From f67401bb318680e6b6844cc009bd7e73e684a826 Mon Sep 17 00:00:00 2001
From: Sanjay Patel <spatel@rotateright.com>
Date: Fri, 16 Aug 2019 23:10:34 +0000
Subject: [PATCH] [CodeGenPrepare] Fix use-after-free
MIME-Version: 1.0
Content-Type: text/plain; charset=utf8
Content-Transfer-Encoding: 8bit

If OptimizeExtractBits() encountered a shift instruction with no operands at all,
it would erase the instruction, but still return false.

This previously didn’t matter because its caller would always return after
processing the instruction, but https://reviews.llvm.org/D63233 changed the
function’s caller to fall through if it returned false, which would then cause
a use-after-free detectable by ASAN.

This change makes OptimizeExtractBits return true if it removes a shift
instruction with no users, terminating processing of the instruction.

Patch by: @brentdax (Brent Royal-Gordon)

Differential Revision: https://reviews.llvm.org/D66330

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369168 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/CodeGenPrepare.cpp                  |  3 ++-
 .../CodeGenPrepare/sink-shift-and-trunc.ll      | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/CodeGen/CodeGenPrepare.cpp b/lib/CodeGen/CodeGenPrepare.cpp
index d4d5fcd48cb..7da810c40c4 100644
--- a/lib/CodeGen/CodeGenPrepare.cpp
+++ b/lib/CodeGen/CodeGenPrepare.cpp
@@ -1682,10 +1682,11 @@ static bool OptimizeExtractBits(BinaryOperator *ShiftI, ConstantInt *CI,
     TheUse = InsertedShift;
   }
 
-  // If we removed all uses, nuke the shift.
+  // If we removed all uses, or there are none, nuke the shift.
   if (ShiftI->use_empty()) {
     salvageDebugInfo(*ShiftI);
     ShiftI->eraseFromParent();
+    MadeChange = true;
   }
 
   return MadeChange;
diff --git a/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll b/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll
index 07ea73b5a5b..929a4f43e4a 100644
--- a/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll
+++ b/test/Transforms/CodeGenPrepare/sink-shift-and-trunc.ll
@@ -58,6 +58,23 @@ return:                                           ; preds = %if.then17, %if.end1
   ret i32 %retval.0, !dbg !63
 }
 
+; CodeGenPrepare was erasing the unused lshr instruction, but then further
+; processing the instruction after it was freed. If this bug is still present,
+; this test will always crash in an LLVM built with ASAN enabled, and may
+; crash even if ASAN is not enabled.
+
+define i32 @shift_unused(i32 %a) {
+; CHECK-LABEL: @shift_unused(
+; CHECK-NEXT:  BB2:
+; CHECK-NEXT:    ret i32 [[A:%.*]]
+;
+  %as = lshr i32 %a, 3
+  br label %BB2
+
+BB2:
+  ret i32 %a
+}
+
 ; CHECK: [[shift1_loc]] = !DILocation(line: 1
 ; CHECK: [[trunc1_loc]] = !DILocation(line: 2
 ; CHECK: [[shift2_loc]] = !DILocation(line: 3
-- 
2.40.0