Re: Fwd: Fwd: X stopped working with 5.14 on iBook

[Finn Thain <fthain@linux-m68k.org>]

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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))