From d94a6320d538bf155f166150597edde26d18a784 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda@apple.com>
Date: Mon, 29 Sep 2025 13:33:49 -0700
Subject: [PATCH 1/2] [llvm] Use the VFS to make path absolute

---
 llvm/lib/Support/VirtualFileSystem.cpp        |  2 +-
 .../Support/VirtualFileSystemTest.cpp         | 23 +++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
index 7ff62d43ba205..64a6607603ff0 100644
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -2204,7 +2204,7 @@ RedirectingFileSystem::create(std::unique_ptr<MemoryBuffer> Buffer,
     //  FS->OverlayFileDir => /<absolute_path_to>/dummy.cache/vfs
     //
     SmallString<256> OverlayAbsDir = sys::path::parent_path(YAMLFilePath);
-    std::error_code EC = llvm::sys::fs::make_absolute(OverlayAbsDir);
+    std::error_code EC = ExternalFS->makeAbsolute(OverlayAbsDir);
     assert(!EC && "Overlay dir final path must be absolute");
     (void)EC;
     FS->setOverlayFileDir(OverlayAbsDir);
diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
index d47a4ee986778..f52f25f93744d 100644
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -1941,6 +1941,29 @@ TEST_F(VFSFromYAMLTest, ReturnsExternalPathVFSHit) {
   EXPECT_EQ(0, NumDiagnostics);
 }
 
+TEST_F(VFSFromYAMLTest, RelativeFileDirWithOverlayRelativeSetting) {
+  auto Lower = makeIntrusiveRefCnt<DummyFileSystem>();
+  Lower->addDirectory("//root/foo/bar");
+  Lower->addRegularFile("//root/foo/bar/a");
+  Lower->setCurrentWorkingDirectory("//root/foo");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS =
+      getFromYAMLString("{\n"
+                        "  'case-sensitive': false,\n"
+                        "  'overlay-relative': true,\n"
+                        "  'roots': [\n"
+                        "    { 'name': '//root/foo/bar/b', 'type': 'file',\n"
+                        "      'external-contents': 'a'\n"
+                        "    }\n"
+                        "  ]\n"
+                        "}",
+                        Lower, "bar/overlay");
+
+  ASSERT_NE(FS.get(), nullptr);
+  ErrorOr<vfs::Status> S = FS->status("//root/foo/bar/b");
+  ASSERT_FALSE(S.getError());
+  EXPECT_EQ("//root/foo/bar/a", S->getName());
+}
+
 TEST_F(VFSFromYAMLTest, RootRelativeToOverlayDirTest) {
   auto Lower = makeIntrusiveRefCnt<DummyFileSystem>();
   Lower->addDirectory("//root/foo/bar");

From d35ad7d1322492139a169fcd9848077afb39a79b Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda@apple.com>
Date: Tue, 30 Sep 2025 09:36:04 -0700
Subject: [PATCH 2/2] Fix Windows CI

---
 llvm/lib/Support/VirtualFileSystem.cpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
index 64a6607603ff0..44d2ee7076fb2 100644
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -1908,7 +1908,12 @@ class llvm::vfs::RedirectingFileSystemParser {
           FullPath = FS->getOverlayFileDir();
           assert(!FullPath.empty() &&
                  "External contents prefix directory must exist");
-          llvm::sys::path::append(FullPath, Value);
+          SmallString<256> AbsFullPath = Value;
+          if (FS->makeAbsolute(FullPath, AbsFullPath)) {
+            error(N, "failed to make 'external-contents' absolute");
+            return nullptr;
+          }
+          FullPath = AbsFullPath;
         } else {
           FullPath = Value;
         }
@@ -2204,7 +2209,7 @@ RedirectingFileSystem::create(std::unique_ptr<MemoryBuffer> Buffer,
     //  FS->OverlayFileDir => /<absolute_path_to>/dummy.cache/vfs
     //
     SmallString<256> OverlayAbsDir = sys::path::parent_path(YAMLFilePath);
-    std::error_code EC = ExternalFS->makeAbsolute(OverlayAbsDir);
+    std::error_code EC = FS->makeAbsolute(OverlayAbsDir);
     assert(!EC && "Overlay dir final path must be absolute");
     (void)EC;
     FS->setOverlayFileDir(OverlayAbsDir);