[fuzzer] Don't hard-code page size in FuzzerUtil.h

Don't hard code the page in FuzzerUtil.h, this breaks on
e.g. LoongArch which defaults to a 16KiB page size.

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D140607

Author: Ami-zhang

compiler-rt/lib/fuzzer/FuzzerUtil.cpp

@@ -19,6 +19,7 @@
 #include <signal.h>
 #include <sstream>
 #include <stdio.h>
+#include <sys/auxv.h>
 #include <sys/types.h>
 #include <thread>
 
@@ -234,4 +235,9 @@ uint64_t SimpleFastHash(const void *Data, size_t Size, uint64_t Initial) {
   return Res;
 }
 
+size_t PageSize() {
+  static size_t PageSizeCached = getauxval(AT_PAGESZ);
+  return PageSizeCached;
+}
+
 }  // namespace fuzzer

compiler-rt/lib/fuzzer/FuzzerUtil.h

@@ -94,7 +94,8 @@ inline size_t Log(size_t X) {
   return static_cast<size_t>((sizeof(unsigned long long) * 8) - Clzll(X) - 1);
 }
 
-inline size_t PageSize() { return 4096; }
+size_t PageSize();
+
 inline uint8_t *RoundUpByPage(uint8_t *P) {
   uintptr_t X = reinterpret_cast<uintptr_t>(P);
   size_t Mask = PageSize() - 1;