mac: System libraries are no longer found on disk on macOS 11

On macOS 11, system libraries are no longer present on disk as
individual Mach-O files, but are shipped exclusively in the dyld shared
cache. This affects tests that verify the list of modules loaded into a
process. The new _dyld_shared_cache_contains_path function can be used
to determine whether a path exists in the dyld shared cache.

Bug: crashpad:347
Test: crashpad_snapshot_test ProcessReaderMac.{Self,Child}Modules
Change-Id: I19402e45701caeb850bdafd701ccbba4ef9c60bc
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2384322
Commit-Queue: Mark Mentovai <mark@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>

snapshot/mac/process_reader_mac_test.cc

@@ -16,17 +16,20 @@
 
 #include <Availability.h>
 #include <OpenCL/opencl.h>
+#include <dlfcn.h>
 #include <errno.h>
 #include <mach-o/dyld.h>
 #include <mach-o/dyld_images.h>
 #include <mach/mach.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/types.h>
 
 #include <map>
 #include <utility>
 
 #include "base/check_op.h"
+#include "base/logging.h"
 #include "base/mac/mach_logging.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/stl_util.h"
@@ -530,6 +533,75 @@ TEST(ProcessReaderMac, ChildSeveralThreads) {
   process_reader_threaded_child.Run();
 }
 
+template <typename T>
+T GetDyldFunction(const char* symbol) {
+  static void* dl_handle = []() -> void* {
+    Dl_info dl_info;
+    if (!dladdr(reinterpret_cast<void*>(dlopen), &dl_info)) {
+      LOG(ERROR) << "dladdr: failed";
+      return nullptr;
+    }
+
+    void* dl_handle =
+        dlopen(dl_info.dli_fname, RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
+    DCHECK(dl_handle) << "dlopen: " << dlerror();
+
+    return dl_handle;
+  }();
+
+  if (!dl_handle) {
+    return nullptr;
+  }
+
+  return reinterpret_cast<T>(dlsym(dl_handle, symbol));
+}
+
+void VerifyImageExistenceAndTimestamp(const char* path, time_t timestamp) {
+  const char* stat_path;
+  bool timestamp_may_be_0;
+
+#if __MAC_OS_X_VERSION_MAX_ALLOWED < __MAC_10_16
+  static auto _dyld_shared_cache_contains_path =
+      GetDyldFunction<bool (*)(const char*)>(
+          "_dyld_shared_cache_contains_path");
+#endif
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunguarded-availability"
+  if (_dyld_shared_cache_contains_path &&
+      _dyld_shared_cache_contains_path(path)) {
+#pragma clang diagnostic pop
+    // The timestamp will either match the timestamp of the dyld_shared_cache
+    // file in use, or be 0.
+    static const char* dyld_shared_cache_file_path = []() -> const char* {
+      auto dyld_shared_cache_file_path_f =
+          GetDyldFunction<const char* (*)()>("dyld_shared_cache_file_path");
+
+      // dyld_shared_cache_file_path should always be present if
+      // _dyld_shared_cache_contains_path is.
+      DCHECK(dyld_shared_cache_file_path_f);
+
+      const char* dyld_shared_cache_file_path = dyld_shared_cache_file_path_f();
+      DCHECK(dyld_shared_cache_file_path);
+
+      return dyld_shared_cache_file_path;
+    }();
+
+    stat_path = dyld_shared_cache_file_path;
+    timestamp_may_be_0 = true;
+  } else {
+    stat_path = path;
+    timestamp_may_be_0 = false;
+  }
+
+  struct stat stat_buf;
+  int rv = stat(stat_path, &stat_buf);
+  EXPECT_EQ(rv, 0) << ErrnoMessage("stat");
+  if (rv == 0 && (!timestamp_may_be_0 || timestamp != 0)) {
+    EXPECT_EQ(timestamp, stat_buf.st_mtime);
+  }
+}
+
 // cl_kernels images (OpenCL kernels) are weird. They’re not ld output and don’t
 // exist as files on disk. On OS X 10.10 and 10.11, their Mach-O structure isn’t
 // perfect. They show up loaded into many executables, so these quirks should be
@@ -695,12 +767,8 @@ TEST(ProcessReaderMac, SelfModules) {
       found_cl_kernels = true;
     } else {
       // Hope that the module didn’t change on disk.
-      struct stat stat_buf;
-      int rv = stat(dyld_image_name, &stat_buf);
-      EXPECT_EQ(rv, 0) << ErrnoMessage("stat");
-      if (rv == 0) {
-        EXPECT_EQ(modules[index].timestamp, stat_buf.st_mtime);
-      }
+      VerifyImageExistenceAndTimestamp(dyld_image_name,
+                                       modules[index].timestamp);
     }
   }
 
@@ -785,12 +853,8 @@ class ProcessReaderModulesChild final : public MachMultiprocess {
         found_cl_kernels = true;
       } else {
         // Hope that the module didn’t change on disk.
-        struct stat stat_buf;
-        int rv = stat(expect_name.c_str(), &stat_buf);
-        EXPECT_EQ(rv, 0) << ErrnoMessage("stat");
-        if (rv == 0) {
-          EXPECT_EQ(modules[index].timestamp, stat_buf.st_mtime);
-        }
+        VerifyImageExistenceAndTimestamp(expect_name.c_str(),
+                                         modules[index].timestamp);
       }
     }