[LLVMbugs] [Bug 21705] New: llvm::sys::fs::file_status::getUniqueID() may fail to return unique IDs on Windows

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Mon Dec 1 14:05:49 PST 2014


            Bug ID: 21705
           Summary: llvm::sys::fs::file_status::getUniqueID() may fail to
                    return unique IDs on Windows
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows XP
            Status: NEW
          Severity: normal
          Priority: P
         Component: Support Libraries
          Assignee: unassignedbugs at nondot.org
          Reporter: thonermann at coverity.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The implementation of llvm::sys::fs::file_status::getUniqueID() in
<llvm>/lib/Support/Windows/Path.inc relies on a call to the Windows
GetFileInformationByHandle() [1][2] function to obtain a unique ID for a file
regardless of the path or file name of the file.  The intent is that the same
unique ID be returned for each hard link, soft link, NTFS junction directory,
etc... corresponding to the same file.  GetFileInformationByHandle() requires
an open file handle and populates a structure [2] with information about the
referenced file.  This information includes a file index ID that is documented
(see the description of nFileIndexLow at [2]) to uniquely identify a file on a
particular volume (see dwVolumeSerialNumber at [2]).

Unfortunately, and the Microsoft documentation is not particularly clear on
this, the unique file index ID that GetFileInformationByHandle() produces is
only guaranteed to be unique so long as the file remains open.  Once all
handles to the file have been closed, the system is free to reuse the file
index ID for another file.

Additionally, Microsoft's documentation is clear that
GetFileInformationByHandle() may not produce a unique file index ID for the
Windows Server 2012 ReFS file system due to its use of 128-bit file
identifiers.  GetFileInformationByHandleEx() [3] may be used to populate a
FILE_ID_INFO [4] structure with a 128-bit file ID, but Windows 8 or Windows
Server 2012 is required.

llvm::sys::fs::file_status::getUniqueID() constructs a unique ID from the
VolumeSerialNumber, FileIndexHigh, and FileIndexLow file_status data members
typically populated by a call to GetFileInformationByHandle().  To see where
problems may occur, code that creates file_status objects must be examined.

file_status objects are typically constructed by calling one of the status()
overloads declared in:
488 std::error_code status(const Twine &path, file_status &result);
491 std::error_code status(int FD, file_status &Result);

The overload taking a file descriptor should be ok.  Internally, the file
handle associated with the file descriptor is obtained to call
GetFileInformationByHandle().  The retrieved file index ID (and thus, the
UniqueID object returned by file_statis::getUniqueID()) is valid until the file
is closed.

The overload taking a path is problematic.  Internally, this overload opens the
file identified by the path, calls a static getStatus() function with the file
handle, and then closes the file handle.  getStatus() calls 
GetFileInformationByHandle() and populates the file_status data members.  The
problem is, by the time status() returns, the data members in the file_status
object that it has populated are no longer guaranteed to uniquely identify a

For files hosted on NTFS and FAT file systems, GetFileInformationByHandle()
seems to reliably return consistent and unique file index IDs.  However, this
is not guaranteed and may simply be an implementation side effect.  For files
hosted on network file servers (specifically, on large Network Appliance file
systems shared via CIFS), re-use of file index IDs has been observed.

Unfortunately, I don't have a test case to share.  I don't have access to the
servers where this was observed and I haven't been able to reproduce problems
in my own local environments.  Regardless, it is clear to me that
GetFileInformationByHandle() does not offer the guarnatees that LLVM is using
it for.


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