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

[Christophe Leroy]

Quoting Christophe Leroy <christophe.leroy at csgroup.eu>:

> 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 ?

It seems to be OK with asm_volatile_goto() with GCC 4.9, and it is  
already what is used in our asm/jump_label.h


diff --git a/arch/powerpc/include/asm/uaccess.h  
b/arch/powerpc/include/asm/uaccess.h
index ef5bbb705c08..501c9a79038c 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -178,7 +178,7 @@ do {								\
   * are no aliasing issues.
   */
  #define __put_user_asm_goto(x, addr, label, op)			\
-	asm volatile goto(					\
+	asm_volatile_goto(					\
  		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
  		EX_TABLE(1b, %l2)				\
  		:						\
@@ -191,7 +191,7 @@ do {								\
  	__put_user_asm_goto(x, ptr, label, "std")
  #else /* __powerpc64__ */
  #define __put_user_asm2_goto(x, addr, label)			\
-	asm volatile goto(					\
+	asm_volatile_goto(					\
  		"1:	stw%X1 %0, %1\n"			\
  		"2:	stw%X1 %L0, %L1\n"			\
  		EX_TABLE(1b, %l2)				\
---

Christophe