[PATCH] 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.  I'm not sure if using a simple
"r" for the input is safe or not here, so just follow the documentation.

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
Cc: Thomas Weißschuh <thomas.weissschuh at linutronix.de>
Cc: Nathan Chancellor <nathan at kernel.org>
Signed-off-by: Xi Ruoyao <xry111 at xry111.site>
---
 arch/riscv/include/asm/vdso/getrandom.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/include/asm/vdso/getrandom.h b/arch/riscv/include/asm/vdso/getrandom.h
index 8dc92441702a..0d84a38e79da 100644
--- a/arch/riscv/include/asm/vdso/getrandom.h
+++ b/arch/riscv/include/asm/vdso/getrandom.h
@@ -18,8 +18,8 @@ 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" (nr), "r" (buffer), "r" (len), "r" (flags)
+		      : "=r" (ret)
+		      : "r" (nr), "0" (buffer), "r" (len), "r" (flags)
 		      : "memory");
 
 	return ret;

base-commit: dc5240f09bca7b5fc72ad8894d6b9321bce51139
-- 
2.49.0