Re: [Qemu-block] [PATCH v3] block/file-posix: do not fail on unlock bytes

[Kevin Wolf]

Am 29.03.2019 um 18:15 hat Max Reitz geschrieben:
> On 29.03.19 12:04, Vladimir Sementsov-Ogievskiy wrote:
> > bdrv_replace_child() calls bdrv_check_perm() with error_abort on
> > loosening permissions. However file-locking operations may fail even
> > in this case, for example on NFS. And this leads to Qemu crash.
> > 
> > Let's avoid such errors. Note, that we ignore such things anyway on
> > permission update commit and abort.
> > 
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> > ---
> >  block/file-posix.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/block/file-posix.c b/block/file-posix.c
> > index db4cccbe51..1cf4ee49eb 100644
> > --- a/block/file-posix.c
> > +++ b/block/file-posix.c
> > @@ -815,6 +815,18 @@ static int raw_handle_perm_lock(BlockDriverState *bs,
> >  
> >      switch (op) {
> >      case RAW_PL_PREPARE:
> > +        if ((s->perm | new_perm) == s->perm &&
> > +            (s->shared_perm & new_shared) == s->shared_perm)
> > +        {
> > +            /*
> > +             * We are going to unlock bytes, it should not fail. If it 
> > fail due
> > +             * to some fs-dependent permission-unrelated reasons (which 
> > occurs
> > +             * sometimes on NFS and leads to abort in bdrv_replace_child) 
> > we
> > +             * can't prevent such errors by any check here. And we ignore 
> > them
> > +             * anyway in ABORT and COMMIT.
> > +             */
> > +            return 0;
> > +        }
> >          ret = raw_apply_lock_bytes(s, s->fd, s->perm | new_perm,
> >                                     ~s->shared_perm | ~new_shared,
> >                                     false, errp);
> 
> Help me understand the exact issue, please.  I understand that there are
> operations like bdrv_replace_child() that pass &error_abort to
> bdrv_check_perm() because they just loosen the permissions, so it should
> not fail.
> 
> However, if the whole effect really would be to loosen permissions,
> raw_apply_lock_bytes() wouldn't have failed here in PREPARE anyway:
> @unlock is passed as false, so no bytes will be unlocked.  And if
> permissions are just loosened (as your condition checks), it should not
> lock any bytes.
> 
> So why does it attempt lock any bytes in the first place?  There must be
> some discrepancy between s->perm and s->locked_perm, or ~s->shared_perm
> and s->locked_shared_perm.  How does that occur?

I suppose raw_check_lock_bytes() is what is failing, not
raw_apply_lock_bytes().

Kevin

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