LoongArch: vDSO: Correctly use asm parameters in syscall wrappers

The syscall wrappers use the "a0" register for two different register
variables, both the first argument and the return value. Here the "ret"
variable is used as both input and output while the argument register is
only used as input. Clang treats the conflicting input parameters as an
undefined behaviour and optimizes away the argument assignment.

The code seems to work by chance for the most part today but that may
change in the future. Specifically clock_gettime_fallback() fails with
clockids from 16 to 23, as implemented by the upcoming auxiliary clocks.

Switch the "ret" register variable to a pure output, similar to the
other architectures' vDSO code. This works in both clang and GCC.

Link: https://lore.kernel.org/lkml/20250602102825-42aa84f0-23f1-4d10-89fc-e8bbaffd291a@linutronix.de/
Link: https://lore.kernel.org/lkml/20250519082042.742926976@linutronix.de/
Fixes: c6b99bed6b8f ("LoongArch: Add VDSO and VSYSCALL support")
Fixes: 18efd0b10e0f ("LoongArch: vDSO: Wire up getrandom() vDSO implementation")
Cc: stable@vger.kernel.org
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Yanteng Si <si.yanteng@linux.dev>
Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

diff --git a/arch/loongarch/include/asm/vdso/getrandom.h b/arch/loongarch/include/asm/vdso/getrandom.h
index 48c43f55b039b42168698614d0479b7a872d20f3..a81724b69f291ee49dd1f46b12d6893fc18442b8 100644 (file)
--- a/arch/loongarch/include/asm/vdso/getrandom.h
+++ b/arch/loongarch/include/asm/vdso/getrandom.h
@@ -20,7 +20,7 @@ static __always_inline ssize_t getrandom_syscall(void *_buffer, size_t _len, uns
 
        asm volatile(
        "      syscall 0\n"
-       : "+r" (ret)
+       : "=r" (ret)
        : "r" (nr), "r" (buffer), "r" (len), "r" (flags)
        : "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7", "$t8",
          "memory");

diff --git a/arch/loongarch/include/asm/vdso/gettimeofday.h b/arch/loongarch/include/asm/vdso/gettimeofday.h
index 88cfcf13311630ed5f1a734d23a2bc3f65d79a88..f15503e3336ca1bdc9675ec6e17bbb77abc35ef4 100644 (file)
--- a/arch/loongarch/include/asm/vdso/gettimeofday.h
+++ b/arch/loongarch/include/asm/vdso/gettimeofday.h
@@ -25,7 +25,7 @@ static __always_inline long gettimeofday_fallback(
 
        asm volatile(
        "       syscall 0\n"
-       : "+r" (ret)
+       : "=r" (ret)
        : "r" (nr), "r" (tv), "r" (tz)
        : "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
          "$t8", "memory");
@@ -44,7 +44,7 @@ static __always_inline long clock_gettime_fallback(
 
        asm volatile(
        "       syscall 0\n"
-       : "+r" (ret)
+       : "=r" (ret)
        : "r" (nr), "r" (clkid), "r" (ts)
        : "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
          "$t8", "memory");
@@ -63,7 +63,7 @@ static __always_inline int clock_getres_fallback(
 
        asm volatile(
        "       syscall 0\n"
-       : "+r" (ret)
+       : "=r" (ret)
        : "r" (nr), "r" (clkid), "r" (ts)
        : "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7",
          "$t8", "memory");