[PATCH] powerpc/uaccess: Don't use "m<>" constraint

[PATCH] powerpc/uaccess: Don't use "m<>" constraint

[Michael Ellerman]

The "m<>" constraint breaks compilation with GCC 4.6.x era compilers.

The use of the constraint allows the compiler to use update-form
instructions, however in practice current compilers never generate
those forms for any of the current uses of __put_user_asm_goto().

We anticipate that GCC 4.6 will be declared unsupported for building
the kernel in the not too distant future. So for now just switch to
the "m" constraint.

Fixes: 334710b1496a ("powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 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 62cc8d7640ec..164112007f54 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -210,7 +210,7 @@ do {								\
 		"1:	" op "%U1%X1 %0,%1	# put_user\n"	\
 		EX_TABLE(1b, %l2)				\
 		:						\
-		: "r" (x), "m<>" (*addr)				\
+		: "r" (x), "m" (*addr)				\
 		:						\
 		: label)
 
-- 
2.25.1

Re: [PATCH] powerpc/uaccess: Don't use "m<>" constraint

[Segher Boessenkool]

On Thu, May 07, 2020 at 10:33:24PM +1000, Michael Ellerman wrote:
> The "m<>" constraint breaks compilation with GCC 4.6.x era compilers.
> 
> The use of the constraint allows the compiler to use update-form
> instructions, however in practice current compilers never generate
> those forms for any of the current uses of __put_user_asm_goto().
> 
> We anticipate that GCC 4.6 will be declared unsupported for building
> the kernel in the not too distant future. So for now just switch to
> the "m" constraint.
> 
> Fixes: 334710b1496a ("powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Acked-by: Segher Boessenkool <segher@kernel.crashing.org>

Thanks!  So much trouble for what looked like such a simple change, all
those years ago :-(


Segher

Re: [PATCH] powerpc/uaccess: Don't use "m<>" constraint

[Michael Ellerman]

On Thu, 2020-05-07 at 12:33:24 UTC, Michael Ellerman wrote:
> The "m<>" constraint breaks compilation with GCC 4.6.x era compilers.
> 
> The use of the constraint allows the compiler to use update-form
> instructions, however in practice current compilers never generate
> those forms for any of the current uses of __put_user_asm_goto().
> 
> We anticipate that GCC 4.6 will be declared unsupported for building
> the kernel in the not too distant future. So for now just switch to
> the "m" constraint.
> 
> Fixes: 334710b1496a ("powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Applied to powerpc topic/uaccess-ppc.

https://git.kernel.org/powerpc/c/e2a8b49e79553bd8ec48f73cead84e6146c09408

cheers