Re: [Qemu-devel] [PATCH/RFC] block: Ensure that block size constraints are considered

[Heinz Graalfs <graalfs@linux.vnet.ibm.com>]

On Tue, 2012-12-11 at 11:30 +0100, Kevin Wolf wrote:
> Am 11.12.2012 10:58, schrieb Heinz Graalfs:
> > Hi Kevin,
> > 
> > I'm using the bdrv_pread() function during boot partition detection ...
> > 
> > In detail: 
> > bdrv_pread() is called to read 32 bytes from a 2048 bytes formatted
> > disk. This results in setting up a read of 512 bytes (1 sector
> > multiplied by 512 current code in paio_submit()), which is wrong for a
> > O_DIRECT opened file, and produces the error.
> 
> So this sounds like the real problem: bdrv_pread/pwrite assume 512 byte
> sectors. May it's better to fix it there instead of just fixing one code
> path in one backend.
> 
> In any case this patch as submitted is wrong as it overflows the buffer
> passed to paio_submit. Test it with this patch:
> 
> --- a/qemu-io.c
> +++ b/qemu-io.c
> @@ -1718,6 +1718,8 @@ static int openfile(char *name, int flags, int
> growable)
>              bs = NULL;
>              return 1;
>          }
> +
> +        bdrv_set_buffer_alignment(bs, 4096);
>      }
> 
>      return 0;
> 
> 
> $ ./qemu-io -n -c 'read -p 0 512' /tmp/foo
> read 512/512 bytes at offset 0
> 512 bytes, 1 ops; 0.0001 sec (3.727 MiB/sec and 7633.5878 ops/sec)
> *** glibc detected *** ./qemu-io: double free or corruption (out):
> 0x00007fa22349b000 ***
> 
> Kevin
> 

Kevin, I tried your fix and it solves the free error...

Here is what I get:

# lsdasd
Bus-ID     Status      Name      Device  Type  BlkSz  Size      Blocks
==============================================================================
0.0.37a1   active      dasdb     94:4    ECKD  2048   6162MB    3155355
0.0.37a0   active      dasdc     94:8    ECKD  512    3594MB    7362495

# ./qemu-io  -c 'read -p 0 512' /dev/disk/by-path/ccw-0.0.37a0
read 512/512 bytes at offset 0
512 bytes, 1 ops; 0.0000 sec (7.512 MiB/sec and 15384.6154 ops/sec)

# ./qemu-io -n  -c 'read -p 0 512' /dev/disk/by-path/ccw-0.0.37a0
read 512/512 bytes at offset 0
512 bytes, 1 ops; 0.0005 sec (904.159 KiB/sec and 1808.3183 ops/sec)


# ./qemu-io   -c 'read -p 0 512' /dev/disk/by-path/ccw-0.0.37a1
read 512/512 bytes at offset 0
512 bytes, 1 ops; 0.0000 sec (7.288 MiB/sec and 14925.3731 ops/sec)

# ./qemu-io -n  -c 'read -p 0 512' /dev/disk/by-path/ccw-0.0.37a1
read failed: Invalid argument
# 

Are you going to fix the rest in bdrv_pread/pwrite too?

Heinz