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

[Kevin Wolf]

Am 06.04.2017 um 23:29 hat Richard W.M. Jones geschrieben:
> 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.

I think for the moment this is actually the best solution. Copy on read
is currently implemented in a way that we can't do things properly with
respect to permissions, so we need to bypass the permission system.

We can't require the callers to have write permissions for a read
request, this would have to be handled internally by the COR code. As
long as it is still implemented directly in block/io.c instead of a
separate filter driver, the COR code doesn't have a BdrvChild and
therefore can't have its own set of permissions on the node.

Kevin