Re: [PATCH 14/15] kallsyms: scripts/kallsyms: Reduce the output file size

[David Laight]

On Tue, 20 Oct 2026 03:50:35 +0800
kernel test robot <lkp@xxxxxxxxx> wrote:

> Hi David,
> 
> kernel test robot noticed the following build warnings:
> 
> [auto build test WARNING on akpm-mm/mm-everything]
> [also build test WARNING on linus/master v7.3]
> [cannot apply to next-20261016 next-20261016]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/David-Laight/kallsyms-scripts-kallsyms-c-Avoid-overrunning-on-stack-buffer/20261018-165152
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20261018155206.132411-15-david.laight.linux%40gmail.com
> patch subject: [PATCH 14/15] kallsyms: scripts/kallsyms: Reduce the output file size
> config: um-allmodconfig (https://download.01.org/0day-ci/archive/20261020/202610200336.6v4SE96n-lkp@xxxxxxxxx/config)
> compiler: clang version 17.0.6 (https://github.com/llvm/llvm-project 6009708b4367171ccdbf4b5905cb6a803753fe18)
> rustc: rustc 1.96.0 (ac68faa20 2026-05-25)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20261020/202610200336.6v4SE96n-lkp@xxxxxxxxx/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202610200336.6v4SE96n-lkp@xxxxxxxxx/
> 
> All warnings (new ones prefixed by >>):
> 
> >> scripts/kallsyms.c:371:26: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]  
>      371 |                         printf("\n\t.short\t" + !i);
>          |                                ~~~~~~~~~~~~~~~^~~~
>    scripts/kallsyms.c:371:26: note: use array indexing to silence this warning

Hmmm it gives the same error for (!i + "string") even though that doesn't even look
like a broken attempt to append to the string.


>      371 |                         printf("\n\t.short\t" + !i);
>          |                                               ^   
>          |                                &              [   ]
> >> scripts/kallsyms.c:371:11: warning: format string is not a string literal (potentially insecure) [-Wformat-security]  
>      371 |                         printf("\n\t.short\t" + !i);
>          |                                ^~~~~~~~~~~~~~~~~~~

The compiler ought to do better there...
Indeed, gcc does.

...
>    366		output_label("kallsyms_token_index");
>    367		for (i = 0; i < 256; i++) {
>    368			if (i & 15)
>    369				printf(",");
>    370			else
>  > 371				printf("\n\t.short\t" + !i);  
>    372			printf("%u", best_idx[i]);
>    373		}

I knew I should have interleaved putchar() and fputs() with the printf().
They are somewhat faster - in this file it is measurable.
gcc does convert printf() to putchar() or puts() but not fputs().

The time taken to write the files halves if I initialise an array with all the
",%u" values and use fputs(str[byte_val] + !comma).
I decided someone would object to the mess.

I'll think of something for v3.

David