commit 2bdf22e992b6f875c31812201cacb3469fe032eb
author Kalesh Singh <kaleshsingh@google.com> Fri Aug 11 15:15:55 2023 -0700
committer Kalesh Singh <kaleshsingh@google.com> Sat Aug 19 00:05:12 2023 +0000
tree f0a9312c8b430da944f97a2ab751ade3a1f25d41
parent 307bae7afe7fb6be06b4a15785aefe4cb51071ac

bionic: libc: WriteProtected use max_page_size()

In the 4k targets there is no fucntional difference since
max_page_size() == page_size() == 4096.

On a 16kb device max_page_size() == 65536 and page_size() == 16384.
However, aligning up does not incur any memory regressions
since the .bss section is still be backes in page-sizeed chunks.

See: go/16k-page-aligned-variables

Bug: 296275298
Test: mma
Change-Id: I41c3e410f3b84c24eeb969c9aeca4b33a8d6170a
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

libc/private/WriteProtected.h

diff --git a/libc/private/WriteProtected.h b/libc/private/WriteProtected.h
index 746f72a..2faaf77 100644
--- a/libc/private/WriteProtected.h
+++ b/libc/private/WriteProtected.h
@@ -25,15 +25,16 @@
 #include <async_safe/log.h>
 
 #include "platform/bionic/macros.h"
+#include "platform/bionic/page.h"
 
 template <typename T>
 union WriteProtectedContents {
   T value;
-  char padding[PAGE_SIZE];
+  char padding[max_page_size()];
 
   WriteProtectedContents() = default;
   BIONIC_DISALLOW_COPY_AND_ASSIGN(WriteProtectedContents);
-} __attribute__((aligned(PAGE_SIZE)));
+} __attribute__((aligned(max_page_size())));
 
 // Write protected wrapper class that aligns its contents to a page boundary,
 // and sets the memory protection to be non-writable, except when being modified
@@ -41,8 +42,8 @@
 template <typename T>
 class WriteProtected {
  public:
-  static_assert(sizeof(T) < PAGE_SIZE,
-                "WriteProtected only supports contents up to PAGE_SIZE");
+  static_assert(sizeof(T) < max_page_size(),
+                "WriteProtected only supports contents up to max_page_size()");
   static_assert(__is_pod(T), "WriteProtected only supports POD contents");
 
   WriteProtected() = default;
@@ -80,7 +81,7 @@
     // ourselves.
     addr = untag_address(addr);
 #endif
-    if (mprotect(reinterpret_cast<void*>(addr), PAGE_SIZE, prot) == -1) {
+    if (mprotect(reinterpret_cast<void*>(addr), max_page_size(), prot) == -1) {
       async_safe_fatal("WriteProtected mprotect %x failed: %s", prot, strerror(errno));
     }
   }