From 2b9518684f8558cd61a7c608cc03a27822cf7b03 Mon Sep 17 00:00:00 2001 From: Xi Ruoyao Date: Fri, 6 Jun 2025 17:24:44 +0800 Subject: [PATCH] RISC-V: vDSO: Correct inline assembly constraints in the getrandom syscall wrapper MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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. Instead use "=r" (only output) for the output parameter and "r" (only input) for the input parameter. While the example from the GCC documentation uses "0" for the input parameter, this is not necessary as confirmed by the GCC developers and "r" matches what the other architectures' vDSO implementations are using. [ alex: Update log to match v2 (Thomas) ] 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 Cc: Nathan Chancellor Signed-off-by: Xi Ruoyao Reviewed-by: Thomas Weißschuh Fixes: ee0d03053e70 ("RISC-V: vDSO: Wire up getrandom() vDSO") Link: https://lore.kernel.org/r/20250606092443.73650-2-xry111@xry111.site Signed-off-by: Alexandre Ghiti Signed-off-by: Palmer Dabbelt --- 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 8dc92441702a9..c6d66895c1f58 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"); -- 2.47.2