From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: /dev/fd/N not synonymous with file descriptor N; it is on Linux
Date: Mon, 28 Jan 2019 16:51:00 -0000 [thread overview]
Message-ID: <20190128165102.GP3912@calimero.vinschen.de> (raw)
In-Reply-To: <c09c2a73bcc7b20fcbcfafe7732053d6@smtp-cloud7.xs4all.net>
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On Jan 28 15:15, Houder wrote:
> On Sun, 27 Jan 2019 22:57:21, Corinna Vinschen wrote:
> >
> > On Jan 27 19:39, Houder wrote:
> > > NO BLODA.
> > >
> > > Ok, for the record (as this is W7, i.e. pre-pre-W10 :-)
> > >
> > > Using my original STC again: (source code included below)
> > >
> > > - create file (in /tmp) write-only, write "Hello, world!" to file, close
> > > fd
> > > - open file once more read-only
> > > - unlink file
> > > - open file, using /dev/fd/N, read-write <==== succeeds (and the handle
> > > shown by fcntl is read-write)
> > > - write "*****" to file (using the fd obtained in the previous line),
> > > lseek to begin of file
> > > - write fails w/ "Permission denied" <==== so ... the file cannot be
> > > written to?
> >
> > Yes, that scenario fails on W7 but works on W10 1709 and later. Keep in
> > mind that the OS doesn't allow to reopen a file which has been deleted.
> > Cygwin tries a best effort by duplicating the handle. A duplicated file
> > handle can't have more permissions than the original handle, so if the
> > original handle was opened for reading only, the duplicated handle can't
> > have write perms.
> [snip]
>
> Yes Corinna, I already got that from one of your previous replies. You gave
> the same explanation here:
>
> https://cygwin.com/ml/cygwin/2019-01/msg00171.html
> (Date: Tue, 22 Jan 2019 10:41:57 +0100)
>
> "A duplicated file handle can't have more permissions than the original
> handle" (i.e what occurs on pre "Windows 10 1709" systems)
>
> The "funny" thing is, in that same post, you showed that the STC succeeded
> on your virtual W7 system ... (contradiction!).
>
> The STC in that post executes the same scenario as above ...
>
> - the difference is that the first 3 steps are carried out by bash, when
> invoked as follows:
>
> @@ ./stca /dev/fd/N N<<EOF
>
> Both STC's (stc.c and stca.c) fail on my W7 (Note: stc.c is the testcase
> that I included in my previous post -- and the one I started this thread
> with).
>
> Mind this: I am NOT upset that the STC's (plural) fail on (my) W7. Not at
> all!
There was a minor difference when I tested it: My shell is not bash
but tcsh, and tcsh opens the here document with different perms.
I added debug output to fhandler_base::open and on bash the ACCESS_MASK
set on the incoming handle is 0x12019F with tcsh and 0x120089 with bash.
Compare with https://msdn.microsoft.com/en-us/library/cc246802.aspx, and
you'll see that tcsh opened the file woth O_RDWR, while bash opened the
file with O_RDONLY.
I didn't notice this before, sorry!
Corinna
--
Corinna Vinschen
Cygwin Maintainer
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next prev parent reply other threads:[~2019-01-28 16:51 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-16 16:31 Houder
2018-12-16 20:28 ` Corinna Vinschen
2018-12-16 20:31 ` Corinna Vinschen
2018-12-16 21:36 ` Wayne Davison
2018-12-16 21:55 ` Corinna Vinschen
2018-12-17 4:30 ` Houder
2018-12-17 9:25 ` Corinna Vinschen
2018-12-17 11:26 ` Corinna Vinschen
2019-01-02 13:56 ` Houder
2018-12-17 3:41 ` Houder
2019-01-06 20:19 ` Corinna Vinschen
2019-01-22 8:50 ` Houder
2019-01-22 8:57 ` Houder
2019-01-22 9:06 ` Corinna Vinschen
2019-01-22 9:25 ` Houder
2019-01-22 9:42 ` Corinna Vinschen
2019-01-22 10:20 ` Houder
2019-01-22 10:39 ` Corinna Vinschen
2019-01-27 18:39 ` Houder
2019-01-27 21:57 ` Corinna Vinschen
2019-01-27 22:12 ` Corinna Vinschen
2019-01-28 14:15 ` Houder
2019-01-28 16:51 ` Corinna Vinschen [this message]
2019-01-22 9:02 ` Corinna Vinschen
2019-01-22 9:07 ` Corinna Vinschen
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