Mail Archives: cygwin/2023/04/18/05:26:07
Hi Bruno,
On Apr 17 20:44, Bruno Haible via Cygwin wrote:
> Hi Corinna,
>
> > > Would it be possible to change Cygwin's posix_spawnp implementation,
> > > so that both tests succeed?
> >
> > Basically, yes, but...
>
> Thanks!
>
> > > Disclaimer: I have done my tests with Cygwin 2.9.0; so, if things have
> > ^^^^^^^^^^^^
> > 2017-09-07
> >
> > I'm a bit puzzled. You quote that only Cygwin 3.1.7 has the fixed
> > posix_spawn, but then you test this with a version three years older?
>
> I have now verified that the findings with Cygwin 3.4.6 are the same as
> with Cygwin 2.9.0. I had expected that, based on browsing through the
> Cygwin git history; I confirm it now.
Thanks a lot! The patch is actually simple.
But I'm not *that* happy with the change yet, for two reasons.
First, the security risk outlined in
https://sourceware.org/bugzilla/show_bug.cgi?id=13134#c0 doesn't
actually exist on Cygwin, because we don't implement setuid/setgid
executables. You can set the mode bits, but they are not acted upon.
Second, the rational section in POSIX explains posix_spawn and
posix_spawnp, but it does *not* actually provide an example
implementation of posix_spawnp, only of posix_spawn.
From the above bugzilla entry I take it that on glibc, both
functions tried to run the shell if the executable isn't recognized
(up to commits d96de9634a33 / 13adfa34aff).
However, on Cygwin, only posix_spawnp does that,but not posix_spawn.
In fact, I read the POSIX descriptions in terms of these functions quite
thoroughly, and at no point I see it mentioned that posix_spawnp shall
*not* work like exevlp/execvp. The crucial difference between posix_spawn
and posixc_spawnp is described in an interestingly vague way:
posix_spawnp() interprets the second parameter more elaborately than
posix_spawn().
If I missed the point in the POSIX docs, please tell me.
So, again, the patch is simple. But it's kind of a pity that the change
in glibc has been made without a bigger discussion. Right now, it looks
like the glibc change to posix_spawn was correct, but the change to
posix_spawnp was arbitrary.
Has anybody attempted to ask the Austin group to define this behaviour
in posix_spawnp more concise? Is there a protocel from the Austin
group? If not, wouldn't it be time to ask the Austin group?
> Btw, there are two more functions in the posix_spawn family meanwhile:
> * posix_spawn_file_actions_addchdir_np
> implemented by glibc [1], musl libc, macOS, FreeBSD [2], Solaris ≥ 11.3
> used by a few packages (Firefox, Chromium, Rust)
> * posix_spawn_file_actions_addfchdir_np
> implemented in glibc, musl libc
> but not used by any package so far [3].
>
> The next POSIX will contain these functions (without the _np suffix).[4]
Thanks for the pointers. I'm not sure I'll have the time to implement
them soon, but I put them on my list for 3.5.0. Patches welcome!
Thanks,
Corinna
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