diff --git a/arch/x86_64/syscall_arch.h b/arch/x86_64/syscall_arch.h
index 074e73c..2b082ab 100644
--- a/arch/x86_64/syscall_arch.h
+++ b/arch/x86_64/syscall_arch.h
@@ -75,10 +75,11 @@ static inline long __attribute__((always_inline))
 __syscall4(long n, long a, long b, long c, long d)
 {
     unsigned long ret;
+    register long r10 __asm__("r10") = d;
 
     __asm__ volatile("syscall"
                      : "=a"(ret)
-                     : "a"(n), "D"(a), "S"(b), "d"(c), "r"(d)
+                     : "a"(n), "D"(a), "S"(b), "d"(c), "r"(r10)
                      : "rcx", "r11", "memory");
     return (long)ret;
 }
@@ -87,10 +88,12 @@ static inline long __attribute__((always_inline))
 __syscall5(long n, long a, long b, long c, long d, long e)
 {
     unsigned long ret;
+    register long r10 __asm__("r10") = d;
+    register long r8 __asm__("r8") = e;
 
     __asm__ volatile("syscall"
                      : "=a"(ret)
-                     : "a"(n), "D"(a), "S"(b), "d"(c), "r"(d), "r"(e)
+                     : "a"(n), "D"(a), "S"(b), "d"(c), "r"(r10), "r"(r8)
                      : "rcx", "r11", "memory");
     return (long)ret;
 }
diff --git a/tests/syscall_test.c b/tests/syscall_test.c
index d4354e5..1928096 100644
--- a/tests/syscall_test.c
+++ b/tests/syscall_test.c
@@ -14,6 +14,9 @@
  *      then point exactly at fake + VLIBC_TCB_ERRNO_OFF, a failing syscall
  *      must land its errno in that slot, and the real thread pointer is
  *      restored afterwards.
+ *   4. register pinning: a 4-arg (SYS_rt_sigprocmask) and a 5-arg
+ *      (SYS_ppoll) syscall whose later arguments are load-bearing must see
+ *      them in r10/r8, where the x86_64 kernel ABI places args 4 and 5.
  *
  * All diagnostics go through raw SYS_write (no printf): between installing
  * and restoring the fake thread pointer the test must not call any libc
@@ -32,6 +35,11 @@
 #define TEST_O_RDONLY 0
 #define TEST_O_CLOEXEC 0x80000
 
+/* Constants the public <signal.h>/<poll.h> will own; test-local copies. */
+#define TEST_SIG_BLOCK 0
+#define TEST_SIG_SETMASK 2
+#define TEST_SIGUSR1 10
+
 /* arch_prctl codes (kernel UAPI). */
 #define TEST_ARCH_SET_FS 0x1002
 #define TEST_ARCH_GET_FS 0x1003
@@ -101,6 +109,54 @@ restore_real_tcb(unsigned long real_fs)
     return syscall_ret(__syscall2(SYS_arch_prctl, TEST_ARCH_SET_FS, (long)real_fs)) != 0;
 }
 
+struct test_timespec
+{
+    long tv_sec;
+    long tv_nsec;
+};
+
+/*
+ * 4-arg regression probe: block SIGUSR1, read the mask back through a NULL
+ * set, restore. The 4th argument (sigsetsize) must reach the kernel in r10;
+ * placed anywhere else it fails with -EINVAL.
+ */
+static int
+probe_rt_sigprocmask(void)
+{
+    unsigned long mask = 1UL << (TEST_SIGUSR1 - 1);
+    unsigned long oldmask = 0;
+    unsigned long readback = 0;
+
+    if (__syscall4(SYS_rt_sigprocmask, TEST_SIG_BLOCK, (long)&mask, (long)&oldmask, 8) != 0)
+    {
+        return 0;
+    }
+    if (__syscall4(SYS_rt_sigprocmask, TEST_SIG_BLOCK, 0, (long)&readback, 8) != 0)
+    {
+        return 0;
+    }
+    if (__syscall4(SYS_rt_sigprocmask, TEST_SIG_SETMASK, (long)&oldmask, 0, 8) != 0)
+    {
+        return 0;
+    }
+    return (readback & (1UL << (TEST_SIGUSR1 - 1))) != 0;
+}
+
+/*
+ * 5-arg regression probe: ppoll with no fds and a zero timeout must return
+ * 0. The 4th argument (sigmask) must reach r10 and the 5th (sigsetsize) r8;
+ * misplaced, the kernel reads a pointer value where the set size belongs and
+ * answers -EINVAL.
+ */
+static int
+probe_ppoll(void)
+{
+    struct test_timespec ts = {0, 0};
+    unsigned long mask = 0;
+
+    return __syscall5(SYS_ppoll, 0, 0, (long)&ts, (long)&mask, 8) == 0;
+}
+
 /*
  * Failure scenario (-f): SYS_openat on a nonexistent path must return -1
  * and leave errno == ENOENT in the TCB slot. Prints the observed result to
@@ -173,6 +229,10 @@ main(int argc, char **argv)
     check(*(int *)((char *)fake_tcb + VLIBC_TCB_ERRNO_OFF) == ENOENT,
           "fake TCB errno slot retains ENOENT after restore");
 
+    /* 4. Register pinning for 4- and 5-argument syscalls (r10/r8). */
+    check(probe_rt_sigprocmask(), "rt_sigprocmask 4-arg r10 pinning");
+    check(probe_ppoll(), "ppoll 5-arg r10/r8 pinning");
+
     /*
      * VLIBC_TCB_ERRNO_OFF addresses slot 1 of whichever TCB the FS thread
      * pointer selects; under the host libc that slot is its private TLS