linux-kernel - Re: [PATCH] perf jvmti: Fix gcc string overflow warning

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Date:   Fri, 31 May 2019 09:05:30 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Ben Gainey <ben.gainey@....com>,
        Stephane Eranian <eranian@...gle.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] perf jvmti: Fix gcc string overflow warning

Em Fri, May 31, 2019 at 10:03:07AM +0200, Jiri Olsa escreveu:
> We are getting fake gcc warning when we compile with gcc9 (9.1.1):
> 
>      CC       jvmti/libjvmti.o
>    In file included from /usr/include/string.h:494,
>                     from jvmti/libjvmti.c:5:
>    In function ‘strncpy’,
>        inlined from ‘copy_class_filename.constprop’ at jvmti/libjvmti.c:166:3:
>    /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound depends on the length of the source argument [-Werror=stringop-overflow=]
>      106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
>          |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    jvmti/libjvmti.c: In function ‘copy_class_filename.constprop’:
>    jvmti/libjvmti.c:165:26: note: length computed here
>      165 |   size_t file_name_len = strlen(file_name);
>          |                          ^~~~~~~~~~~~~~~~~
>    cc1: all warnings being treated as errors
> 
> First I wanted to disable the check, but now I think the code
> could be more straight forward. There's no need to check the
> source size, strncpy will do that. We just need to make sure
> the string is correctly terminated.
> 
> Cc: Ben Gainey <ben.gainey@....com>
> Cc: Stephane Eranian <eranian@...gle.com>
> Link: http://lkml.kernel.org/n/tip-sve3b63c550wr907e6ui6gx5@git.kernel.org
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  tools/perf/jvmti/libjvmti.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/jvmti/libjvmti.c b/tools/perf/jvmti/libjvmti.c
> index aea7b1fe85aa..00fa0b7f1ad9 100644
> --- a/tools/perf/jvmti/libjvmti.c
> +++ b/tools/perf/jvmti/libjvmti.c
> @@ -162,8 +162,8 @@ copy_class_filename(const char * class_sign, const char * file_name, char * resu
>  		result[i] = '\0';
>  	} else {
>  		/* fallback case */
> -		size_t file_name_len = strlen(file_name);
> -		strncpy(result, file_name, file_name_len < max_length ? file_name_len : max_length);
> +		strncpy(result, file_name, max_length - 1);
> +		result[max_length - 1] = 0;

The usual idiom here, if we stay with strncpy is:

		strncpy(result, file_name, max_length - 1)[max_length - 1] = 0;

one line instead of two, but there are other possibilities, what I've
done int these cases in tools/perf/ is to switch to strlcpy, so just
flip that 'n' to a 'l' and it should be enough.

For that we just have a copy of the kernel's strlcpy() implementation in
lib/string.c, and it has this doc:

/**
 * strlcpy - Copy a C-string into a sized buffer
 * @dest: Where to copy the string to
 * @src: Where to copy the string from
 * @size: size of destination buffer
 *
 * Compatible with ``*BSD``: the result is always a valid
 * NUL-terminated string that fits in the buffer (unless,
 * of course, the buffer size is zero). It does not pad
 * out the result like strncpy() does.
 */

The kernel folks moved beyond that and in lib/string.c we have:

/**
 * strscpy - Copy a C-string into a sized buffer
 * @dest: Where to copy the string to
 * @src: Where to copy the string from
 * @count: Size of destination buffer
 *
 * Copy the string, or as much of it as fits, into the dest buffer.  The
 * behavior is undefined if the string buffers overlap.  The destination
 * buffer is always NUL terminated, unless it's zero-sized.
 *
 * Preferred to strlcpy() since the API doesn't require reading memory
 * from the src string beyond the specified "count" bytes, and since
 * the return value is easier to error-check than strlcpy()'s.
 * In addition, the implementation is robust to the string changing out
 * from underneath it, unlike the current strlcpy() implementation.
 *
 * Preferred to strncpy() since it always returns a valid string, and
 * doesn't unnecessarily force the tail of the destination buffer to be
 * zeroed.  If zeroing is desired please use strscpy_pad().
 *
 * Return: The number of characters copied (not including the trailing
 *         %NUL) or -E2BIG if the destination buffer wasn't big enough.
 */
ssize_t strscpy(char *dest, const char *src, size_t count)

I think for these needs flipping that 'n' into a 'l' is good enough.

- Arnaldo

>  	}
>  }
>  
> -- 
> 2.21.0

-- 

- Arnaldo