[PATCH v5.10] powerpc/uaccess: Fix build errors seen with GCC 13/14

[PATCH v5.10] powerpc/uaccess: Fix build errors seen with GCC 13/14

[Michael Ellerman]

commit 2d43cc701b96f910f50915ac4c2a0cae5deb734c upstream.

Building ppc64le_defconfig with GCC 14 fails with assembler errors:

    CC      fs/readdir.o
  /tmp/ccdQn0mD.s: Assembler messages:
  /tmp/ccdQn0mD.s:212: Error: operand out of domain (18 is not a multiple of 4)
  /tmp/ccdQn0mD.s:226: Error: operand out of domain (18 is not a multiple of 4)
  ... [6 lines]
  /tmp/ccdQn0mD.s:1699: Error: operand out of domain (18 is not a multiple of 4)

A snippet of the asm shows:

  # ../fs/readdir.c:210:         unsafe_copy_dirent_name(dirent->d_name, name, namlen, efault_end);
         ld 9,0(29)       # MEM[(u64 *)name_38(D) + _88 * 1], MEM[(u64 *)name_38(D) + _88 * 1]
  # 210 "../fs/readdir.c" 1
         1:      std 9,18(8)     # put_user       # *__pus_addr_52, MEM[(u64 *)name_38(D) + _88 * 1]

The 'std' instruction requires a 4-byte aligned displacement because
it is a DS-form instruction, and as the assembler says, 18 is not a
multiple of 4.

A similar error is seen with GCC 13 and CONFIG_UBSAN_SIGNED_WRAP=y.

The fix is to change the constraint on the memory operand to put_user(),
from "m" which is a general memory reference to "YZ".

The "Z" constraint is documented in the GCC manual PowerPC machine
constraints, and specifies a "memory operand accessed with indexed or
indirect addressing". "Y" is not documented in the manual but specifies
a "memory operand for a DS-form instruction". Using both allows the
compiler to generate a DS-form "std" or X-form "stdx" as appropriate.

Unfortunately clang doesn't support the "Y" constraint so that has to be
behind an ifdef.

Although the build error is only seen with GCC 13/14, that appears
to just be luck. The constraint has been incorrect since it was first
added.

Fixes: c20beffeec3c ("powerpc/uaccess: Use flexible addressing with __put_user()/__get_user()")
Suggested-by: Kewen Lin <linkw@gcc.gnu.org>
[mpe: Drop CONFIG_PPC_KERNEL_PREFIXED ifdef for backport]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240529123029.146953-1-mpe@ellerman.id.au
---
 arch/powerpc/include/asm/uaccess.h | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 6b808bcdecd5..6df110c1254e 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -186,9 +186,20 @@ do {								\
 		:						\
 		: label)
 
+#ifdef CONFIG_CC_IS_CLANG
+#define DS_FORM_CONSTRAINT "Z<>"
+#else
+#define DS_FORM_CONSTRAINT "YZ<>"
+#endif
+
 #ifdef __powerpc64__
-#define __put_user_asm2_goto(x, ptr, label)			\
-	__put_user_asm_goto(x, ptr, label, "std")
+#define __put_user_asm2_goto(x, addr, label)			\
+	asm goto ("1: std%U1%X1 %0,%1	# put_user\n"		\
+		EX_TABLE(1b, %l2)				\
+		:						\
+		: "r" (x), DS_FORM_CONSTRAINT (*addr)		\
+		:						\
+		: label)
 #else /* __powerpc64__ */
 #define __put_user_asm2_goto(x, addr, label)			\
 	asm_volatile_goto(					\
-- 
2.45.1

Re: [PATCH v5.10] powerpc/uaccess: Fix build errors seen with GCC 13/14

[Greg KH]

On Fri, Jun 14, 2024 at 09:27:14PM +1000, Michael Ellerman wrote:
> commit 2d43cc701b96f910f50915ac4c2a0cae5deb734c upstream.
> 
> Building ppc64le_defconfig with GCC 14 fails with assembler errors:
> 

All backports now queued up,t hanks!

greg k-h

Re: [PATCH v5.10] powerpc/uaccess: Fix build errors seen with GCC 13/14

[Segher Boessenkool]

On Fri, Jun 14, 2024 at 09:27:14PM +1000, Michael Ellerman wrote:
> commit 2d43cc701b96f910f50915ac4c2a0cae5deb734c upstream.
> The 'std' instruction requires a 4-byte aligned displacement because
> it is a DS-form instruction, and as the assembler says, 18 is not a
> multiple of 4.

You learn something new every day :-)

> A similar error is seen with GCC 13 and CONFIG_UBSAN_SIGNED_WRAP=y.
> 
> The fix is to change the constraint on the memory operand to put_user(),
> from "m" which is a general memory reference to "YZ".
> 
> The "Z" constraint is documented in the GCC manual PowerPC machine
> constraints, and specifies a "memory operand accessed with indexed or
> indirect addressing". "Y" is not documented in the manual but specifies
> a "memory operand for a DS-form instruction". Using both allows the
> compiler to generate a DS-form "std" or X-form "stdx" as appropriate.

https://gcc.gnu.org/PR115289
It will be documented soon, thanks for the report!

> Although the build error is only seen with GCC 13/14, that appears
> to just be luck. The constraint has been incorrect since it was first
> added.

Yes, "m" allows any memory operand, an unaligned one is just fine.

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


Segher