Re: [Qemu-devel] [PATCH for-4.1] block/copy-on-read: Fix permissions for inactive node

[Eric Blake]

On 7/29/19 5:53 AM, Kevin Wolf wrote:
> The copy-on-read drive must not request the WRITE_UNCHANGED permission
> for its child if the node is inactive, otherwise starting a migration
> destination with -incoming will fail because the child cannot provide
> write access yet:
> 
>   qemu-system-x86_64: -blockdev copy-on-read,file=img,node-name=cor: Block 
> node is read-only
> 
> Earlier QEMU versions additionally ran into an abort() on the migration
> source side: bdrv_inactivate_recurse() failed to update permissions.
> This is silently ignored today because it was only supposed to loosen
> restrictions. This is the symptom that was originally reported here:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=1733022
> 
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block/copy-on-read.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)

Do any of the iotests cover this?  Should they, especially if you are
trying to get this in for -rc3 tomorrow?

> 
> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
> index 22f24fd0db..6631f30205 100644
> --- a/block/copy-on-read.c
> +++ b/block/copy-on-read.c
> @@ -56,16 +56,14 @@ static void cor_child_perm(BlockDriverState *bs, 
> BdrvChild *c,
>                             uint64_t perm, uint64_t shared,
>                             uint64_t *nperm, uint64_t *nshared)
>  {
> -    if (c == NULL) {
> -        *nperm = (perm & PERM_PASSTHROUGH) | BLK_PERM_WRITE_UNCHANGED;
> -        *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
> -        return;
> -    }
> +    *nperm = perm & PERM_PASSTHROUGH;
> +    *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
>  
> -    *nperm = (perm & PERM_PASSTHROUGH) |
> -             (c->perm & PERM_UNCHANGED);
> -    *nshared = (shared & PERM_PASSTHROUGH) |
> -               (c->shared_perm & PERM_UNCHANGED);

The old code unconditionally returned one set of permissions when c ==
NULL, or made a choice based on c's existing permissions on whether to
pass in those two bits.

> +    /* We must not request write permissions for an inactive node, the child
> +     * cannot provide it. */
> +    if (!(bs->open_flags & BDRV_O_INACTIVE)) {
> +        *nperm |= BLK_PERM_WRITE_UNCHANGED;
> +    }

The new code changes the condition for or'ing in WRITE_UNCHANGED to
*nperm (it is no longer dependent on whether c == NULL, but whether the
drive is inactive), which matches your commit message.

But the new code also changes to always pass in the PERM_UNCHANGED to
*nshared; that used to be skipped if c was non-NULL and did not already
have the permission.  I don't follow that change from the commit
message, am I missing something?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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