[compiler-rt] Reland "[scudo] Apply filling when realloc shrinks and re-grows a block in-place" (PR #95838)

[Fabio D'Urso via llvm-commits]

https://github.com/fabio-d created https://github.com/llvm/llvm-project/pull/95838

Reland of #93212, which has been reverted in
commit bddd8eae17df6511aee789744ccdc158de817081.


>From e3c9ce6602142062103432d43175d1d7e40159ff Mon Sep 17 00:00:00 2001
From: Fabio D'Urso <fdurso at google.com>
Date: Mon, 17 Jun 2024 17:04:27 +0200
Subject: [PATCH] Reland "[scudo] Apply filling when realloc shrinks and
 re-grows a block in-place"

Reland of #93212, which has been reverted in
commit bddd8eae17df6511aee789744ccdc158de817081.
---
 compiler-rt/lib/scudo/standalone/combined.h   | 14 +++++++++++
 .../scudo/standalone/tests/combined_test.cpp  | 23 +++++++++++++++----
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/compiler-rt/lib/scudo/standalone/combined.h b/compiler-rt/lib/scudo/standalone/combined.h
index f9ed36581f8d3..73da686287747 100644
--- a/compiler-rt/lib/scudo/standalone/combined.h
+++ b/compiler-rt/lib/scudo/standalone/combined.h
@@ -565,6 +565,20 @@ class Allocator {
             storeSecondaryAllocationStackMaybe(Options, OldPtr, NewSize);
           }
         }
+
+        // If we have reduced the size, set the extra bytes to the fill value
+        // so that we are ready to grow it again in the future.
+        if (NewSize < OldSize) {
+          const FillContentsMode FillContents =
+              TSDRegistry.getDisableMemInit() ? NoFill
+                                              : Options.getFillContentsMode();
+          if (FillContents != NoFill) {
+            memset(reinterpret_cast<char *>(OldTaggedPtr) + NewSize,
+                   FillContents == ZeroFill ? 0 : PatternFillByte,
+                   OldSize - NewSize);
+          }
+        }
+
         return OldTaggedPtr;
       }
     }
diff --git a/compiler-rt/lib/scudo/standalone/tests/combined_test.cpp b/compiler-rt/lib/scudo/standalone/tests/combined_test.cpp
index 1a36155bcd423..655dc87cbac64 100644
--- a/compiler-rt/lib/scudo/standalone/tests/combined_test.cpp
+++ b/compiler-rt/lib/scudo/standalone/tests/combined_test.cpp
@@ -447,19 +447,32 @@ SCUDO_TYPED_TEST(ScudoCombinedDeathTest, ReallocateSame) {
   // returns the same chunk. This requires that all the sizes we iterate on use
   // the same block size, but that should be the case for MaxSize - 64 with our
   // default class size maps.
-  constexpr scudo::uptr ReallocSize =
+  constexpr scudo::uptr InitialSize =
       TypeParam::Primary::SizeClassMap::MaxSize - 64;
-  void *P = Allocator->allocate(ReallocSize, Origin);
   const char Marker = 'A';
-  memset(P, Marker, ReallocSize);
+  Allocator->setFillContents(scudo::PatternOrZeroFill);
+
+  void *P = Allocator->allocate(InitialSize, Origin);
+  scudo::uptr CurrentSize = InitialSize;
   for (scudo::sptr Delta = -32; Delta < 32; Delta += 8) {
+    memset(P, Marker, CurrentSize);
     const scudo::uptr NewSize =
-        static_cast<scudo::uptr>(static_cast<scudo::sptr>(ReallocSize) + Delta);
+        static_cast<scudo::uptr>(static_cast<scudo::sptr>(InitialSize) + Delta);
     void *NewP = Allocator->reallocate(P, NewSize);
     EXPECT_EQ(NewP, P);
-    for (scudo::uptr I = 0; I < ReallocSize - 32; I++)
+
+    // Verify that existing contents have been preserved.
+    for (scudo::uptr I = 0; I < scudo::Min(CurrentSize, NewSize); I++)
       EXPECT_EQ((reinterpret_cast<char *>(NewP))[I], Marker);
+
+    // Verify that new bytes are set according to FillContentsMode.
+    for (scudo::uptr I = CurrentSize; I < NewSize; I++) {
+      EXPECT_EQ((reinterpret_cast<unsigned char *>(NewP))[I],
+                scudo::PatternFillByte);
+    }
+
     checkMemoryTaggingMaybe(Allocator, NewP, NewSize, 0);
+    CurrentSize = NewSize;
   }
   Allocator->deallocate(P, Origin);
 }