Re: [Buildroot] [PATCH] package/busybox: disable stack optimization for i386 target

[Peter Korsgaard <peter@korsgaard.com>]

>>>>> "Romain" == Romain Naour <romain.naour@smile.fr> writes:

 > Hello Thomas, All,
 > Le 11/02/2023 à 11:10, Romain Naour a écrit :
 >> Hello Thomas,
 >> 
 >> Le 11/02/2023 à 10:49, Thomas Petazzoni via buildroot a écrit :
 >>> On Sat, 11 Feb 2023 00:36:58 +0100
 >>> Romain Naour <romain.naour@gmail.com> wrote:
 >>> 
 >>>> The toolchain-builder project reported an issue with Qemu 7.2.0 for
 >>>> x86-core2--glibc--bleeding-edge toolchain [1]:
 >>>> 
 >>>> Run /sbin/init as init process
 >>>> random: fast init done
 >>>> EXT4-fs (vda): warning: mounting unchecked fs, running e2fsck is recommended
 >>>> EXT4-fs (vda): re-mounted. Opts: (null). Quota mode: disabled.
 >>>> Starting syslogd: OK
 >>>> traps: syslogd[52] general protection fault ip:b7e21465
 >>>> sp:bfe59e6c error:0 in libc.so.6[b7d9b000+123000]
 >>>> Starting klogd: OK
 >>>> traps: klogd[56] general protection fault ip:b7e94465
 >>>> sp:bf8f069c error:0 in libc.so.6[b7e0e000+123000]
 >>>> Running sysctl: traps: logger[62] general protection fault
 >>>> ip:b7e48b6c sp:bfd7d194 error:0 in libc.so.6[b7e05000+123000]
 >>>> Segmentation fault
 >>>> traps: logger[64] general protection fault ip:b7dd3b6c
 >>>> sp:bf9b8604 error:0 in libc.so.6[b7d90000+123000]
 >>>> Segmentation fault
 >>>> 
 >>>> (Followed by a kernel panic.)
 >>>> 
 >>>> Testing with the pevious Qemu release (7.1.0) allows to boot the
 >>>> system without any problem.
 >>>> 
 >>>> Building qemu sources between 7.1.0 and 7.2.0 allows to identify
 >>>> the first "bad" commit [2] and
 >>>> report to the Qemu project [3].
 >>>> 
 >>>> Thanks to Qemu maintainers review, several issues was noticed:
 >>>> 
 >>>> "The default i386 busybox build config does not respect glibc's
 >>>> requirements around stack alignment
 >>>> (see [4] for previous discussions and a workaround)."
 >>>> 
 >>>> Disabling CONFIG_STACK_OPTIMIZATION_386 option (as suggested in
 >>>> the Gentoo bug report) fixed the issue!
 >>>> 
 >>>> This option has been added and enabled by default in buxybox
 >>>> 1_29_0, so it was used since then the for
 >>>> Buildroot's qemu defconfig.
 >>>> 
 >>>> Note: The x86-i686--glibc--bleeding-edge (generic x86) doesn't trigger the issue with
 >>>> CONFIG_STACK_OPTIMIZATION_386 enabled.
 >>>> 
 >>>> Fixes:
 >>>> https://gitlab.com/buildroot.org/toolchains-builder/-/jobs/3731683337
 >>>> 
 >>>> [1] https://gitlab.com/buildroot.org/toolchains-builder/-/jobs/3731683337
 >>>> [2] https://gitlab.com/qemu-project/qemu/-/commit/958e1dd1300f37f18b2161dfb4eb806fc8c19b44
 >>>> [3] https://gitlab.com/qemu-project/qemu/-/issues/1478
 >>>> [4] https://bugs.gentoo.org/725674
 >>> 
 >>> Thanks a lo for the great investigation. Do we understand precisely
 >>> what is happening? The link at [4] does not really have an explanation,
 >>> it only has experimental observations that lead to the conclusion that
 >>> disabling CONFIG_STACK_OPTIMIZATION_386 is a work-around, but it does
 >>> not really explain what is happening.
 >> 
 >> Actually there are two different issues that contribute to this issue:
 >> 
 >> 1) An existing latent Busybox bug on i386 (busybox compiled with
 >> -mpreferred-stack-boundary=2)
 >> 
 >> https://lists.debian.org/debian-boot/2018/01/msg00352.html
 >> 
 >> 2) A Qemu improvement that trigger an exception on unaligned memory accesses
 >> that require 16-byte alignment.
 >> 
 >> https://gitlab.com/qemu-project/qemu/-/commit/958e1dd1300f37f18b2161dfb4eb806fc8c19b44
 >> 
 >> I didn't digging further the root cause of the issue.

 > Several bug report are confirming a stack problem due to the i386 GCC ABI which
 > assumes the stack is 16-byte aligned [1] [2]. The gcc's default ABI for
 > i386-linux-gnu was quietly changed [3] (maybe between gcc 9 and gcc 10).

 > Note: When the option was added to Busybox and enabled by default, the help text
 > explains that this option may not work with some libc versions:

 > "This option makes for smaller code, but some libc versions
 > do not work with it (they use SSE instructions without
 > ensuring stack alignment)."

 > This problem break the test of the x86 core2 toolchain on toolchain-builder.

 > [1] https://bugs.gentoo.org/725674#c30
 > [2] https://lists.debian.org/debian-boot/2018/01/msg00352.html
 > [3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=886506;msg=97
 > [4]
 > https://git.busybox.net/busybox/commit/?id=2c9970281083a99acfa3aec8c6d41db955cb583d

Committed to 2023.02.x, thanks.

-- 
Bye, Peter Korsgaard
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