Re: [PATCHSET 0/3] xfs: fix ascii-ci problems with userspace

["Darrick J. Wong" <djwong@kernel.org>]

Hi Linus,

My finger slipped and I accidentally added you to the To: list on this
new series.  This series needs to go through review on linux-xfs; when
this is ready to go I (or Dave) will send you a pull request.

Sorry about the noise.

--D

On Tue, Apr 04, 2023 at 10:07:00AM -0700, Darrick J. Wong wrote:
> Hi all,
> 
> Last week, I was fiddling around with the metadump name obfuscation code
> while writing a debugger command to generate directories full of names
> that all have the same hash name.  I had a few questions about how well
> all that worked with ascii-ci mode, and discovered a nasty discrepancy
> between the kernel and glibc's implementations of the tolower()
> function.
> 
> I discovered that I could create a directory that is large enough to
> require separate leaf index blocks.  The hashes stored in the dabtree
> use the ascii-ci specific hash function, which uses a library function
> to convert the name to lowercase before hashing.  If the kernel and C
> library's versions of tolower do not behave exactly identically,
> xfs_ascii_ci_hashname will not produce the same results for the same
> inputs.  xfs_repair will deem the leaf information corrupt and rebuild
> the directory.  After that, lookups in the kernel will fail because the
> hash index doesn't work.
> 
> The kernel's tolower function will convert extended ascii uppercase
> letters (e.g. A-with-umlaut) to extended ascii lowercase letters (e.g.
> a-with-umlaut), whereas glibc's will only do that if you force LANG to
> ascii.  Tiny embedded libc implementations just plain won't do it at
> all, and the result is a mess.  Stabilize the behavior of the hash
> function by encoding the kernel's tolower function in libxfs, add it to
> the selftest, and fix xfs_scrub not handling this correctly.
> 
> If you're going to start using this mess, you probably ought to just
> pull from my git trees, which are linked below.
> 
> This is an extraordinary way to destroy everything.  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-asciici-tolower-6.3
> ---
>  fs/xfs/libxfs/xfs_dir2.c |    4 -
>  fs/xfs/libxfs/xfs_dir2.h |   20 ++++
>  fs/xfs/scrub/dir.c       |    7 +-
>  fs/xfs/xfs_dahash_test.c |  211 ++++++++++++++++++++++++----------------------
>  4 files changed, 139 insertions(+), 103 deletions(-)
>