builtins test: Move clear_cache_test.c from a mprotect()ed global to a mmap()ed variable

[thakis]

Summary

ld64 in the macOS 10.15 SDK gives __DATA a maxprot of 3, meaning it can't be made executable at runtime by default.

Change clear_cache_test.c to use mmap()ed data that's mapped as writable and executable from the beginning, instead of trying to mprotect()ing a __DATA variable as executable. This fixes the test on macOS with the 10.15 SDK.

PR43407.

[thakis]

delcypher: ping :)

[delcypher]

compiler-rt/test/builtins/Unit/clear_cache_test.c

46–58

start, and end aren't being used anymore. We should just remove them.

If we decide not to use page_size we should remove it too (and the code that computes `page_size).

48

I don't like repetition of the 128 constant. We could define a global const (e.g. kExecutionBufferSize) and use it here.
However, we aren't using page_size anymore here and arguably we could use it here instead of 128 (which seems like a really arbitrary size). However, mmap has to round up to the page size anyway so maybe dropping page_size and the code that computes it is the cleanest thing to do?

[thakis]

comments

[thakis]

Thanks, good comments. All done.

[delcypher]

I just realised. It's kind of weird that this test has code to support ARM but the test is marked as unsupported for arm. Anyway fixing that is out of scope for this patch so LGTM.

compiler-rt/test/builtins/Unit/clear_cache_test.c

 // REQUIRES: native-run
 // UNSUPPORTED: arm, aarch64
 // RUN: %clang_builtins %s %librt -o %t && %run %t
 //===-- clear_cache_test.c - Test clear_cache -----------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 
 #include <stdio.h>
 #include <string.h>
 #include <stdint.h>
+
 #if defined(_WIN32)
 #include <windows.h>
-static uintptr_t get_page_size() {
-    SYSTEM_INFO si;
-    GetSystemInfo(&si);
-    return si.dwPageSize;
-}
 #else
 #include <unistd.h>
 #include <sys/mman.h>
-
-static uintptr_t get_page_size() {
-    return sysconf(_SC_PAGE_SIZE);
-}
 #endif
 
 extern void __clear_cache(void* start, void* end);
 
-
 typedef int (*pfunc)(void);
 
 // Make these static to avoid ILT jumps for incremental linking on Windows.
 static int func1() { return 1; }
 static int func2() { return 2; }
 
 void *__attribute__((noinline))
 memcpy_f(void *dst, const void *src, size_t n) {
 // ARM and MIPS nartually align functions, but use the LSB for ISA selection
 // (THUMB, MIPS16/uMIPS respectively).  Ensure that the ISA bit is ignored in
 // the memcpy
 #if defined(__arm__) || defined(__mips__)
   return (void *)((uintptr_t)memcpy(dst, (void *)((uintptr_t)src & ~1), n) |
                   ((uintptr_t)src & 1));
 #else
   return memcpy(dst, (void *)((uintptr_t)src), n);
 #endif
 }
 
-unsigned char execution_buffer[128] __attribute__((aligned(8)));
-
 int main()
 {
-    // make executable the page containing execution_buffer 
-    uintptr_t page_size = get_page_size();
-    char* start = (char*)((uintptr_t)execution_buffer & (-page_size));
-    char* end = (char*)((uintptr_t)(&execution_buffer[128+page_size]) & (-page_size));
-#if defined(_WIN32)
-    DWORD dummy_oldProt;
-    MEMORY_BASIC_INFORMATION b;
-    if (!VirtualQuery(start, &b, sizeof(b)))
-        return 1;
-    if (!VirtualProtect(b.BaseAddress, b.RegionSize, PAGE_EXECUTE_READWRITE, &b.Protect))
+    const int kSize = 128;
+#if !defined(_WIN32)
+    uint8_t *execution_buffer = mmap(0, kSize,
+                                     PROT_READ | PROT_WRITE | PROT_EXEC,
+                                     MAP_ANON | MAP_PRIVATE, 0, 0);
+    if (execution_buffer == MAP_FAILED)
 #else
-    if (mprotect(start, end-start, PROT_READ|PROT_WRITE|PROT_EXEC) != 0)
-#endif
+    HANDLE mapping = CreateFileMapping(INVALID_HANDLE_VALUE, NULL,
+                                       PAGE_EXECUTE_READWRITE, 0, kSize, NULL);
+    if (mapping == NULL)
         return 1;
 
+    uint8_t* execution_buffer = MapViewOfFile(
+        mapping, FILE_MAP_ALL_ACCESS | FILE_MAP_EXECUTE, 0, 0, 0);
+    if (execution_buffer == NULL)
+        return 1;
+#endif
+
     // verify you can copy and execute a function
-    pfunc f1 = (pfunc)memcpy_f(execution_buffer, func1, 128);
-    __clear_cache(execution_buffer, &execution_buffer[128]);
+    pfunc f1 = (pfunc)memcpy_f(execution_buffer, func1, kSize);
+    __clear_cache(execution_buffer, execution_buffer + kSize);
     if ((*f1)() != 1)
         return 1;
 
     // verify you can overwrite a function with another
-    pfunc f2 = (pfunc)memcpy_f(execution_buffer, func2, 128);
-    __clear_cache(execution_buffer, &execution_buffer[128]);
+    pfunc f2 = (pfunc)memcpy_f(execution_buffer, func2, kSize);
+    __clear_cache(execution_buffer, execution_buffer + kSize);
     if ((*f2)() != 2)
         return 1;
 
     return 0;
 }