[llvm] [NFCI][msan] Show shadow incorrectly computed for partially undefined vectors (PR #143823)

[Thurston Dang via llvm-commits]

https://github.com/thurstond created https://github.com/llvm/llvm-project/pull/143823

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.

>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] [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>
+}