Bug 1479960 - Give SharedStringMap a magic number so that all zeroes isn't a valid instance. r=kmag
authorJed Davis <jld@mozilla.com>
Wed, 14 Aug 2019 22:48:38 +0000
changeset 488049 1fa598bf26998154cfc2c189a2a5f7fb862c7640
parent 488048 0f466f2a18c0fcf9e552ea07158e847f88ca9950
child 488050 c3ca7014893cd993088b4e6ad5f333ef7eed10a9
push id36434
push usercbrindusan@mozilla.com
push dateThu, 15 Aug 2019 09:44:30 +0000
treeherdermozilla-central@144fbfb409b7 [default view] [failures only]
perfherder[talos] [build metrics] [platform microbench] (compared to previous push)
bugs1479960, 1525803
first release with
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last release without
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nightly linux64
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Bug 1479960 - Give SharedStringMap a magic number so that all zeroes isn't a valid instance. r=kmag There was a strange bug when converting SharedStringMap to use shared memory: on Android, some tests would fail because a pref wasn't set or there was something wrong with an expected error message. The root cause was that mapping ashmem with MAP_PRIVATE results in all zeroes (see bug 1525803), but that was read as a valid SharedStringMap with length 0. To prevent any possible future issues like that, this patch adds a nonzero magic number to the header. It fits into padding on 64-bit and the cost of setting and checking it should be essentially free. Depends on D26744 Differential Revision: https://phabricator.services.mozilla.com/D26745
--- a/dom/ipc/SharedStringMap.cpp
+++ b/dom/ipc/SharedStringMap.cpp
@@ -16,35 +16,39 @@ using namespace mozilla::loader;
 namespace mozilla {
 using namespace ipc;
 namespace dom {
 namespace ipc {
+static constexpr uint32_t kSharedStringMapMagic = 0x9e3779b9;
 static inline size_t GetAlignmentOffset(size_t aOffset, size_t aAlign) {
   auto mod = aOffset % aAlign;
   return mod ? aAlign - mod : 0;
 SharedStringMap::SharedStringMap(const FileDescriptor& aMapFile,
                                  size_t aMapSize) {
   auto result = mMap.initWithHandle(aMapFile, aMapSize);
+  MOZ_RELEASE_ASSERT(GetHeader().mMagic == kSharedStringMapMagic);
   // We return literal nsStrings and nsCStrings pointing to the mapped data,
   // which means that we may still have references to the mapped data even
   // after this instance is destroyed. That means that we need to keep the
   // mapping alive until process shutdown, in order to be safe.
 SharedStringMap::SharedStringMap(SharedStringMapBuilder&& aBuilder) {
   auto result = aBuilder.Finalize(mMap);
+  MOZ_RELEASE_ASSERT(GetHeader().mMagic == kSharedStringMapMagic);
 mozilla::ipc::FileDescriptor SharedStringMap::CloneFileDescriptor() const {
   return mMap.cloneHandle();
 bool SharedStringMap::Has(const nsCString& aKey) {
@@ -87,17 +91,17 @@ Result<Ok, nsresult> SharedStringMapBuil
   MOZ_ASSERT(mEntries.Count() == mKeyTable.Count());
   nsTArray<nsCString> keys(mEntries.Count());
   for (auto iter = mEntries.Iter(); !iter.Done(); iter.Next()) {
-  Header header = {uint32_t(keys.Length())};
+  Header header = {kSharedStringMapMagic, uint32_t(keys.Length())};
   size_t offset = sizeof(header);
   offset += GetAlignmentOffset(offset, alignof(Header));
   offset += keys.Length() * sizeof(SharedStringMap::Entry);
   header.mKeyStringsOffset = offset;
   header.mKeyStringsSize = mKeyTable.Size();
--- a/dom/ipc/SharedStringMap.h
+++ b/dom/ipc/SharedStringMap.h
@@ -54,16 +54,17 @@ class SharedStringMap {
    * - Optional alignment padding for char16_t[]
    * - StringTable<nsString>:
    *   A region of flat, null-terminated UTF-16 strings. Entry value strings are
    *   encoded as character (*not* byte) offsets into this region.
   struct Header {
+    uint32_t mMagic;
     // The number of entries in this map.
     uint32_t mEntryCount;
     // The raw byte offset of the beginning of the key string table, from the
     // start of the shared memory region, and its size in bytes.
     size_t mKeyStringsOffset;
     size_t mKeyStringsSize;