linux-kernel - Re: [PATCH] Fix overflow in vma length when copying mmap on clone

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Date:	Wed, 25 Apr 2012 01:10:03 +0530
From:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Jens Axboe <axboe@...nel.dk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Fix overflow in vma length when copying mmap on clone

On 25 April 2012 00:31, Hugh Dickins <hughd@...gle.com> wrote:
> But I didn't (try very hard to) work out what your demo program shows
> - though I am amused by your sense of humour in using %d for a pointer
> there!  I wonder what setting of /proc/sys/vm/overcommit_memory is
> needed for it to behave as you intend?

That was supposed to be errno, not the pointer. I had added my own
syscall wrappers to eliminate glibc and then reverted it to the
original smaller-to-read reproducer and this got left behind in the
process. The demo program is supposed to show "successed" for
iterations 16383 to 16390 since the overflow happens at 16TB. All
iterations before it (and after) show a fork failure.

/proc/sys/vm/overcommit_memory is 0.

Perhaps a cleaner demo program would have been:

#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>
#include <errno.h>

#define GIG 1024 * 1024 * 1024L
#define EXTENT 16393

int main(void)
{
        int i, r;
        void *m;
        char buf[1024];
        int prev_failed = 0;

        for (i = 0; i < EXTENT; i++) {
                m = mmap(NULL, (size_t) 1 * 1024 * 1024 * 1024L,
                         PROT_READ | PROT_WRITE, MAP_PRIVATE |
MAP_ANONYMOUS, 0, 0);

                if (m == (void *)-1) {
                        printf("MMAP Failed: %d\n", errno);
                        return 1;
                }

                r = fork();

                if (r == 0) {
                        return 0;
                } else if (r < 0) {
                        prev_failed = 1;
                        /* Fork failed as expected */
                }
                else if (r > 0 && prev_failed) {
                        printf("Unexpected success at %d\n", i);
                        wait(NULL);
                        return 1;
                }
        }
        return 0;
}


-- 
Siddhesh Poyarekar
http://siddhesh.in
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