Re: Writing to mmaped region cause segfault

[Otto Moerbeek]

On Wed, Nov 03, 2010 at 01:18:51PM +0200, Alexey Suslikov wrote:

> Hello t...@.
> 
> On OpenBSD/amd64, doing something like
>     char *buf = mmap(NULL, len + 1, PROT_READ | PROT_WRITE,
> MAP_PRIVATE, some.fd, 0);
>     buf[len] = '\0';
> causes segfault on buf[len] = '\0' assignment if len = 16384.
> 
> However doing
>     char *buf = mmap(NULL, len + 1, PROT_READ | PROT_WRITE,
> MAP_PRIVATE, some.fd, 0);
>     char *nbuf = malloc(len + 1);
>     memcpy(nbuf, buf, len);
>     nbuf[len] = '\0';
> does not lead to a crash.
> 
> Is it expected behavior of mmap (alignment?) or usage of mmap is wrong?
> 
> Thanks.
> 
> Alexey

This (complete!) program does not show the behahaviour. Please post a
complete testcase. Did you include sys/mman.h?

        -Otto

#include <sys/types.h>
#include <sys/mman.h>

#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>


int
main()
{
        int fd;
        char *buf;
        size_t len;

        fd = open("file", O_RDWR, 0);
        if (fd == -1)
                err(1, NULL);

        len = 16384;
        buf = mmap(NULL, len + 1, PROT_READ | PROT_WRITE, MAP_PRIVATE,
            fd, (off_t)0);
        if (buf == MAP_FAILED)
                err(1, NULL);
        buf[len] = '\0';
}