busybox sh broken on i386 with glibc 2.26, leads to kernel panic

[Raphael Hertzog <hertzog@debian.org>]

Control: reassign -1 src:glibc 2.26-1
Control: retitle -1 busybox sh broken on i386 with glibc 2.26, leads to kernel panic
Control: severity -1 serious
Control: affects -1 + busybox src:linux

Hello,

on i386 with glibc 2.26-4, busybox sh is broken:

    $ busybox sh
    [...]
    BusyBox v1.27.2 (Debian 1:1.27.2-2) built-in shell (ash)
    Enter 'help' for a list of built-in commands.

    Segmentation fault

In the kernel messages, you see this:
[1097712.640730] traps: busybox[3288] general protection ip:f7e9a51d sp:ff8da68c error:0 in libc-2.26.so[f7d48000+1cd000]

There's a work-around (the same as the one described in
#887169):

    $ GLIBC_TUNABLES=glibc.tune.hwcaps=-SSE4_2 busybox sh
    [...]
    BusyBox v1.27.2 (Debian 1:1.27.2-2) built-in shell (ash)
    Enter 'help' for a list of built-in commands.

    ~ $

Given that busybox's sh is used in the initrd and that the init
command is a shell script, this will lead to the kernel panic
shown earlier in this bug report.

Possible work-arounds in the mean time:
- disable busybox in the initrd by setting BUSYBOX=n in
  /etc/initramfs-tools/initramfs.conf (but this is not
  possible if you use cryptsetup)
- you can add the "GLIBC_TUNABLES=glibc.tune.hwcaps=-SSE4_2" to the kernel
  command line so that it's set in the environment of the init script
  (this will at least let you boot once to fix it permanently)
- install busybox-static instead of busybox (since it was built with
  an earlier version of glibc) and rebuild your initrd.

Aurélien Jaron commented on IRC that this was strange that busybox
was affected by this bug since the analysis made in #887169 lead to
believe that only binaries compiled with -mstack-align=4 would be
affected.

I see that busybox uses things like this:

include/platform.h:# define ALIGN1 __attribute__((aligned(1)))
include/platform.h:# define ALIGN2 __attribute__((aligned(2)))
include/platform.h:# define ALIGN4 __attribute__((aligned(4)))

Could those attributes have a similar effect than -mstack-align=4 ?

Cheers,
-- 
Raphaël Hertzog ◈ Debian Developer

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