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Date: Wed, 27 Aug 2014 08:52:56 -0400
From: Ken Brown <kbrown@cornell.edu>
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To: cygwin@cygwin.com
Subject: Re: (call-process ...) hangs in emacs
References: <20140805184047.GC13601@calimero.vinschen.de> <53E3685B.8050508@cornell.edu> <53E39BAD.3010004@redhat.com> <53E3CB46.1020909@cornell.edu> <53E3F2AE.7030608@redhat.com> <53E4D01B.9010005@cornell.edu> <53F1F154.1020702@cornell.edu> <53FB87DC.2050908@cornell.edu> <87wq9v9j2y.fsf@Rainer.invalid> <53FD0662.5050208@cornell.edu> <20140827084245.GD20700@calimero.vinschen.de>
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On 8/27/2014 4:42 AM, Corinna Vinschen wrote:
> On Aug 26 18:12, Ken Brown wrote:
>> On 8/26/2014 2:55 PM, Achim Gratz wrote:
>>> 2) Files that have no POSIX permissions (filemode 0000) and where access
>>> is granted via ACL only get always opened as "read-only" and you have to
>>> C-x C-q them before saving.  It appears that this is Cygwin specific
>>> since on Linux the same version copes with that situation correctly
>>> (however, the mask bits in the ACL get displayed in the group portion of
>>> the file mode, which I've never seen happen on Cygwin, so this may be
>>> something that Cygwin needs to do -- maybe that'd even solve the
>>> problems that Perl has in the same situation).
>>
>> AFAICT, emacs decides whether the file is writable via the system call
>> faccessat.  (See the function 'check_writable' in src/fileio.c.)  This is
>> not Cygwin specific.  So faccessat must be returning failure in the scenario
>> you described.  I don't know if that's a Cygwin bug or not.
>
> faccessat/access/eaccess don't try to be intelligent by themselves.
> Rather they just call a Windows function if the filesystem is mounted
> with "acl" mount flags:
>
> - Fetch file's security descriptor
> - Create process impersonation token.
> - Call NtAccessCheck
> - If NtAccessCheck returns "not allowed", check for backup/restore
>    privileges via NtPrivilegeCheck.
>
> In "noacl" mode or on filesystems not supporting ACLs, access uses the
> st_mode flags from stat() to figure out the permissions.
>
> The relevant parts of the implementation are the check_file_access and
> subsequently called check_access functions in security.cc.
>
> If you see a bug there, please let me know.

Achim, could you send me a recipe for reproducing the problem so that I 
can test further?  Please be very detailed; I have no experience with ACLs.

>> BTW, emacs on Cygwin doesn't directly check ACLs, because the relevant
>> configure test fails.
>
> Works for vim.  Does the Emacs configure test only check for POSIX
> ACL functions and not for Solaris ACL functions, by any chance?

I spoke too soon.  It does detect that Cygwin has certain ACL functions. 
  But the feature that Achim was asking about seems to get used only on 
systems that have acl_get_file.  I guess that's a POSIX ACL function.

Ken

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