Corrected D27428: Do not use the alignment-rounded-up size with secondary

Summary:
I atually had an integer overflow on 32-bit with D27428 that didn't reproduce
locally, as the test servers would manage allocate addresses in the 0xffffxxxx
range, which led to some issues when rounding addresses.

At this point, I feel that Scudo could benefit from having its own combined
allocator, as we don't get any benefit from the current one, but have to work
around some hurdles (alignment checks, rounding up that is no longer needed,
extraneous code).

Reviewers: kcc, alekseyshl

Subscribers: llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D27681

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@289572 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 36 insertions, 19 deletions

diff --git a/lib/scudo/scudo_allocator.cpp b/lib/scudo/scudo_allocator.cpp
index 57a2be494..4f3b05ffb 100644
--- a/lib/scudo/scudo_allocator.cpp
+++ b/lib/scudo/scudo_allocator.cpp
@@ -402,12 +402,18 @@ struct Allocator {
       Size = 1;
     if (Size >= MaxAllowedMallocSize)
       return BackendAllocator.ReturnNullOrDieOnBadRequest();
-    uptr RoundedSize = RoundUpTo(Size, MinAlignment);
-    uptr NeededSize = RoundedSize + AlignedChunkHeaderSize;
+
+    uptr NeededSize = RoundUpTo(Size, MinAlignment) + AlignedChunkHeaderSize;
     if (Alignment > MinAlignment)
       NeededSize += Alignment;
     if (NeededSize >= MaxAllowedMallocSize)
       return BackendAllocator.ReturnNullOrDieOnBadRequest();
+
+    // Primary backed and Secondary backed allocations have a different
+    // treatment. We deal with alignment requirements of Primary serviced
+    // allocations here, but the Secondary will take care of its own alignment
+    // needs, which means we also have to work around some limitations of the
+    // combined allocator to accommodate the situation.
     bool FromPrimary = PrimaryAllocator::CanAllocate(NeededSize, MinAlignment);
 
     void *Ptr;
@@ -426,8 +432,11 @@ struct Allocator {
     // If the allocation was serviced by the secondary, the returned pointer
     // accounts for ChunkHeaderSize to pass the alignment check of the combined
     // allocator. Adjust it here.
-    if (!FromPrimary)
+    if (!FromPrimary) {
       AllocBeg -= AlignedChunkHeaderSize;
+      if (Alignment > MinAlignment)
+        NeededSize -= Alignment;
+    }
 
     uptr ActuallyAllocatedSize = BackendAllocator.GetActuallyAllocatedSize(
         reinterpret_cast<void *>(AllocBeg));
diff --git a/lib/scudo/scudo_allocator_secondary.h b/lib/scudo/scudo_allocator_secondary.h
index d3468f840..b984f0db4 100644
--- a/lib/scudo/scudo_allocator_secondary.h
+++ b/lib/scudo/scudo_allocator_secondary.h
@@ -32,32 +32,39 @@ class ScudoLargeMmapAllocator {
   void *Allocate(AllocatorStats *Stats, uptr Size, uptr Alignment) {
     // The Scudo frontend prevents us from allocating more than
     // MaxAllowedMallocSize, so integer overflow checks would be superfluous.
-    uptr HeadersSize = sizeof(SecondaryHeader) + AlignedChunkHeaderSize;
-    uptr MapSize = RoundUpTo(Size + sizeof(SecondaryHeader), PageSize);
+    uptr MapSize = Size + SecondaryHeaderSize;
+    MapSize = RoundUpTo(MapSize, PageSize);
     // Account for 2 guard pages, one before and one after the chunk.
     MapSize += 2 * PageSize;
-    // Adding an extra Alignment is not required, it was done by the frontend.
+    // The size passed to the Secondary comprises the alignment, if large
+    // enough. Subtract it here to get the requested size, including header.
+    if (Alignment > MinAlignment)
+      Size -= Alignment;
+
     uptr MapBeg = reinterpret_cast<uptr>(MmapNoAccess(MapSize));
     if (MapBeg == ~static_cast<uptr>(0))
       return ReturnNullOrDieOnOOM();
     // A page-aligned pointer is assumed after that, so check it now.
     CHECK(IsAligned(MapBeg, PageSize));
     uptr MapEnd = MapBeg + MapSize;
+    // The beginning of the user area for that allocation comes after the
+    // initial guard page, and both headers. This is the pointer that has to
+    // abide by alignment requirements.
     uptr UserBeg = MapBeg + PageSize + HeadersSize;
-    // In the event of larger alignments, we will attempt to fit the mmap area
-    // better and unmap extraneous memory. This will also ensure that the
+
+    // In the rare event of larger alignments, we will attempt to fit the mmap
+    // area better and unmap extraneous memory. This will also ensure that the
     // offset and unused bytes field of the header stay small.
     if (Alignment > MinAlignment) {
       if (UserBeg & (Alignment - 1))
         UserBeg += Alignment - (UserBeg & (Alignment - 1));
       CHECK_GE(UserBeg, MapBeg);
-      uptr NewMapBeg = UserBeg - HeadersSize;
-      NewMapBeg = RoundDownTo(NewMapBeg, PageSize) - PageSize;
+      uptr NewMapBeg = RoundDownTo(UserBeg - HeadersSize, PageSize) - PageSize;
       CHECK_GE(NewMapBeg, MapBeg);
-      uptr NewMapSize = RoundUpTo(MapSize - Alignment, PageSize);
-      uptr NewMapEnd = NewMapBeg + NewMapSize;
+      uptr NewMapEnd = RoundUpTo(UserBeg + (Size - AlignedChunkHeaderSize),
+                                 PageSize) + PageSize;
       CHECK_LE(NewMapEnd, MapEnd);
-      // Unmap the extra memory if it's large enough.
+      // Unmap the extra memory if it's large enough, on both sides.
       uptr Diff = NewMapBeg - MapBeg;
       if (Diff > PageSize)
         UnmapOrDie(reinterpret_cast<void *>(MapBeg), Diff);
@@ -65,14 +72,13 @@ class ScudoLargeMmapAllocator {
       if (Diff > PageSize)
         UnmapOrDie(reinterpret_cast<void *>(NewMapEnd), Diff);
       MapBeg = NewMapBeg;
-      MapSize = NewMapSize;
       MapEnd = NewMapEnd;
+      MapSize = NewMapEnd - NewMapBeg;
     }
-    uptr UserEnd = UserBeg - AlignedChunkHeaderSize + Size;
-    // For larger alignments, Alignment was added by the frontend to Size.
-    if (Alignment > MinAlignment)
-      UserEnd -= Alignment;
+
+    uptr UserEnd = UserBeg + (Size - AlignedChunkHeaderSize);
     CHECK_LE(UserEnd, MapEnd - PageSize);
+    // Actually mmap the memory, preserving the guard pages on either side.
     CHECK_EQ(MapBeg + PageSize, reinterpret_cast<uptr>(
         MmapFixedOrDie(MapBeg + PageSize, MapSize - 2 * PageSize)));
     uptr Ptr = UserBeg - AlignedChunkHeaderSize;
@@ -84,7 +90,7 @@ class ScudoLargeMmapAllocator {
     // the guard pages.
     Stats->Add(AllocatorStatAllocated, MapSize - 2 * PageSize);
     Stats->Add(AllocatorStatMapped, MapSize - 2 * PageSize);
-    CHECK(IsAligned(UserBeg, Alignment));
+
     return reinterpret_cast<void *>(UserBeg);
   }
 
@@ -173,6 +179,8 @@ class ScudoLargeMmapAllocator {
     return getHeader(reinterpret_cast<uptr>(Ptr));
   }
 
+  const uptr SecondaryHeaderSize = sizeof(SecondaryHeader);
+  const uptr HeadersSize = SecondaryHeaderSize + AlignedChunkHeaderSize;
   uptr PageSize;
   atomic_uint8_t MayReturnNull;
 };