Re: [PATCH 1/2] xfs: transactionless xfs_bunmapi shouldn't do format conversion

On Tue, Jun 19, 2018 at 03:27:59PM +1000, Dave Chinner wrote:

On Mon, Jun 18, 2018 at 09:54:05PM -0700, Darrick J. Wong wrote:

quoted

On Tue, Jun 19, 2018 at 12:41:27PM +1000, Dave Chinner wrote:

quoted

From: Dave Chinner <redacted>

If we are punching out a delalloc extent, xfs_bunmapi() does not
have a transaction context and should not ever need to convert the
on-disk extent format. If such a thing is attempted (e.g. via a
corrupt inode extent count in extent format) then we should abort
with an EFSCORRUPTED error. Unfortunately, we don't do that and
crash instead:

 XFS (loop0): page discard on page 0000000005fd24f3, inode 0x75e5, offset 0.
 ==================================================================
 BUG: KASAN: null-ptr-deref in xfs_alloc_get_freelist+0x115/0x350
 Read of size 8 at addr 0000000000000028 by task a.out/1406
 CPU: 0 PID: 1406 Comm: a.out Not tainted 4.17.0-rc4-kasan #2
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
 Call Trace:
  dump_stack+0x7b/0xb5
  kasan_report+0x10c/0x390
  __asan_load8+0x54/0x90
  xfs_alloc_get_freelist+0x115/0x350
  xfs_alloc_fix_freelist+0x35b/0x830
  xfs_alloc_vextent+0x215/0x990
  xfs_bmap_extents_to_btree+0x30d/0x940
.....

By returning an error here, we avoid such crashes when punching out
a delalloc page because we don't try to fix up an AG freelist
without a transaction. Hence we get an error like so:

Um, isn't erroring out here leaving a dirty bomb in the in-core metadata?

Not that I can tell. We've already trashed the dirty page state by
this point, so the page cache can safely reclaim the page and the
delalloc range over it will never get written.  And the XFS inode
cleanup code didn't have any issues with the way the error was
handled, either, because the delalloc range was actually removed
before the fork format error was triggered.

IOWs, there is no dirty, stale page state or delalloc extents
hanging around if this error fires.

Hmmm, well I guess I'll pull this one in and look for problems.

I wonder, is there a <cough> testcase for this?  Or a fuzz-o-matic to
turn all these things into regression tests?

(Yeah, I know there won't be one for syzbot, I dug through its code and
had to reset my brain by reading mballoc.c. :P)

quoted

Like you say:

quoted

XFS (loop0): page discard on page ffffea00040ae640, inode 0x75e5, offset 0.
XFS (loop0): page discard unable to remove delalloc mapping.

We know the fs is corrupt, we might as well shut down now rather than
let this burp out later.

xfs_bunmapi() doesn't do shutdowns - the higher level code does a
shutdown on error if it is necessary, otherwise it just propagates
the error. In this case it has cleaned up correctly, propagates the
error and it gets back to userspace on the next fsync, and we're
fine to continue onwards as there was no unrecoverable error....

Fair enough.

quoted

I get that people don't want to touch well seasoned code, but
xfs_bunmapi is this big unwieldly function that's crying out for a
refactor.  It's 330 lines long and can be called from various contexts
(data/attr fork, punch delalloc, etc.)...

...it's also weird that xfs_bmap_punch_delalloc_range calls xfs_bunmapi
with no transaction and a xfs_defer that we dump on the ground.

Yes, and yes.

quoted

So yes, I think the patch does fix the crash, but it's kinda gross.

Yes, it is.

But OTOH, I don't want to risk a bunch of filesystem corrupting
regressions across the entire XFS userbase just to fix a trivially
simple crash that requires an extremely unlikely co-ordinated
corruption of an inode data fork and an AGFL, and to simultaneously
have ENOSPC in every other AGF in the filesystem.

Put "refactor xfs_bunmapi()" on the list of "things to do when
there's nothing else to do"...

So in 2066 after the polar ice caps melt after the XFS LOGHAMMER attack
has finally been put down?  Ok. :)

(But no, seriously, if anyone's looking for a little refactoring +
domain knowledge enhancement of the bmapi code...)

--D

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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