[PATCH] powerpc/uaccess: Fix __get_user() with CONFIG_CC_HAS_ASM_GOTO_OUTPUT

[PATCH] powerpc/uaccess: Fix __get_user() with CONFIG_CC_HAS_ASM_GOTO_OUTPUT

[Christophe Leroy]

Building kernel mainline with GCC 11 leads to following failure
when starting 'init':

  init[1]: bad frame in sys_sigreturn: 7ff5a900 nip 001083cc lr 001083c4
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

This is an issue due to a segfault happening in
__unsafe_restore_general_regs() in a loop copying registers from user
to kernel:

  10:	7d 09 03 a6 	mtctr   r8
  14:	80 ca 00 00 	lwz     r6,0(r10)
  18:	80 ea 00 04 	lwz     r7,4(r10)
  1c:	90 c9 00 08 	stw     r6,8(r9)
  20:	90 e9 00 0c 	stw     r7,12(r9)
  24:	39 0a 00 08 	addi    r8,r10,8
  28:	39 29 00 08 	addi    r9,r9,8
  2c:	81 4a 00 08 	lwz     r10,8(r10)  <== r10 is clobbered here
  30:	81 6a 00 0c 	lwz     r11,12(r10)
  34:	91 49 00 08 	stw     r10,8(r9)
  38:	91 69 00 0c 	stw     r11,12(r9)
  3c:	39 48 00 08 	addi    r10,r8,8
  40:	39 29 00 08 	addi    r9,r9,8
  44:	42 00 ff d0 	bdnz    14 <__unsafe_restore_general_regs+0x14>

As shown above, this is due to r10 being re-used by GCC. This didn't
happen with CLANG.

This is fixed by tagging 'x' output as an earlyclobber operand in
__get_user_asm2_goto().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/asm/uaccess.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index a09e4240c5b1..22c79ab40006 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -157,7 +157,7 @@ do {								\
 		"2:	lwz%X1 %L0, %L1\n"			\
 		EX_TABLE(1b, %l2)				\
 		EX_TABLE(2b, %l2)				\
-		: "=r" (x)					\
+		: "=&r" (x)					\
 		: "m" (*addr)					\
 		:						\
 		: label)
-- 
2.25.0

Re: [PATCH] powerpc/uaccess: Fix __get_user() with CONFIG_CC_HAS_ASM_GOTO_OUTPUT

[Michael Ellerman]

On Sat, 8 May 2021 09:25:32 +0000 (UTC), Christophe Leroy wrote:
> Building kernel mainline with GCC 11 leads to following failure
> when starting 'init':
> 
>   init[1]: bad frame in sys_sigreturn: 7ff5a900 nip 001083cc lr 001083c4
>   Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> 
> This is an issue due to a segfault happening in
> __unsafe_restore_general_regs() in a loop copying registers from user
> to kernel:
> 
> [...]

Applied to powerpc/fixes.

[1/1] powerpc/uaccess: Fix __get_user() with CONFIG_CC_HAS_ASM_GOTO_OUTPUT
      https://git.kernel.org/powerpc/c/7315e457d6bc342d06ba0b7ee498221c5237a547

cheers