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List:       kde-bugs-dist
Subject:    [valgrind] [Bug 326955] New: 64 bit false positive  move depends on uninitialised value wcscpy
From:       David Dyck <david.dyck () fluke ! com>
Date:       2013-10-31 17:28:52
Message-ID: bug-326955-17878 () http ! bugs ! kde ! org/
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https://bugs.kde.org/show_bug.cgi?id=326955

            Bug ID: 326955
           Summary: 64 bit false positive  move depends on uninitialised
                    value wcscpy
    Classification: Unclassified
           Product: valgrind
           Version: 3.7.0
          Platform: openSUSE RPMs
                OS: Linux
            Status: UNCONFIRMED
          Severity: major
          Priority: NOR
         Component: memcheck
          Assignee: jseward@acm.org
          Reporter: david.dyck@fluke.com

Created attachment 83253
  --> https://bugs.kde.org/attachment.cgi?id=83253&action=edit
sample C program that demonstrates false positives

I've attached the sample program valgrind_bug_wcscpy.c
that shows false positives that get in the way of finding
real errors in our code.

The bug does not show up if compiled as a 32 bit program on linux,
or if I use memset() to pre-initialize the buffer.

The bug does not show up if the same program uses the <string.h> functions
 strcpy and strlen which is my main argument as to why this is a false
positive.

---------------------
/*
  The following program causes valgrind to report an error ( false positive )
  Compile:
        gcc -m64 -Wall -O0 -g valgrind_bug_wcscpy.c
  Execute:
        valgrind --track-origins=yes ./a.out

  Unexpected and undesirable results (false positive):

==15062== Conditional jump or move depends on uninitialised value(s)
==15062==    at 0x4F75F05: __wcscpy_ssse3 (in /lib64/libc-2.15.so)
==15062==    by 0x4005B7: main (valgrind_bug.c:31)
==15062==  Uninitialised value was created by a stack allocation
==15062==    at 0x40058C: main (valgrind_bug.c:23)

*/

// #include <stdio.h>
#include <wchar.h>
#include <string.h>

int main()
{
        wchar_t stackbuf1[10];                     // if we init on allocation
( = { 0 }) no error

//        memset(stackbuf1, 0, sizeof(stackbuf1)); // if we pre-initialize the
buffer, no error

        wcscpy(stackbuf1,L"hello");     // initialize (part of) stackbuf1

        wchar_t stackbuf2[10];
        wcscpy(stackbuf2, stackbuf1);   // valgrind reports stackbuf1
unitialized

        return wcslen(stackbuf2);
}

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