Add getauxval() compat for NetBSD

[krytarowski]

Summary

getauxval() is not available on NetBSD and there is no a direct equivalent.

Add a function that implements the same semantics with NetBSD internals.

Reorder the GetPageSize() functions to prefer the sysctl approach for NetBSD.
It no longer makes a difference which approach is better. Avoid changing
conditional code path.

[vitalybuka]

lib/sanitizer_common/sanitizer_linux.cpp

1073

now we can't get to SANITIZER_USE_GETAUXVAL for SANITIZER_NETBSD

[krytarowski]

lib/sanitizer_common/sanitizer_linux.cpp

1073

sysctl returns the same value and it does not make difference. I want to keep it closer to No-Functional-Change here.

SANITIZER_USE_GETAUXVAL is wanted in LSan to get AT_BASE.

lib/sanitizer_common/sanitizer_getauxval.h

 //===-- sanitizer_getauxval.h -----------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Common getauxval() guards and definitions.
 // getauxval() is not defined until glibc version 2.16, or until API level 21
 // for Android.
+// Implement the getauxval() compat function for NetBSD.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef SANITIZER_GETAUXVAL_H
 #define SANITIZER_GETAUXVAL_H
 
 #include "sanitizer_platform.h"
 

 # else
 // The weak getauxval definition allows to check for the function at runtime.
 // This is useful for Android, when compiled at a lower API level yet running
 // on a more recent platform that offers the function.
 extern "C" SANITIZER_WEAK_ATTRIBUTE
 unsigned long getauxval(unsigned long type);  // NOLINT
 # endif
 
-#endif // SANITIZER_LINUX || SANITIZER_FUCHSIA
+#elif SANITIZER_NETBSD
+
+#define SANITIZER_USE_GETAUXVAL 1
+
+#include <dlfcn.h>
+#include <elf.h>
+
+static inline decltype(AuxInfo::a_v) getauxval(decltype(AuxInfo::a_type) type) {
+  for (const AuxInfo *aux = (const AuxInfo *)_dlauxinfo();
+       aux->a_type != AT_NULL; ++aux) {
+    if (type == aux->a_type)
+      return aux->a_v;
+  }
+
+  return 0;
+}
+
+#endif
 
 #endif // SANITIZER_GETAUXVAL_H

lib/sanitizer_common/sanitizer_linux.cpp

 #endif
   return addr;
 }
 
 #if !SANITIZER_ANDROID
 uptr GetPageSize() {
 #if SANITIZER_LINUX && (defined(__x86_64__) || defined(__i386__))
   return EXEC_PAGESIZE;
-#elif SANITIZER_USE_GETAUXVAL
-  return getauxval(AT_PAGESZ);
 #elif SANITIZER_FREEBSD || SANITIZER_NETBSD
 // Use sysctl as sysconf can trigger interceptors internally.
   int pz = 0;
   uptr pzl = sizeof(pz);
   int mib[2] = {CTL_HW, HW_PAGESIZE};
   int rv = internal_sysctl(mib, 2, &pz, &pzl, nullptr, 0);
   CHECK_EQ(rv, 0);
   return (uptr)pz;
+#elif SANITIZER_USE_GETAUXVAL
+  return getauxval(AT_PAGESZ);
 #else
   return sysconf(_SC_PAGESIZE);  // EXEC_PAGESIZE may not be trustworthy.
 #endif
 }
 #endif // !SANITIZER_ANDROID
 
 #if !SANITIZER_OPENBSD
 uptr ReadBinaryName(/*out*/char *buf, uptr buf_len) {