linux-ext4 - [Bug 205957] Ext4 64 bit hash breaks 32 bit glibc 2.28+

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Date:   Fri, 02 Oct 2020 10:03:42 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...r.kernel.org
Subject: [Bug 205957] Ext4 64 bit hash breaks 32 bit glibc 2.28+

https://bugzilla.kernel.org/show_bug.cgi?id=205957

Danny Milosavljevic (dannym+a@...atchpost.org) changed:

           What    |Removed                     |Added
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                 CC|                            |dannym+a@...atchpost.org

--- Comment #17 from Danny Milosavljevic (dannym+a@...atchpost.org) ---
The right place to fix this is in the distributions enabling LFS support, not
in ext4, not in qemu (which is completely blameless and should not be changed)
and only marginally in glibc.

See also my added comments on
https://sourceware.org/bugzilla/show_bug.cgi?id=23960 and also on
https://lists.gnu.org/archive/html/guix-patches/2020-10/msg00059.html , the
latter is where the actual testing goes on.

Summary:

* glibc calls getdents64 and then is surprised (and fails when
_FILE_OFFSET_BITS < 64) when it gets a 64 bit result back
* The distribution does not enable _FILE_OFFSET_BITS=64
* It is possible for anyone to make it return a 64 bit result by writing a FUSE
filesystem, without being root
* readdir is thus unreliable in these environments, and it depends on
filesystem internals when the first value > 2**32 is returned, at which point
readdir stops reading and sets errno.
* Nobody reads errno in those cases.  Even if they did, what are they supposed
to do in those cases?
* X86_32 syscall emulation does not exist on aarch64 and other 64 bit
archs--they couldn't use the kernel workaround even where it does exist.

Solution:

* The distribution should globally enable _FILE_OFFSET_BITS=64

Extra:

* Now (after 15 years of 64 bit) glibc should be made to emit a warning or
error if users try to use file stuff (like readdir) and _FILE_OFFSET_BITS!=64.

There is no need to fiddle with ext4, qemu or kernel syscalls in order to fix
this problem.

If distributions didn't enable LFS (large file support) in the last 15 years,
they are weird.  Everyone has drives > 4 GiB, usually > 1000 GiB, nowadays. 
But without LFS users can't even create a file > 4 GiB on those.  How are there
still distributions which actually have this problem?

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