[PATCHSET,0/3] xfs: fix realtime geometry integer overflows

Message

Darrick J. Wong Dec. 3, 2023, 7 p.m. UTC

Hi all,

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.

If you're going to start using this code, I strongly recommend pulling
from my git trees, which are linked below.

This has been lightly tested with fstests.  Enjoy!
Comments and questions are, as always, welcome.

--D

kernel git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=fix-rtmount-overflows-6.7
---
 fs/xfs/libxfs/xfs_rtbitmap.c |   12 ++++++++++++
 fs/xfs/libxfs/xfs_rtbitmap.h |   14 ++++++++++++++
 fs/xfs/libxfs/xfs_sb.c       |    6 ++++--
 fs/xfs/xfs_rtalloc.c         |    6 ++++--
 4 files changed, 34 insertions(+), 4 deletions(-)