On Thu, 4 Nov 2021, Christophe Leroy wrote:
> Le 02/11/2021 à 03:20, Finn Thain a écrit :
> > Hi Christopher,
> >
> > After many builds and tests, Stan and I were able to determine that this
> > regression only affects builds with CONFIG_USER_NS=y. That is,
> >
> > d3ccc9781560 + CONFIG_USER_NS=y --> fail
> > d3ccc9781560 + CONFIG_USER_NS=n --> okay
> > d3ccc9781560~ + CONFIG_USER_NS=y --> okay
> > d3ccc9781560~ + CONFIG_USER_NS=n --> okay
> >
> > Stan also tested a PowerMac G3 system and found that the regression is not
> > present there. Thus far, only PowerMac G4 systems are known to be affected
> > (Stan's Cube and Riccardo's PowerBook).
> >
> > I asked Stan to try v5.15-rc after reverting commit d3ccc9781560.
> > Unexpectedly, this build had the same issue. So, it appears there are
> > multiple bad commits that produce this Xorg failure, of which d3ccc9781560
> > is just the first.
> >
> > But there's no easy way to identify the other bad commits using bisection.
> > So I've addressed this message to you. Can you help fix this regression?
> >
>
> I'm wondering if this commit is really the cause of the problem.
>
> Are you using GCC 11 ?
>
> If yes, I think it could be a false positive, fixed by
> https://github.com/linuxppc/linux/commit/7315e457d6bc
>
> Can you try with GCC 10 or older ?
>
AFAIK, all of Stan's builds were made with gcc 10.
> Can you cherry pick 7315e457d6bc ("powerpc/uaccess: Fix __get_user() with
> CONFIG_CC_HAS_ASM_GOTO_OUTPUT") on top of d3ccc9781560 and see what happens ?
>
$ git checkout d3ccc9781560
$ git cherry-pick 7315e457d6bc
Auto-merging arch/powerpc/include/asm/uaccess.h
CONFLICT (content): Merge conflict in arch/powerpc/include/asm/uaccess.h
error: could not apply 7315e457d6bc... powerpc/uaccess: Fix __get_user() with
CONFIG_CC_HAS_ASM_GOTO_OUTPUT
There is no __get_user_asm2_goto in this tree, and __get_user_asm2 already
has the "=&r" constraint:
#define __get_user_asm2(x, addr, err) \
__asm__ __volatile__( \
"1: lwz%X2 %1, %2\n" \
"2: lwz%X2 %L1, %L2\n" \
"3:\n" \
".section .fixup,\"ax\"\n" \
"4: li %0,%3\n" \
" li %1,0\n" \
" li %1+1,0\n" \
" b 3b\n" \
".previous\n" \
EX_TABLE(1b, 4b) \
EX_TABLE(2b, 4b) \
: "=r" (err), "=&r" (x) \
: "m" (*addr), "i" (-EFAULT), "0" (err))
