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Date: Tue, 01 Nov 2016 23:34:36 +0000
From: Thierry Moreau <thierry.moreau@connotech.com>
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To: linux-kernel@vger.kernel.org
Subject: Re: Bus error on shm_open/ftruncate/mmap after changing kernel boot
 partition: FIXED
References: <5818F39A.30702@connotech.com>
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Fixed (but still puzzled) -- see below.

On 01/11/16 07:57 PM, Thierry Moreau wrote:
> Strange bug after changing the boot partition (the previous was on a
> hard disk with a few media errors).
>

Ah!

mounting tmpfs on /dev/shm (manually or through fstab) fixes this.

I am somehow puzzled as I think I observe the test program working 
without this mount on some kernel instances.

> I must apologize that the old partition is re-formatted. (As an excuse
> the other services on this server system work fine with the new partition.)
>
> Anyway, the bogus code sequence is below. Should work fine everywhere,
> modulo write permission on /dev/shm.
>
> Same observation with privileged execution.
>
> uname -a reports:
>
> Linux hostname 4.1.3 #15 SMP Sun Aug 30 03:01:12 UTC 2015 x86_64
> Intel(R) Pentium(R) CPU G3250 @ 3.20GHz GenuineIntel GNU/Linux
>
> Obviously, any help appreciated.
>
> Regards
>
> - Thierry Moreau
>
>
> ============
> /* mmap_test.cpp --
> */
>
> #include <cstdio>
> #include <cstring>
> #include <cerrno>
> #include <sys/mman.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <unistd.h>
>
> int main(int argc, char *argv[])
> {
>    int shm_fd;
>    shm_fd=shm_open(/*"/dev/shm/"*/"test",
>           O_RDWR | O_CREAT | O_EXCL,
>           S_IRUSR|S_IWUSR | S_IRGRP|S_IWGRP);
>    if (shm_fd>=0) {
>      if (ftruncate(shm_fd,0x20000)==0) {
>        void *region=mmap(0,0x20000,
>                     PROT_READ|PROT_WRITE,MAP_SHARED,
>                     shm_fd,0);
>        if (MAP_FAILED!=region) {
>      fprintf(stderr,"%d %p\n",__LINE__,region);
>      fprintf(stderr,"%d %d\n",__LINE__,*((int *)region));
>          /* ===== this triggers a bus error on the kernel instance */
>        }
>        else {
>      fprintf(stderr,"%d %d %s\n",__LINE__,errno,strerror(errno));
>        }
>      }
>      else {
>        fprintf(stderr,"%d %d %s\n",__LINE__,errno,strerror(errno));
>      }
>    }
>    else {
>      fprintf(stderr,"%d %d %s\n",__LINE__,errno,strerror(errno));
>    }
> return errno;
> }
> ============
>