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 <[email protected]> > > --- > > 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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