[llvm-dev] clang-format sets executable permission on windows (openNativeFile ignores mode on Windows)

[Adrian McCarthy via llvm-dev]

I don't claim to understand NTFS permissions fully, but this mostly sounds
like a problem of how the NTFS permissions are presented as a Unix-style
mode.

When you create a new file without specifying explicit permissions (as most
tools do), Windows (or NTFS) grants the new file the same permissions as
the folder that contains it.  Folders generally have the "Read & Execute"
permission, since that's what lets a user navigate the filesystem.
Whatever tool you're using to translate Windows/NTFS permissions into a
Unix-style mode is probably showing x when the file has R&E.

I'm curious how your source files were originally created without R&E.

On something like a text file or a source file, R&E is common and harmless.

Common:  If I create a text file from Explorer or Notepad or Visual Studio,
that file gets R&E.  I have source files that have never been touched by
clang-format, and they have the same permissions, including R&E, as the
ones that have.

Harmless:  Since text files aren't executable, having R&E doesn't grant
anything beyond Read.  A possible exception might be batch files (.BAT).
Does the command interpreter check R&E for those?  I don't know offhand.
If it does, would you want to force the user to change permissions of a
.BAT file they had just written in a text editor before they try it?

That said, I don't entirely understand the permission model has both "Read"
and "Read & Execute".  I'd guess that it's because having orthogonal "Read"
and "Execute" permissions would allow a nonsense state that marks a file as
executable but not readable.

The flags like FILE_GENERIC_READ and FILE_GENERIC_EXECUTE don't map
one-to-one with the Windows/NTFS file permissions.  Rather, they are mostly
used to specify the type of access a particular operation needs (to a call
like CreateFileW, which doesn't necessarily create a file but often opens
one instead).  The system checks the requested access against what's
allowed by the permissions granted in the file's security descriptor (as
well as types of shared access allowed by others who already have the file
open).

On Fri, Apr 17, 2020 at 4:53 PM Chris Tetreault via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
>
>
>                 I’m having an issue where clang-format is setting the
> executable bit on all source files it modifies when using the -i parameter.
> I spent some time troubleshooting this issue today, and I found that
> clang-format create a new temporary file, writes the formatted source into
> that file, then copies it over the old file. Deep in the bowels of
> openNativeFile in lib/Support/Windows/Path.inc, in openNativeFileInternal,
> CreateFileW is called with a SECURITY_ATTRIBUTES with lpSecurityDescriptor
> = nullptr. The result of this is you get a new file with the default
> permissions based on whatever NTFS decides to do. On my machine, this ends
> up being a file with 755 mode. This is happening because the mode parameter
> to openNativeFile is unused. This issue occurs in clang-format 9, and on
> HEAD of master.
>
>
>
>                 I spent some time thinking about how to improve this state
> of affairs. I feel like just letting files modified by clang-format get
> their permissions changed severely limits the convenience of the tool. Just
> some thoughts I had:
>
>
>
>    1. Would it be so terrible if files created by openNativeFile on
>    windows queried the default SECURITY_ATTRIBUTES and stripped executable
>    permissions? Does anything actually need llvm to produce files with
>    executable permissions? I did a quick search but can’t find anywhere in the
>    codebase that actually sets a value for mode.
>    2. How would we even map unix-style permissions to windows? I see in
>    the MS docs that there are coarse-grained permission types that map to unix
>    permissions (FILE_GENERIC_READ, FILE_GENERIC_WRITE, and
>    FILE_GENERIC_EXECUTE). For file creation, the current user could be the
>    owner. Perhaps all groups the user is a member of could get the group
>    permissions, and maybe Authenticated Users for other?
>    3. On my previous project, we used clang-format, and I never had this
>    issue. I was using a very old version though, so I don’t know if my
>    configuration is just different, or if this behavior changed at some point
>
>
>
> Thanks,
>
>                 Christopher Tetreault
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