xfs: fix negative array access in xfs_getbmap

In commit 8ee81ed5, Ye Bin complained about an ASSERT in the bmapx
code that trips if we encounter a delalloc extent after flushing the
pagecache to disk.  The ioctl code does not hold MMAPLOCK so it's
entirely possible that a racing write page fault can create a delalloc
extent after the file has been flushed.  The proposed solution was to
replace the assertion with an early return that avoids filling out the
bmap recordset with a delalloc entry if the caller didn't ask for it.

At the time, I recall thinking that the forward logic sounded ok, but
felt hesitant because I suspected that changing this code would cause
something /else/ to burst loose due to some other subtlety.

syzbot of course found that subtlety.  If all the extent mappings found
after the flush are delalloc mappings, we'll reach the end of the data
fork without ever incrementing bmv->bmv_entries.  This is new, since
before we'd have emitted the delalloc mappings even though the caller
didn't ask for them.  Once we reach the end, we'll try to set
BMV_OF_LAST on the -1st entry (because bmv_entries is zero) and go
corrupt something else in memory.  Yay.

I really dislike all these stupid patches that fiddle around with debug
code and break things that otherwise worked well enough.  Nobody was
complaining that calling XFS_IOC_BMAPX without BMV_IF_DELALLOC would
return BMV_OF_DELALLOC records, and now we've gone from "weird behavior
that nobody cared about" to "bad behavior that must be addressed
immediately".

Maybe I'll just ignore anything from Huawei from now on for my own sake.

Reported-by: syzbot+c103d3808a0de5faaf80@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-xfs/20230412024907.GP360889@frogsfrogsfrogs/
Fixes: 8ee81ed5 ("xfs: fix BUG_ON in xfs_getbmap()")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_bmap_util.c

@@ -558,7 +558,9 @@ xfs_getbmap(
 		if (!xfs_iext_next_extent(ifp, &icur, &got)) {
 			xfs_fileoff_t	end = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
 
-			out[bmv->bmv_entries - 1].bmv_oflags |= BMV_OF_LAST;
+			if (bmv->bmv_entries > 0)
+				out[bmv->bmv_entries - 1].bmv_oflags |=
+								BMV_OF_LAST;
 
 			if (whichfork != XFS_ATTR_FORK && bno < end &&
 			    !xfs_getbmap_full(bmv)) {