[compiler-rt] [scudo] Split the code paths which enable memory tagging (PR #83493)

[Christopher Ferris via llvm-commits]

================
@@ -1148,10 +1035,176 @@ class Allocator {
            reinterpret_cast<uptr>(Ptr) - SizeOrUnusedBytes;
   }
 
+  ALWAYS_INLINE void *initChunk(const uptr ClassId, const Chunk::Origin Origin,
+                                void *Block, const uptr UserPtr,
+                                const uptr SizeOrUnusedBytes,
+                                const FillContentsMode FillContents) {
+    Block = addHeaderTag(Block);
+    // Only do content fill when it's from primary allocator because secondary
+    // allocator has filled the content.
+    if (ClassId != 0 && UNLIKELY(FillContents != NoFill)) {
+      // This condition is not necessarily unlikely, but since memset is
+      // costly, we might as well mark it as such.
+      memset(Block, FillContents == ZeroFill ? 0 : PatternFillByte,
+             PrimaryT::getSizeByClassId(ClassId));
+    }
+
+    Chunk::UnpackedHeader Header = {};
+
+    const uptr DefaultAlignedPtr =
+        reinterpret_cast<uptr>(Block) + Chunk::getHeaderSize();
+    if (UNLIKELY(DefaultAlignedPtr != UserPtr)) {
+      const uptr Offset = UserPtr - DefaultAlignedPtr;
+      DCHECK_GE(Offset, 2 * sizeof(u32));
+      // The BlockMarker has no security purpose, but is specifically meant for
+      // the chunk iteration function that can be used in debugging situations.
+      // It is the only situation where we have to locate the start of a chunk
+      // based on its block address.
+      reinterpret_cast<u32 *>(Block)[0] = BlockMarker;
+      reinterpret_cast<u32 *>(Block)[1] = static_cast<u32>(Offset);
+      Header.Offset = (Offset >> MinAlignmentLog) & Chunk::OffsetMask;
+    }
+
+    Header.ClassId = ClassId & Chunk::ClassIdMask;
+    Header.State = Chunk::State::Allocated;
+    Header.OriginOrWasZeroed = Origin & Chunk::OriginMask;
+    Header.SizeOrUnusedBytes = SizeOrUnusedBytes & Chunk::SizeOrUnusedBytesMask;
+    Chunk::storeHeader(Cookie, reinterpret_cast<void *>(addHeaderTag(UserPtr)),
+                       &Header);
+
+    return reinterpret_cast<void *>(UserPtr);
+  }
+
+  NOINLINE void *
+  initChunkWithMemoryTagging(const uptr ClassId, const Chunk::Origin Origin,
+                             void *Block, const uptr UserPtr, const uptr Size,
+                             const uptr SizeOrUnusedBytes,
+                             const FillContentsMode FillContents) {
+    const Options Options = Primary.Options.load();
+    DCHECK(useMemoryTagging<Config>(Options));
+
+    void *Ptr = reinterpret_cast<void *>(UserPtr);
+    void *TaggedPtr = Ptr;
+
+    if (LIKELY(ClassId)) {
+      // We only need to zero or tag the contents for Primary backed
+      // allocations. We only set tags for primary allocations in order to avoid
+      // faulting potentially large numbers of pages for large secondary
+      // allocations. We assume that guard pages are enough to protect these
+      // allocations.
+      //
+      // FIXME: When the kernel provides a way to set the background tag of a
+      // mapping, we should be able to tag secondary allocations as well.
+      //
+      // When memory tagging is enabled, zeroing the contents is done as part of
+      // setting the tag.
+
+      uptr PrevUserPtr;
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cferris1000 wrote:

Super small nit, but it would probably be best if this was down with the TaggedUserPtr declaration.

https://github.com/llvm/llvm-project/pull/83493