diff --git a/llvm/lib/Transforms/Utils/BuildLibCalls.cpp b/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
--- a/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
@@ -166,6 +166,14 @@
   return Changed;
 }
 
+static bool setArgNoUndef(Function &F, unsigned ArgNo) {
+  if (F.hasParamAttribute(ArgNo, Attribute::NoUndef))
+    return false;
+  F.addParamAttr(ArgNo, Attribute::NoUndef);
+  ++NumNoUndef;
+  return true;
+}
+
 static bool setRetAndArgsNoUndef(Function &F) {
   return setRetNoUndef(F) | setArgsNoUndef(F);
 }
@@ -323,8 +331,10 @@
     Changed |= setDoesNotThrow(F);
     Changed |= setDoesNotCapture(F, 0);
     return Changed;
-  case LibFunc_strdup:
   case LibFunc_strndup:
+    Changed |= setArgNoUndef(F, 1);
+    LLVM_FALLTHROUGH;
+  case LibFunc_strdup:
     Changed |= setOnlyAccessesInaccessibleMemOrArgMem(F);
     Changed |= setDoesNotThrow(F);
     Changed |= setRetDoesNotAlias(F);
@@ -383,7 +393,7 @@
   case LibFunc_malloc:
   case LibFunc_vec_malloc:
     Changed |= setOnlyAccessesInaccessibleMemory(F);
-    Changed |= setRetNoUndef(F);
+    Changed |= setRetAndArgsNoUndef(F);
     Changed |= setDoesNotThrow(F);
     Changed |= setRetDoesNotAlias(F);
     Changed |= setWillReturn(F);
@@ -471,10 +481,12 @@
     Changed |= setRetDoesNotAlias(F);
     Changed |= setWillReturn(F);
     Changed |= setDoesNotCapture(F, 0);
+    Changed |= setArgNoUndef(F, 1);
     return Changed;
   case LibFunc_reallocf:
     Changed |= setRetNoUndef(F);
     Changed |= setWillReturn(F);
+    Changed |= setArgNoUndef(F, 1);
     return Changed;
   case LibFunc_read:
     // May throw; "read" is a valid pthread cancellation point.
@@ -517,7 +529,7 @@
     return Changed;
   case LibFunc_aligned_alloc:
     Changed |= setOnlyAccessesInaccessibleMemory(F);
-    Changed |= setRetNoUndef(F);
+    Changed |= setRetAndArgsNoUndef(F);
     Changed |= setDoesNotThrow(F);
     Changed |= setRetDoesNotAlias(F);
     Changed |= setWillReturn(F);
@@ -549,7 +561,7 @@
   case LibFunc_calloc:
   case LibFunc_vec_calloc:
     Changed |= setOnlyAccessesInaccessibleMemory(F);
-    Changed |= setRetNoUndef(F);
+    Changed |= setRetAndArgsNoUndef(F);
     Changed |= setDoesNotThrow(F);
     Changed |= setRetDoesNotAlias(F);
     Changed |= setWillReturn(F);
@@ -833,7 +845,7 @@
     return Changed;
   case LibFunc_valloc:
     Changed |= setOnlyAccessesInaccessibleMemory(F);
-    Changed |= setRetNoUndef(F);
+    Changed |= setRetAndArgsNoUndef(F);
     Changed |= setDoesNotThrow(F);
     Changed |= setRetDoesNotAlias(F);
     Changed |= setWillReturn(F);
@@ -908,8 +920,10 @@
     Changed |= setRetAndArgsNoUndef(F);
     Changed |= setDoesNotCapture(F, 3);
     return Changed;
-  case LibFunc_dunder_strdup:
   case LibFunc_dunder_strndup:
+    Changed |= setArgNoUndef(F, 1);
+    LLVM_FALLTHROUGH;
+  case LibFunc_dunder_strdup:
     Changed |= setDoesNotThrow(F);
     Changed |= setRetDoesNotAlias(F);
     Changed |= setWillReturn(F);
diff --git a/llvm/test/Transforms/InferFunctionAttrs/annotate.ll b/llvm/test/Transforms/InferFunctionAttrs/annotate.ll
--- a/llvm/test/Transforms/InferFunctionAttrs/annotate.ll
+++ b/llvm/test/Transforms/InferFunctionAttrs/annotate.ll
@@ -178,7 +178,7 @@
 ; CHECK: declare x86_fp80 @acosl(x86_fp80) [[NOFREE_NOUNWIND_WILLRETURN]]
 declare x86_fp80 @acosl(x86_fp80)
 
-; CHECK: declare noalias noundef i8* @aligned_alloc(i64, i64) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND:#[0-9]+]]
+; CHECK: declare noalias noundef i8* @aligned_alloc(i64 noundef, i64 noundef) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND:#[0-9]+]]
 declare i8* @aligned_alloc(i64, i64)
 
 ; CHECK: declare double @asin(double) [[NOFREE_NOUNWIND_WILLRETURN]]
@@ -250,7 +250,7 @@
 ; CHECK: declare void @bzero(i8* nocapture writeonly, i64)  [[ARGMEMONLY_NOFREE_NOUNWIND:#[0-9]+]]
 declare void @bzero(i8*, i64)
 
-; CHECK: declare noalias noundef i8* @calloc(i64, i64) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN:#[0-9]+]]
+; CHECK: declare noalias noundef i8* @calloc(i64 noundef, i64 noundef) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN:#[0-9]+]]
 declare i8* @calloc(i64, i64)
 
 ; CHECK: declare double @cbrt(double) [[NOFREE_NOUNWIND_WILLRETURN]]
@@ -610,7 +610,7 @@
 ; CHECK-LINUX: declare noundef i32 @lstat64(i8* nocapture noundef readonly, %opaque* nocapture noundef) [[NOFREE_NOUNWIND]]
 declare i32 @lstat64(i8*, %opaque*)
 
-; CHECK: declare noalias noundef i8* @malloc(i64) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
+; CHECK: declare noalias noundef i8* @malloc(i64 noundef) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
 declare i8* @malloc(i64)
 
 ; CHECK-LINUX: declare noalias noundef i8* @memalign(i64, i64) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
@@ -727,10 +727,10 @@
 ; CHECK: declare noundef i64 @readlink(i8* nocapture noundef readonly, i8* nocapture noundef, i64 noundef) [[NOFREE_NOUNWIND]]
 declare i64 @readlink(i8*, i8*, i64)
 
-; CHECK: declare noalias noundef i8* @realloc(i8* nocapture, i64) [[NOUNWIND]]
+; CHECK: declare noalias noundef i8* @realloc(i8* nocapture, i64 noundef) [[NOUNWIND]]
 declare i8* @realloc(i8*, i64)
 
-; CHECK: declare noundef i8* @reallocf(i8*, i64)
+; CHECK: declare noundef i8* @reallocf(i8*, i64 noundef)
 declare i8* @reallocf(i8*, i64)
 
 ; CHECK: declare noundef i8* @realpath(i8* nocapture noundef readonly, i8* noundef) [[NOFREE_NOUNWIND]]
@@ -871,7 +871,7 @@
 ; CHECK: declare i8* @strncpy(i8* noalias returned writeonly, i8* noalias nocapture readonly, i64) [[ARGMEMONLY_NOFREE_NOUNWIND]]
 declare i8* @strncpy(i8*, i8*, i64)
 
-; CHECK: declare noalias i8* @strndup(i8* nocapture readonly, i64) [[INACCESSIBLEMEMORARGONLY_NOFREE_NOUNWIND_WILLRETURN]]
+; CHECK: declare noalias i8* @strndup(i8* nocapture readonly, i64 noundef) [[INACCESSIBLEMEMORARGONLY_NOFREE_NOUNWIND_WILLRETURN]]
 declare i8* @strndup(i8*, i64)
 
 ; CHECK: declare i64 @strnlen(i8*, i64) [[NOFREE_NOUNWIND_WILLRETURN]]
@@ -979,7 +979,7 @@
 ; CHECK: declare noundef i32 @utimes(i8* nocapture noundef readonly, %opaque* nocapture noundef readonly) [[NOFREE_NOUNWIND]]
 declare i32 @utimes(i8*, %opaque*)
 
-; CHECK: declare noalias noundef i8* @valloc(i64) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
+; CHECK: declare noalias noundef i8* @valloc(i64 noundef) [[INACCESSIBLEMEMONLY_NOFREE_NOUNWIND_WILLRETURN]]
 declare i8* @valloc(i64)
 
 ; CHECK: declare noundef i32 @vfprintf(%opaque* nocapture noundef, i8* nocapture noundef readonly, %opaque* noundef) [[NOFREE_NOUNWIND]]