[PATCH] powerpc: Don't use asm goto for put_user() with GCC 4.9

[Michael Ellerman]

Christophe Leroy <christophe.leroy at csgroup.eu> writes:
> Le 03/11/2020 à 14:29, Michael Ellerman a écrit :
>> Andreas reported that commit ee0a49a6870e ("powerpc/uaccess: Switch
>> __put_user_size_allowed() to __put_user_asm_goto()") broke
>> CLONE_CHILD_SETTID.
>> 
>> Further inspection showed that the put_user() in schedule_tail() was
>> missing entirely, the store not emitted by the compiler.
>> 
>> Notice there are no stores other than to the stack. There should be a
>> stw in there for the store to current->set_child_tid.
>> 
>> This is only seen with GCC 4.9 era compilers (tested with 4.9.3 and
>> 4.9.4), and only when CONFIG_PPC_KUAP is disabled.
>> 
>> When CONFIG_PPC_KUAP=y, the memory clobber that's part of the isync()
>> and mtspr() inlined via allow_user_access() seems to be enough to
>> avoid the bug.
>> 
>> For now though let's just not use asm goto with GCC 4.9, to avoid this
>> bug and any other issues we haven't noticed yet. Possibly in future we
>> can find a smaller workaround.
>
> Is that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670 ?
>
> Should we use asm_volatile_goto() defined in include/linux/compiler-gcc.h ?

Ugh. I knew of that work around, but thought we'd dropped it when we
moved up to GCC 4.9. I should have looked more closely.

I'll send a patch to switch to it.

cheers