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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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 ?

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 ?

Thanks
Christophe