Re: [Qemu-devel] [PULL 43/46] block: Assertions for write permissions

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Apr 06, 2017 at 04:23:19PM -0500, Eric Blake wrote:
> On 04/06/2017 04:15 PM, Richard W.M. Jones wrote:
> 
> >>>> +++ b/block/io.c
> >>>> @@ -945,6 +945,8 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
> >>>>      size_t skip_bytes;
> >>>>      int ret;
> >>>>  
> >>>> +    assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE));
> >>>
> 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1439922
> > 
> > There is also a minimal reproducer in comment 2.
> 
> If it helps, backtrace shows:
> 
> #4  0x0000555555c09f10 in bdrv_co_do_copy_on_readv
> (child=0x555556990180, offset=0, bytes=512, qiov=0x7fffffffd0c0) at
> block/io.c:948
> #5  0x0000555555c0a33d in bdrv_aligned_preadv (child=0x555556990180,
> req=0x7fffb6dffec0, offset=0, bytes=512, align=1, qiov=0x7fffffffd0c0,
> flags=1)
>     at block/io.c:1058
> #6  0x0000555555c0a8c3 in bdrv_co_preadv (child=0x555556990180,
> offset=0, bytes=512, qiov=0x7fffffffd0c0, flags=BDRV_REQ_COPY_ON_READ)
> at block/io.c:1166
> #7  0x0000555555bf7ccf in blk_co_preadv (blk=0x555556a2bc20, offset=0,
> bytes=512, qiov=0x7fffffffd0c0, flags=0) at block/block-backend.c:927
> #8  0x0000555555bf7e19 in blk_read_entry (opaque=0x7fffffffd0e0)
>     at block/block-backend.c:974
> #9  0x0000555555cc3015 in coroutine_trampoline (i0=1485566720, i1=21845)
>     at util/coroutine-ucontext.c:79

The patch that fixes this is simply:

diff --git a/block/io.c b/block/io.c
index 2709a7007f..4f1a730e45 100644
--- a/block/io.c
+++ b/block/io.c
@@ -945,8 +945,6 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
     size_t skip_bytes;
     int ret;
 
-    assert(child->perm & (BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE));
-
     /* Cover entire cluster so no additional backing file I/O is required when
      * allocating cluster in the image file.
      */

It has no ill effects that I can see, and fixes the libguestfs test
suite, but I'm assuming the assert was added for a reason so I'm not
proposing this as a patch.

Rich.

-- 
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