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

[Aurelien Jarno <aurel32@debian.org>]

control: reassign -1 busybox
control: retitle -1 busybox: wrongly compiled with -mpreferred-stack-boundary=2 on i386

On 2018-01-17 12:08, Raphael Hertzog wrote:
> 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.

busybox is compiled with -mpreferred-stack-boundary=2 on i386 which has
the same effect of reducing the default stack alignment from 16 bytes to
2 bytes. This comes from arch/i386/Makefile:

|  # -mpreferred-stack-boundary=2 is essential in preventing gcc 4.2.x
|  # from aligning stack to 16 bytes. (Which is gcc's way of supporting SSE).
|  CFLAGS += $(call cc-option,-march=i386 -mpreferred-stack-boundary=2,)

I don't really get why it is essential to prevent gcc from aligning
stack to 16 bytes, anyway this is a bad idea. Removing this option just
fixes the issue.

I am therefore reassigning the bug to busybox.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net