Return errc::no_such_file_or_directory in fs::access if GetFileAttributesW fails

[z2oh]

Fixes #83046

There is a race condition when calling GetFileAttributesW that can cause it to return ERROR_ACCESS_DENIED on a path which exists, which is unexpected for callers using this function to check for file existence by passing AccessMode::Exist. This was manifesting as a compiler crash on Windows downstream in the Swift compiler when using the -index-store-path flag (more information in apple#8224).

I looked for alternate APIs to avoid bringing in shlwapi.h, but didn't see any good candidates. I'm not tied at all to this solution, any feedback and alternative approaches are more than welcome.

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[llvmbot]

@llvm/pr-subscribers-llvm-support

@llvm/pr-subscribers-platform-windows

Author: Jeremy Day (z2oh)

Changes

Fixes #83046

There is a race condition when calling GetFileAttributesW that can cause it to return ERROR_ACCESS_DENIED on a path which exists, which is unexpected for callers using this function to check for file existence by passing AccessMode::Exist. This was manifesting as a compiler crash on Windows downstream in the Swift compiler when using the -index-store-path flag (more information in apple#8224).

I looked for alternate APIs to avoid bringing in shlwapi.h, but didn't see any good candidates. I'm not tied at all to this solution, any feedback and alternative approaches are more than welcome.

---

Full diff: https://github.com/llvm/llvm-project/pull/83495.diff

2 Files Affected:

• (modified) llvm/lib/Support/CMakeLists.txt (+1-1)
• (modified) llvm/lib/Support/Windows/Path.inc (+9)

diff --git a/llvm/lib/Support/CMakeLists.txt b/llvm/lib/Support/CMakeLists.txt
index 1f2d82427552f7..83889535d88f1b 100644
--- a/llvm/lib/Support/CMakeLists.txt
+++ b/llvm/lib/Support/CMakeLists.txt
@@ -40,7 +40,7 @@ endif()
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.
-  set(system_libs ${system_libs} psapi shell32 ole32 uuid advapi32 ws2_32)
+  set(system_libs ${system_libs} psapi shell32 shlwapi ole32 uuid advapi32 ws2_32)
 elseif( CMAKE_HOST_UNIX )
   if( HAVE_LIBRT )
     set(system_libs ${system_libs} rt)
diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
index 3e4c1f74161c6f..03befdd5412628 100644
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -27,6 +27,7 @@
 #include "llvm/Support/Windows/WindowsSupport.h"
 #include <shellapi.h>
 #include <shlobj.h>
+#include <shlwapi.h>
 
 #undef max
 
@@ -620,6 +621,14 @@ std::error_code access(const Twine &Path, AccessMode Mode) {
   if (std::error_code EC = widenPath(Path, PathUtf16))
     return EC;
 
+  if (Mode == AccessMode::Exist) {
+    if (::PathFileExistsW(PathUtf16.begin())) {
+      return std::error_code();
+    } else {
+      return errc::no_such_file_or_directory;
+    }
+  }
+
   DWORD Attributes = ::GetFileAttributesW(PathUtf16.begin());
 
   if (Attributes == INVALID_FILE_ATTRIBUTES) {

[mstorsjo]

There is a race condition when calling GetFileAttributesW that can cause it to return ERROR_ACCESS_DENIED on a path which exists

Can you elaborate a bit more about how this race condition occurs, and how the suggested solution avoids the issue? (I guess it's explained somewhere in the linked issues, but in order to evaluate the suggested fix, it would be good to have that analysis spelled out here as well.)

[z2oh]

Unfortunately I haven't been able to pin down the exact source of the race here, only that sometimes there is a call to fs::access(_, AccessMode::Exist) that unexpectedly fails with ERROR_ACCESS_DENIED. We have been able to reproduce this call failing in this way in isolation, where GetFileAttributesW fails with ERROR_ACCESS_DENIED called on an open handle that has been marked for deletion.

I've evaluated the veracity of this patch experimentally, but after seeing Wine's implementation of PathFileExistsW linked above I'm thinking it works by simply treating any failure of GetFileAttributesW as the evidence of the path not existing. This actually seems to be the semantics of semantics of the fs::exists function, so I may be able to just use this instead in downstream code. Thanks for the pointers everyone! I'll report back after verifying this.

[aganea]

Regardless I think it would still be valuable if we returned errc::no_such_file_or_directory on failure, when calling access with AccessMode::Exists, and not some other Windows error code, ie:

if (Attributes == INVALID_FILE_ATTRIBUTES) {
  if (Mode == AccessMode::Exists)
    return errc::no_such_file_or_directory;
...

[z2oh]

Okay, that makes sense. I've updated the PR. Thanks for your insight and reviews @aganea!

[aganea]

LGTM, thanks!

[z2oh]

After a rebase, CI is passing. I believe this is ready to merge!

[mstorsjo]

LGTM

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