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Re: [PATCH v2 1/2] tools: use memcpy instead of strncpy in getBridge

On 07.10.20 10:56, Bertrand Marquis wrote:
Hi Jurgen,

On 7 Oct 2020, at 09:39, Jürgen Groß <jgross@xxxxxxxx> wrote:

On 07.10.20 10:28, Bertrand Marquis wrote:
Use memcpy in getBridge to prevent gcc warnings about truncated
strings. We know that we might truncate it, so the gcc warning
here is wrong.
Revert previous change changing buffer sizes as bigger buffers
are not needed.
Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Changes in v2:
  Use MIN between string length of de->d_name and resultLen to copy only
  the minimum size required and prevent crossing to from an unallocated
  tools/libs/stat/xenstat_linux.c | 11 +++++++++--
  1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/tools/libs/stat/xenstat_linux.c b/tools/libs/stat/xenstat_linux.c
index d2ee6fda64..0ace03af1b 100644
--- a/tools/libs/stat/xenstat_linux.c
+++ b/tools/libs/stat/xenstat_linux.c
@@ -29,6 +29,7 @@
  #include <string.h>
  #include <unistd.h>
  #include <regex.h>
+#include <xen-tools/libs.h>
    #include "xenstat_priv.h"
  @@ -78,7 +79,13 @@ static void getBridge(char *excludeName, char *result, 
size_t resultLen)
                                sprintf(tmp, "/sys/class/net/%s/bridge", 
                                if (access(tmp, F_OK) == 0) {
-                                       strncpy(result, de->d_name, resultLen);
+                                       /*
+                                        * Do not use strncpy to prevent 
compiler warning with
+                                        * gcc >= 10.0
+                                        * If de->d_name is longer then 
resultLen we truncate it


Will fix

+                                        */
+                                       memcpy(result, de->d_name, 
sizeof(de->d_name)),resultLen - 1));

You can't use sizeof(de->d_name) here, as AFAIK there is no guarantee
that de->d_name isn't e.g. defined like "char d_name[]".

My suggestion to use NAME_MAX as upper boundary for the length was
really meant to be used that way.

And additionally you might want to add 1 to the strnlen() result in
order to copy the trailing 0-byte, too (or you should zero out the
result buffer before and omit writing the final zero byte).

Thinking more about it zeroing the result buffer is better as it even
covers the theoretical case of NAME_MAX being shorter than resultLen.

Setting the result buffer completely to 0 and doing after a copy sounds like
a big complexity.

How about:
copysize = MIN(strnlen(de->d_name,NAME_MAX), resultLen - 1);
memcpy(result, de->d_name, copysize);
result[copysize + 1] = 0

result[copysize] = 0;

This would cover the case where NAME_MAX is shorter then resultLen.

Why is:

memset(result, 0, resultLen);
memcpy(result, de->d_name, MIN(strnlen(de->d_name,NAME_MAX), resultLen - 1));

A big complexity?

In the end both variants are fine.




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