linux-ext4 - Re: [PATCH] ext4: Give 32bit personalities 32bit hashes

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Date:   Tue, 17 Mar 2020 12:52:04 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, qemu-devel@...gnu.org,
        Peter Maydell <peter.maydell@...aro.org>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] ext4: Give 32bit personalities 32bit hashes

* Linus Walleij:

> It was brought to my attention that this bug from 2018 was
> still unresolved: 32 bit emulators like QEMU were given
> 64 bit hashes when running 32 bit emulation on 64 bit systems.
>
> The personality(2) system call supports to let processes
> indicate that they are 32 bit Linux to the kernel. This
> was suggested by Teo in the original thread, so I just wired
> it up and it solves the problem.
>
> Programs that need the 32 bit hash only need to issue the
> personality(PER_LINUX32) call and things start working.
>
> I made a test program like this:
>
>   #include <dirent.h>
>   #include <errno.h>
>   #include <stdio.h>
>   #include <string.h>
>   #include <sys/types.h>
>   #include <sys/personality.h>
>
>   int main(int argc, char** argv) {
>     DIR* dir;
>     personality(PER_LINUX32);
>     dir = opendir("/boot");
>     printf("dir=%p\n", dir);
>     printf("readdir(dir)=%p\n", readdir(dir));
>     printf("errno=%d: %s\n", errno, strerror(errno));
>     return 0;
>   }
>
> This was compiled with an ARM32 toolchain from Bootlin using
> glibc 2.28 and thus suffering from the bug.

Just be sure: Is it possible to move the PER_LINUX32 setting into QEMU?
(I see why not.)

However, this does not solve the issue with network file systems and
other scenarios.  I still think need to add a workaround to the glibc
implementation.