https://bugs.kde.org/show_bug.cgi?id=381299
Bug ID: 381299
Summary: false uninit on new page via sbrk(n)
Product: valgrind
Version: 3.13 SVN
Platform: Compiled Sources
OS: Linux
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: memcheck
Assignee: jsew...@acm.org
Reporter: jrei...@bitwagon.com
Target Milestone: ---
Memcheck generates a false positive uninitialized complaint when the target
program uses a value from a new page that was [just] allocated via sbrk(n).
The operating system guarantees that new pages are all zero, so memcheck should
not complain.
===== vg-brk.c test case
#include <stdio.h>
#include <unistd.h>
#define PAGE_SIZE (1u<<12)
#define PAGE_MASK -PAGE_SIZE
int
main(int argc, char *argv[])
{
void *p0 = sbrk(0);
printf("p0=%p from sbrk(0)\n", p0);
void *p1 = (void *)(PAGE_MASK & (-1+ PAGE_SIZE + (long)p0));
int r1 = brk(p1);
printf("p1=%p p0 rounded up to page boundary r1=%d\n", p1, r1);
void *p2 = sbrk(0x1000);
printf("p2=%p new page was allocated here\n", p2);
void *p3 = sbrk(0x1000);
printf("p3=%p new page was allocated here\n", p3);
printf("\n");
printf("will access %p\n", p2);
printf("%d\n", *(int *)p2);
return 0;
}
=====
$ gcc -g -o vg-brk vg-brk.c
$ valgrind-3.13.0/bin/valgrind --track-origins=yes ./vg-brk
==18003== Memcheck, a memory error detector
==18003== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==18003== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==18003== Command: ./vg-brk
==18003==
p0=0x4228000 from sbrk(0)
p1=0x4228000 p0 rounded up to page boundary r1=0
p2=0x4228000 new page was allocated here
p3=0x4229000 new page was allocated here
will access 0x4228000
==18003== Conditional jump or move depends on uninitialised value(s)
==18003== at 0x4E8844A: vfprintf (in /usr/lib64/libc-2.24.so)
==18003== by 0x4E906D8: printf (in /usr/lib64/libc-2.24.so)
==18003== by 0x4006C2: main (vg-brk.c:21)
==18003== Uninitialised value was created
==18003== at 0x4F37579: brk (in /usr/lib64/libc-2.24.so)
==18003== by 0x4F37658: sbrk (in /usr/lib64/libc-2.24.so)
==18003== by 0x40064D: main (vg-brk.c:15)
==18003==
[[snip]]
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