.h:374,
>                  from ./arch/x86/include/asm/page_32.h:18,
>                  from ./arch/x86/include/asm/page.h:14,
>                  from ./arch/x86/include/asm/thread_info.h:12,
>                  from ./include/linux/thread_info.h:60,
>                  from ./include/linux/spinlock.h:60,
>                  from ./include/linux/mmzone.h:8,
>                  from ./include/linux/gfp.h:7,
>                  from ./include/linux/mm.h:7,
>                  from ./include/linux/kallsyms.h:13,
>                  from kernel/kallsyms.c:15:
> kernel/kallsyms.c: In function ‘__sprint_symbol’:
> ./include/linux/fortify-string.h:122:33: error: ‘__builtin_strcpy’ source argument is the same as destination [-Werror=restrict]
>   122 | #define __underlying_strcpy     __builtin_strcpy
>       |                                 ^
> ./include/linux/fortify-string.h:787:24: note: in expansion of macro ‘__underlying_strcpy’
>   787 |                 return __underlying_strcpy(p, q);
>       |                        ^~~~~~~~~~~~~~~~~~~

But the code is already checking "name != buffer". Does anyone know
what is actually happening here where we have an _overlap_, but not
identical strings?

-Kees

[1] https://lore.kernel.org/lkml/202406271127.CEAE5F4E@keescook/

-- 
Kees Cook