[PATCH v2] RISC-V: vDSO: Correct inline assembly constraints in the getrandom syscall wrapper

[Xi Ruoyao]

As recently pointed out by Thomas, if a register is forced for two
different register variables, among them one is used as "+" (both input
and output) and another is only used as input, Clang would treat the
conflicting input parameters as undefined behaviour and optimize away
the argument assignment.

Per an example in the GCC documentation, for this purpose we can use "="
(only output) for the output, and "0" for the input for that we must
reuse the same register as the output.  And GCC developers have
confirmed using a simple "r" (that we use for most vDSO implementations)
instead of "0" is also fine.

Link: https://lore.kernel.org/all/20250603-loongarch-vdso-syscall-v1-1-6d12d6dfbdd0@linutronix.de/
Link: https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/Local-Register-Variables.html
Link: https://gcc.gnu.org/pipermail/gcc-help/2025-June/144266.html
Cc: Thomas Weißschuh <thomas.weissschuh at linutronix.de>
Cc: Nathan Chancellor <nathan at kernel.org>
Signed-off-by: Xi Ruoyao <xry111 at xry111.site>
---

v1 -> v2: Keep using "r" for buffer to follow the existing convention
(that the GCC developers have confirmed fine).

 arch/riscv/include/asm/vdso/getrandom.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/vdso/getrandom.h b/arch/riscv/include/asm/vdso/getrandom.h
index 8dc92441702a..c6d66895c1f5 100644
--- a/arch/riscv/include/asm/vdso/getrandom.h
+++ b/arch/riscv/include/asm/vdso/getrandom.h
@@ -18,7 +18,7 @@ static __always_inline ssize_t getrandom_syscall(void *_buffer, size_t _len, uns
 	register unsigned int flags asm("a2") = _flags;
 
 	asm volatile ("ecall\n"
-		      : "+r" (ret)
+		      : "=r" (ret)
 		      : "r" (nr), "r" (buffer), "r" (len), "r" (flags)
 		      : "memory");
 

base-commit: dc5240f09bca7b5fc72ad8894d6b9321bce51139
-- 
2.49.0