diff --git a/compiler-rt/lib/safestack/safestack_platform.h b/compiler-rt/lib/safestack/safestack_platform.h
index 77eeb9cda6e15..68d26813bef4a 100644
--- a/compiler-rt/lib/safestack/safestack_platform.h
+++ b/compiler-rt/lib/safestack/safestack_platform.h
@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 //
 // This file implements platform specific parts of SafeStack runtime.
+// Don't use equivalent functionality from sanitizer_common to avoid dragging
+// a large codebase into security sensitive code.
 //
 //===----------------------------------------------------------------------===//
 
@@ -45,6 +47,19 @@ extern "C" void *__mmap(void *, size_t, int, int, int, int, off_t);
 #  include <thread.h>
 #endif
 
+// Keep in sync with sanitizer_linux.cpp.
+//
+// Are we using 32-bit or 64-bit Linux syscalls?
+// x32 (which defines __x86_64__) has SANITIZER_WORDSIZE == 32
+// but it still needs to use 64-bit syscalls.
+#if SANITIZER_LINUX &&                                \
+    (defined(__x86_64__) || defined(__powerpc64__) || \
+     SANITIZER_WORDSIZE == 64 || (defined(__mips__) && _MIPS_SIM == _ABIN32))
+#  define SANITIZER_LINUX_USES_64BIT_SYSCALLS 1
+#else
+#  define SANITIZER_LINUX_USES_64BIT_SYSCALLS 0
+#endif
+
 namespace safestack {
 
 #if SANITIZER_NETBSD
@@ -117,7 +132,8 @@ inline int TgKill(pid_t pid, ThreadId tid, int sig) {
 #elif SANITIZER_FREEBSD
   return syscall(SYS_thr_kill2, pid, tid, sig);
 #else
-  return syscall(SYS_tgkill, pid, tid, sig);
+  // tid is pid_t (int), not ThreadId (uint64_t).
+  return syscall(SYS_tgkill, pid, (pid_t)tid, sig);
 #endif
 }
 
@@ -129,8 +145,13 @@ inline void *Mmap(void *addr, size_t length, int prot, int flags, int fd,
   return (void *)__syscall(SYS_mmap, addr, length, prot, flags, fd, offset);
 #elif SANITIZER_SOLARIS
   return _REAL64(mmap)(addr, length, prot, flags, fd, offset);
-#else
+#elif SANITIZER_LINUX_USES_64BIT_SYSCALLS
   return (void *)syscall(SYS_mmap, addr, length, prot, flags, fd, offset);
+#else
+  // mmap2 specifies file offset in 4096-byte units.
+  SFS_CHECK(IsAligned(offset, 4096));
+  return (void *)syscall(SYS_mmap2, addr, length, prot, flags, fd,
+                         offset / 4096);
 #endif
 }
 
diff --git a/compiler-rt/lib/safestack/safestack_util.h b/compiler-rt/lib/safestack/safestack_util.h
index da3f11b54cabc..ca724312c3eec 100644
--- a/compiler-rt/lib/safestack/safestack_util.h
+++ b/compiler-rt/lib/safestack/safestack_util.h
@@ -33,6 +33,10 @@ inline size_t RoundUpTo(size_t size, size_t boundary) {
   return (size + boundary - 1) & ~(boundary - 1);
 }
 
+inline constexpr bool IsAligned(size_t a, size_t alignment) {
+  return (a & (alignment - 1)) == 0;
+}
+
 class MutexLock {
  public:
   explicit MutexLock(pthread_mutex_t &mutex) : mutex_(&mutex) {