linux-kernel - Re: [ammarfaizi2-block:rostedt/linux-trace/ftrace/core 33/41] include/linux/fortify-string.h:47:30: warning: '__builtin_strncat' output truncated before terminating nul copying as many bytes from a string as its length

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Date:   Tue, 8 Mar 2022 10:36:58 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     kernel test robot <lkp@...el.com>
Cc:     kbuild-all@...ts.01.org,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        linux-kernel@...r.kernel.org, Tom Zanussi <zanussi@...nel.org>
Subject: Re: [ammarfaizi2-block:rostedt/linux-trace/ftrace/core 33/41]
 include/linux/fortify-string.h:47:30: warning: '__builtin_strncat' output
 truncated before terminating nul copying as many bytes from a string as its
 length

On Tue, 8 Mar 2022 21:15:29 +0800
kernel test robot <lkp@...el.com> wrote:

> All warnings (new ones prefixed by >>):
> 
>    In file included from include/linux/string.h:253,
>                     from include/linux/bitmap.h:11,
>                     from include/linux/cpumask.h:12,
>                     from arch/x86/include/asm/cpumask.h:5,
>                     from arch/x86/include/asm/msr.h:11,
>                     from arch/x86/include/asm/processor.h:22,
>                     from arch/x86/include/asm/timex.h:5,
>                     from include/linux/timex.h:65,
>                     from include/linux/time32.h:13,
>                     from include/linux/time.h:60,
>                     from include/linux/stat.h:19,
>                     from include/linux/module.h:13,
>                     from kernel/trace/trace_events_hist.c:8:
>    In function 'strncat',
>        inlined from 'last_cmd_set' at kernel/trace/trace_events_hist.c:760:2,
>        inlined from 'event_hist_trigger_parse' at kernel/trace/trace_events_hist.c:6191:3:
> >> include/linux/fortify-string.h:47:30: warning: '__builtin_strncat' output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]  
>       47 | #define __underlying_strncat __builtin_strncat
>          |                              ^
>    include/linux/fortify-string.h:191:10: note: in expansion of macro '__underlying_strncat'
>      191 |   return __underlying_strncat(p, q, count);
>          |          ^~~~~~~~~~~~~~~~~~~~
>    kernel/trace/trace_events_hist.c: In function 'event_hist_trigger_parse':
>    include/linux/fortify-string.h:46:29: note: length computed here
>       46 | #define __underlying_strlen __builtin_strlen
>          |                             ^
>    include/linux/fortify-string.h:102:10: note: in expansion of macro '__underlying_strlen'
>      102 |   return __underlying_strlen(p);
>          |          ^~~~~~~~~~~~~~~~~~~

I see my mistake. The code it's talking about is this:

	len = sizeof(HIST_PREFIX) + strlen(str) + 1;
	kfree(last_cmd);
	last_cmd = kzalloc(len, GFP_KERNEL);
	if (!last_cmd)
		return;

	strcpy(last_cmd, HIST_PREFIX);
	len -= sizeof(HIST_PREFIX) + 1;
	strncat(last_cmd, str, len);


Where according to the man page of strncat:

     If src contains n or more bytes, strncat() writes n+1 bytes to dest (n
     from src plus the terminating null byte).  Therefore, the size of dest
     must be at least strlen(dest)+n+1.

The above did: len -= sizeof(HIST_PREFIX) + 1; when it meant to do:

  len -= strlen(HIST_PREFX) + 1

Or it could just do:

  len -= sizeof(HIST_PREFIX)

as sizeof(HIST_PREFIX) contains the nul byte.

Heck, the allocation could be:

	len = sizeof(HIST_PREFIX) + strlen(str);

As the sizeof already contains the nul byte.

Will fix.

Thanks,

-- Steve