Re: /dev/fd/N not synonymous with file descriptor N; it is on Linux

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

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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