Merge tag 'fix-rtmount-overflows-6.8_2023-12-06' of https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux into xfs-6.8-mergeA

xfs: fix realtime geometry integer overflows

While reading through the realtime geometry support code in xfsprogs, I
noticed a discrepancy between the sb_rextslog computation used when
writing out the superblock during mkfs and the validation code used in
xfs_repair.  This discrepancy would lead to system failure for a runt rt
volume having more than 1 rt block but zero rt extents in length.  Most
people aren't going to configure a 1M extent size for their 360k rt
floppy disk volume, but I did!

In the process of studying that code, it occurred to me that there is a
second bug in the computation -- the use of highbit32 for a 64-bit
value means that the upper 32 bits are not considered in the search for
a high bit.  This causes the creation of a realtime summary file that is
the wrong length.  If rextents is a multiple of U32_MAX then this will
appear to work fine because highbit32 returns -1 for an input of 0; but
for all other cases the rt summary is undersized, leading to failures.

Fix the first problem by standardizing the computation with a helper in
libxfs; and the second problem by correcting the computation.  This will
cause any existing rt volumes larger than 2^32 blocks to fail validation
but they probably were already crashing anyway.

This has been lightly tested with fstests.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

* tag 'fix-rtmount-overflows-6.8_2023-12-06' of https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux:
  xfs: don't allow overly small or large realtime volumes
  xfs: fix 32-bit truncation in xfs_compute_rextslog
  xfs: make rextslog computation consistent with mkfs

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