linux-kernel - [PATCHv2] perf jvmti: Fix gcc string overflow warning

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Date:   Fri, 31 May 2019 15:13:21 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, 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: [PATCHv2] perf jvmti: Fix gcc string overflow warning

On Fri, May 31, 2019 at 09:05:30AM -0300, Arnaldo Carvalho de Melo wrote:

SNIP

> 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.

ok, I forgot there's strlcpy.. v2 attached

thanks,
jirka


---
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

As per Arnaldo's suggestion using strlcpy, which does
the same thing and keeps gcc silent.

Cc: Ben Gainey <ben.gainey@....com>
Cc: Stephane Eranian <eranian@...gle.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@...hat.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..c441a34cb1c0 100644
--- a/tools/perf/jvmti/libjvmti.c
+++ b/tools/perf/jvmti/libjvmti.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/compiler.h>
+#include <linux/string.h>
 #include <sys/types.h>
 #include <stdio.h>
 #include <string.h>
@@ -162,8 +163,7 @@ 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);
+		strlcpy(result, file_name, max_length);
 	}
 }
 
-- 
2.21.0