On Fri, Jan 25, 2019 at 09:19:17AM +1100, Dave Chinner wrote: > On Thu, Jan 24, 2019 at 11:08:46AM -0800, Darrick J. Wong wrote: > > On Thu, Jan 24, 2019 at 10:54:40AM -0500, Brian Foster wrote: > > > Add a field to specify the v4 and v5 magic values in xfs_buf_ops. > > > This allows otherwise identical verifiers to distinguish between > > > and verify different magic values (inobt vs. finobt buffers). This > > > also facilitates verification of the appropriate magic value based > > > on superblock version. > > > > > > The magic field is optional and is free to be used as appropriate > > > for each particular verifier. > > > > > > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> > > > --- > > > > > > Hi all, > > > > > > What do folks think of something like this as a lightweight (and > > > untested) means to do proper [f]inobt magic verification? For reference, > > > the initial version of this put together to help root cause a user > > > report is here[1]. I was hoping to do the same thing with less code > > > duplication. A couple things that come to mind: > > > > > > 1. I know scrub has at least one place where we invoke the verifier with > > > ->b_ops == NULL, which will cause this to explode. Could we fix that up > > > to assign and reset ->b_ops to accommodate something like this, or is > > > that problematic? > > > > IIRC one of the scrub findroot reviewers didn't like the idea of scrub > > setting b_ops until it was absolutely sure it wanted to. I think it's > > actually ok to patch it in temporarily while running the read verifier > > since we have the buffer locked and patch it out afterwards. > > How does this interact with xfs_buf_ensure_ops()? > > [ side note: the comments about this function are poor - I have no > idea what problem it is avoiding from reading the code. Yes, I know > it protects against transactions with buffers and no ops, but the > comments don't tell me *how or when that occurs* so I do not know > where to go looking for potential issues here. ] > I think the when and how behind this logic is the scrub case (i.e., xrep_findroot_block()) called out above: we read the buffer with a NULL b_ops param because we don't know which buf_ops actually applies. If a ->b_ops is not ultimately attached, the buf sits around in cache without ->b_ops and is never verified (even if read with a non-NULL b_ops) until it cycles out of cache. So without this logic, the aforementioned case would have to drop the buffer from the cache if it was ultimately read with a NULL b_ops. With regard to verifiers depending on ->b_ops != NULL, I don't think this would change anything at this level. The higher level scrub code would just be required to assign ->b_ops in order to run a verifier and thus would have to make sure to reset ->b_ops in the event of a failure. > > > 2. We have some other verifiers around that actually use the buffer > > > magic to set a more specific verifier. See xfs_da3_node_read_verify() > > > for an example. I'm not sure this is all that useful for such higher > > > level verifiers, but I think we'd at least be able to use it for the > > > underlying verifiers. That might provide some extra sb version vs. magic > > > sanity checking for places that might not already look at the sb version > > > (otherwise it's just refactoring). > > > > > > Thoughts or other ideas before I try to apply this more broadly? Thanks. > > > > Hmm... not sure if I like the idea that you have to find the b_ops > > declaration to figure out which magic number the verifier function is > > checking, but I don't really have a cogent objection. > > Yeah, I don't really like it either (especially the added CPU > overhead that we avoided by doing compile time byte swapping), > but I'm struggling to come up with a better option. > I suppose we could store the on-disk magics in the xfs_buf_ops structures (it works on x86_64 at least, I'd have to verify other arches), but that is pretty ugly. Given all of the other conversions and checks, I'm not sure it's worth it. Hmm, I suppose we could also define a separate set of on-disk magic directives: #define XFS_FIBT_CRC_MAGIC_DISK cpu_to_be32(XFS_FIBT_CRC_MAGIC) ... and start using those in various places to avoid the ugliness. I think that's a separate change though (and again, it's not immediately clear to me the benefit justifies the additional code). Brian > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
