The syscall wrappers use the "a0" register for two different register variables, both the first argument and the return value. 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 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-e8bbaffd... 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 Signed-off-by: Thomas Weißschuh thomas.weissschuh@linutronix.de --- arch/loongarch/include/asm/vdso/getrandom.h | 2 +- arch/loongarch/include/asm/vdso/gettimeofday.h | 6 +++--- 2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/arch/loongarch/include/asm/vdso/getrandom.h b/arch/loongarch/include/asm/vdso/getrandom.h index 48c43f55b039b42168698614d0479b7a872d20f3..a81724b69f291ee49dd1f46b12d6893fc18442b8 100644 --- 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", "$t8", "memory"); diff --git a/arch/loongarch/include/asm/vdso/gettimeofday.h b/arch/loongarch/include/asm/vdso/gettimeofday.h index 88cfcf13311630ed5f1a734d23a2bc3f65d79a88..f15503e3336ca1bdc9675ec6e17bbb77abc35ef4 100644 --- a/arch/loongarch/include/asm/vdso/gettimeofday.h +++ b/arch/loongarch/include/asm/vdso/gettimeofday.h @@ -25,7 +25,7 @@ static __always_inline long gettimeofday_fallback(
asm volatile( " syscall 0\n" - : "+r" (ret) + : "=r" (ret) : "r" (nr), "r" (tv), "r" (tz) : "$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");
--- base-commit: 546b1c9e93c2bb8cf5ed24e0be1c86bb089b3253 change-id: 20250603-loongarch-vdso-syscall-f585a99bea03
Best regards,