[issue46506] [Windows] wrap CreateFile to support follow_symlinks

[Eryk Sun]

Eryk Sun <eryksun at gmail.com> added the comment:

> switch to the newer CreateFile2() function

Apparently we need to allow FILE_SHARE_READ if CreateFile2() is used to implement os.stat(). It's not a big deal, but this is a design flaw.

CreateFile2() always uses the kernel I/O flag FILE_DISALLOW_EXCLUSIVE [1] when it calls NtCreateFile(). The intent of this flag is to avoid exclusive read access on the first open of a file when a user lacks write permission. Thus unprivileged users aren't allowed to prevent read access to critical system files. 

However, the implementation is flawed because it denies access even if the open doesn't request data access. It's also flawed because it allows exclusive read access when there are previous opens even if the previous opens have no data access.

Classically, IoCheckShareAccess() only checks for a sharing violation when an open requests read, write, or delete data access (i.e. FILE_READ_DATA, FILE_EXECUTE, FILE_WRITE_DATA, FILE_APPEND_DATA, DELETE). This is clearly explained in [MS-FSA] [2]. An open that only requests metadata access can never cause a sharing violation and poses no problem with regard to blocking access to a file.

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[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntddk/nf-ntddk-iocreatefileex
[2] https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fsa/8c0e3f4f-0729-49f4-a14d-7f7add593819

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