[PATCH RFC] riscv: Use 64-bit variable for output in __get_user_asm

[Nathan Chancellor]

After commit f6bff7827a48 ("riscv: uaccess: use 'asm_goto_output' for
get_user()"), which was the first commit that started using asm goto
with outputs on RISC-V, builds of clang built with assertions enabled
start crashing in certain files that use get_user() with:

  clang: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:12743: Register FollowCopyChain(MachineRegisterInfo &, Register): Assertion `MI->getOpcode() == TargetOpcode::COPY && "start of copy chain MUST be COPY"' failed.

Internally, LLVM generates an addiw instruction when the output of the
inline asm (which may be any scalar type) needs to be sign extended for
ABI reasons, such as a later function call, so that basic block does not
have to do it.

Use a temporary 64-bit variable as the output of the inline assembly in
__get_user_asm() and explicitly cast it to truncate it if necessary,
avoiding the addiw that triggers the assertion.

Link: https://github.com/ClangBuiltLinux/linux/issues/2092
Signed-off-by: Nathan Chancellor <nathan at kernel.org>
---
I did this unconditionally but I did not do much investigation into how
the code generation changes, as I never have good luck with figuring out
what is real vs. noise. If this is a worry, I can just duplicate this
under a CONFIG_CC_IS_CLANG block. Alternatively, we could mark
CONFIG_CC_HAS_ASM_GOTO_OUTPUT as broken with clang when targeting
RISC-V.
---
 arch/riscv/include/asm/uaccess.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index b88a6218b7f2..ff162db20521 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -98,12 +98,16 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigne
 
 #ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
 #define __get_user_asm(insn, x, ptr, label)			\
+do {								\
+	u64 __tmp;						\
 	asm_goto_output(					\
 		"1:\n"						\
 		"	" insn " %0, %1\n"			\
 		_ASM_EXTABLE_UACCESS_ERR(1b, %l2, %0)		\
-		: "=&r" (x)					\
-		: "m" (*(ptr)) : : label)
+		: "=&r" (__tmp)					\
+		: "m" (*(ptr)) : : label);			\
+	(x) = (__typeof__(x))__tmp;				\
+} while (0)
 #else /* !CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
 #define __get_user_asm(insn, x, ptr, label)			\
 do {								\

---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250811-riscv-wa-llvm-asm-goto-outputs-assertion-failure-6aefa26338cc

Best regards,
--  
Nathan Chancellor <nathan at kernel.org>