Hi, Valgrind reports an invalid read, while I think the program is valid. The program uses stpncpy() instead of strcpy():
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char **argv) {
char *dst = (char *) calloc(4, 1);
char *src = (char *) calloc(4, 1);
src[0] = 'a';
src[1] = 'b';
src[2] = 'c';
src[3] = '\0';
dst = stpncpy(dst, src, 3);
printf("%s %s\n", src, dst);
}
The error is the following:
$ valgrind ./a.out
==21946== Memcheck, a memory error detector
==21946== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==21946== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==21946== Command: ./a.out
==21946==
==21946== Invalid read of size 8
==21946== at 0x4ECA554: __stpncpy_sse2_unaligned
(strcpy-sse2-unaligned.S:297)
==21946== by 0x400608: main (in /data/francis/workspace/augeas/a.out)
==21946== Address 0x51ef090 is 0 bytes inside a block of size 4 alloc'd
==21946== at 0x4C29DB4: calloc (in
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==21946== by 0x4005C4: main (in /data/francis/workspace/augeas/a.out)
==21946==
==21946== Invalid read of size 8
==21946== at 0x4ECA558: __stpncpy_sse2_unaligned
(strcpy-sse2-unaligned.S:298)
==21946== by 0x400608: main (in /data/francis/workspace/augeas/a.out)
==21946== Address 0x51ef0a0 is 12 bytes after a block of size 4 alloc'd
==21946== at 0x4C29DB4: calloc (in
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==21946== by 0x4005C4: main (in /data/francis/workspace/augeas/a.out)
==21946==
abc
In the stpncpy routine, there are two key instructions that are executed:
<__stpncpy_sse2_unaligned+45> pcmpeqb (%rsi), xmm1
<__stpncpy_sse2_unaligned+49> pmovmskb %xmm1,%edx
The result in %edx is: 0xfff8, that shows that all bytes are zero,
except the last 3. The whole point here is: this instruction reads 8
bytes, no matter the size of the string (here 4). Could it mislead
valgrind? Does stpncpy assumes the memory is word aligned, that mean
even with a string of 4 bytes, 8 are actually allocated, and that it's
safe to read those 8 bytes?
Thanks!
Francis Giraldeau
smime.p7s
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