[Ocfs2-test-devel] [PATCH 42/59] mmap-test: fix segment fault

[Eric Ren]

Hi Junxiao,

After learning the whole thing about mmap test, I found more issues.

1. In my test, segment faults didn't happen. I did the following
====
steps:
##
eric at laptop:~> gcc mmap_test.c   ===> copy from ocfs2-test directly
eric at laptop:~> dd if=/dev/zero of=test bs=4096 count=2
2+0 records in
2+0 records out
8192 bytes (8.2 kB) copied, 0.000187554 s, 43.7 MB/s
eric at laptop:~> ls -sh test
8.0K test
eric at laptop:~> ./a.out test
file size: 8192         ===> add printf
page size: 4096
buf = 0x7fc277f15000, ptr = 0x7fc277f17000, size = 8192, offset = 0,
remain = 4096
tail of ptr: ""
tail of page: "tail of page: "
5000, ptr = 0x7f000, ptr = 0x7fe = 8192, offset = 8192, offset 096
6
"
##

2. How test suits use mmap_test
==============
eric at laptop:~/ocfs2-test> vim programs/python_common/single_run-WIP.sh +711
 699 run_mmap(){
...
 725 
 726   dd if=/dev/zero of=${workfile} bs=1M count=1024
 727   mmap_test ${workfile} >${outlog} 2>&1
 728   RC=$?
...
}

This way, workfile=1G is always multiple size of page size(4096).
I think bs=1M is bad.

3. What mmap_test want to do
===========================
eric at laptop:~> cd ocfs2-test/
eric at laptop:~/ocfs2-test> vim programs/mmap_test/mmap_test.c
...
75     buf = mmap(NULL, stat_buf.st_size, PROT_READ, MAP_SHARED, fd, 0);
 76     if (buf == MAP_FAILED)
 77     {
 78         perror("MMap");
 79         return 1;
 80     }
 81 
 82     offset = stat_buf.st_size % page_size;
 83     ptr = buf + (stat_buf.st_size - offset);
 84     remain = page_size - offset;
 85 
 86     fprintf(stdout, "buf = %p, ptr = %p, size = %lu, offset = %d, remain = %d\n",
 87             buf, ptr, stat_buf.st_size, offset, remain);
 88 
 89     fprintf(stdout, "tail of ptr: \"");
 90     fwrite(ptr, sizeof(char), offset, stdout);
 91     fprintf(stdout, "\"\n");
 92 
 93     fprintf(stdout, "tail of page: \"");
 94     fwrite(ptr + offset, sizeof(char), remain, stdout);
 95     fprintf(stdout, "\"\n");
...

Looks like it want output the front part of the last mapping page, and the remaining of
the last mapping page, right?

I think "tail of ptr" should be "head of last mapping page", and "tail of page" should
be "tail of last mapping page".

But it didn't handle the case when offset==0. 

4. How to fix
=============
Frist,this line of code:
ptr = buf + (stat_buf.st_size - offset);
should be changed to
ptr = buf + (stat_buf.st_size - offset -1);

Then, when offset==0, the rest of code actually doesn't make any sense. Could we just
return, or output the last whole page?

Thanks,
Eric

On Mon, Sep 14, 2015 at 10:44:28AM +0800, Junxiao Bi wrote: 
> When test file size is multiple of page size, there will be write out of
> mmap range and cause segment fault.
> 
> Signed-off-by: Junxiao Bi <junxiao.bi at oracle.com>
> ---
>  programs/mmap_test/mmap_test.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/programs/mmap_test/mmap_test.c b/programs/mmap_test/mmap_test.c
> index 9edbb58..711bb81 100644
> --- a/programs/mmap_test/mmap_test.c
> +++ b/programs/mmap_test/mmap_test.c
> @@ -80,6 +80,14 @@ int main(int argc, char *argv[])
>      }
>  
>      offset = stat_buf.st_size % page_size;
> +
> +    /* fix offset if the file size is multiple of page size */
> +    if (offset == 0)
> +	    offset = page_size;
> +
> +    if (offset > stat_buf.st_size)
> +	    offset = stat_buf.st_size;
> +
>      ptr = buf + (stat_buf.st_size - offset);
>      remain = page_size - offset;
>  
> -- 
> 1.7.9.5
> 
> 
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