[llvm] [NFCI][msan] Show shadow incorrectly computed for partially undefined vectors (PR #143823)
Thurston Dang via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 11 19:55:22 PDT 2025
https://github.com/thurstond updated https://github.com/llvm/llvm-project/pull/143823
>From 7245c87ca801003839db877f4e7cec67ea42097f Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston at google.com>
Date: Thu, 12 Jun 2025 02:12:29 +0000
Subject: [PATCH 1/3] [NFCI][msan] Show shadow incorrectly computed for
partially undefined vectors
This happens because `getShadow(Value *V)` has a special case for fully
undefined/poisoned values, but partially undefined values fall-through
and are given a clean shadow.
Showing that the same flaw happens for other composite types is left as
an exercise for the reader.
---
.../MemorySanitizer/partial-poison.ll | 77 +++++++++++++++++++
1 file changed, 77 insertions(+)
create mode 100644 llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
diff --git a/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll b/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
new file mode 100644
index 0000000000000..89ceda67a04d0
--- /dev/null
+++ b/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
@@ -0,0 +1,77 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -S -passes='msan' 2>&1 | FileCheck %s
+;
+; Test case to show that MSan incorrectly computes shadows for partially poisoned vectors
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define <2 x i64> @left_poison(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @left_poison(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> <i64 poison, i64 42>
+;
+ ret <2 x i64> <i64 poison, i64 42>
+}
+
+define <2 x i64> @right_poison(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @right_poison(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> <i64 42, i64 poison>
+;
+ ret <2 x i64> <i64 42, i64 poison>
+}
+
+define <2 x i64> @full_poison(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @full_poison(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> splat (i64 -1), ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> poison
+;
+ ret <2 x i64> <i64 poison, i64 poison>
+}
+
+define <2 x i64> @no_poison_or_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @no_poison_or_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> splat (i64 42)
+;
+ ret <2 x i64> <i64 42, i64 42>
+}
+
+define <2 x i64> @left_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @left_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> <i64 undef, i64 42>
+;
+ ret <2 x i64> <i64 undef, i64 42>
+}
+
+define <2 x i64> @right_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @right_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> zeroinitializer, ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> <i64 42, i64 undef>
+;
+ ret <2 x i64> <i64 42, i64 undef>
+}
+
+define <2 x i64> @full_undef(ptr %add.ptr) sanitize_memory {
+; CHECK-LABEL: define <2 x i64> @full_undef(
+; CHECK-SAME: ptr [[ADD_PTR:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT: call void @llvm.donothing()
+; CHECK-NEXT: store <2 x i64> splat (i64 -1), ptr @__msan_retval_tls, align 8
+; CHECK-NEXT: ret <2 x i64> undef
+;
+ ret <2 x i64> <i64 undef, i64 undef>
+}
>From c9115c6240e8aaf79b145051717f2c5fe92c4dc8 Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston at google.com>
Date: Thu, 12 Jun 2025 02:52:37 +0000
Subject: [PATCH 2/3] Add TODO in getShadow
---
llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index c2315d5de7041..4a618cf42c559 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -2085,6 +2085,10 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
assert(ShadowPtr && "Could not find shadow for an argument");
return ShadowPtr;
}
+
+ // TODO: partially undefined vectors are not correctly handled
+ // (see partial-poison.ll)
+
// For everything else the shadow is zero.
return getCleanShadow(V);
}
>From dae762cefea21b84bf5e1483df3d399bcbde4438 Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston at google.com>
Date: Thu, 12 Jun 2025 02:54:56 +0000
Subject: [PATCH 3/3] Clarify that this leads to false negatives
---
llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp | 4 ++--
llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll | 3 ++-
2 files changed, 4 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index 4a618cf42c559..d3c6a7151ec37 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -2086,8 +2086,8 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
return ShadowPtr;
}
- // TODO: partially undefined vectors are not correctly handled
- // (see partial-poison.ll)
+ // TODO: Partially undefined vectors are handled by the fall-through case
+ // below (see partial-poison.ll); this causes false negatives.
// For everything else the shadow is zero.
return getCleanShadow(V);
diff --git a/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll b/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
index 89ceda67a04d0..5164441c17e10 100644
--- a/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll
@@ -1,7 +1,8 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt < %s -S -passes='msan' 2>&1 | FileCheck %s
;
-; Test case to show that MSan incorrectly computes shadows for partially poisoned vectors
+; Test case to show that MSan computes shadows for partially poisoned vectors
+; as fully initialized, resulting in false negatives.
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
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