Re: [PATCH 2/3] core_pattern: allow passing of arguments to user mode helper when core_pattern is a pipe

[Neil Horman <nhorman@tuxdriver.com>]

On Sat, Jul 28, 2007 at 06:17:25PM +0200, Martin Pitt wrote:
> Hi Neil,
> 
> Neil Horman [2007-07-28  9:46 -0400]:
> > > I just want to mention a potential problem with this: If you first
> > > expand the macros (from pattern to corename) and then split
> > > corename into an argv, then this breaks macro expansions
> > > containing spaces.  This mostly affects the executable name, of
> > > course.
> > > 
> > I never intended for this core_pattern argument passing to be able
> > to expand macros, other than the macros specified by the
> > core_pattern code.  If you want it to do that, we can address that
> > with a later patch.
> 
> No, I specifically mean the standard ones provided by
> format_corename(), such as %p (pid), %s (signal), or %e (executable
> name). I don't see a reason to provide additional functionality.
> 
Ahh, well then you should have nothing to worry about, this patch expands them
just fine.  And none of those macros will ever have spaces in them.  I suppose
it would be possible for the executable name to have spaces in it, but honestly,
thats going rather out of your way to do something that you arguably shouldn't
do anyway.

> > > In fact we considered this macro approach when doing the original
> > > patches in the Ubuntu kernel, but we eventually used environment
> > > variables because they are much easier and more robust to
> > > implement than doing a robust macro expansion (i. e. first split
> > > core_pattern into an argv and then call the macro expansion for
> > > each element).
> > > 
> > I disagree. While it might be nice to be able to specify environment
> > variables as command line arguments, it would be much easier to just
> > let the core_pattern executable inherit the crashing processes
> > environment.  we don't do that currently, but we easily could.  That
> > way any information that the crashing process wants the dying
> > process to know can be inherited and vetted easily by apport (or
> > whatever the core_pattern points to).  I'll do a patch later for
> > that if you don't like it.
> 
> I don't think that this will be necessary. After all, the crash
> handler can read all the environment from /proc/<pid>/ (and that's
> indeed what apport does to figure out relevant parts from the
> environment like the locale).
> 
Agreed, /proc/<pid of crashing process>/* will be available while the helper app
runs.

> It seems we misunderstood each other, I don't expect or want any new
> functionality in core_pattern. AN example might make it more clear:
> 
I think you're correct, I misundersood you previously.  Apologies.

> The original problem that we are trying to solve is the current
> behaviour of core_pattern expansion with pipes:
> 
>   |/bin/crash --pid %p
> 
> would try to execute the program '/bin/crash --pid 1234' instead of
> calling /bin/crash with ['--pid', '1234'] as argv, right? Your patch
> achieves the latter by splitting the formatted core dump string into
> an array (at spaces).
> 
Yes, that is exactly correct.

> I pointed out that this leads to problems when macro values contain
> spaces. This currently affects hostname (%h) (although this really
> should not happen in practice) and executable name (%e) (rare, but at
> least valid).  I. e. for an executable name "foo bar" your patch would
> expand
> 
>   |/bin/crash %e
> 
> to ['/bin/crash', 'foo', 'bar'] instead of ['/bin/crash', 'foo bar'].
> 
Also correct.  Thats a pretty big corner case, and I personally don't think an
executable name with spaces is/should be valid anyway, but it can be done.

> Of course this is a corner case, and personally I don't really care.
> I strive to keep the assumptions about the interface at a minimum, so
> right now Apport's only required input is the core dump itself (over
> stdin); signal and pid can be read from the environment, and if not
> present, they are read from the core dump.
> 

Jeremy asked that I make a patch next week to address split_argv's requirement
that the argc parameter be non-NULL.  I'll be fixing that next week, and what I
can do is further enhance it such that it ignores spaces in quoted strings,
which should address the case that concerns you.  I.E I can make split_argv
behave such that:
echo "|\"foo bar\" --pid %p" > /proc/sys/kernel/core_pattern
results in the following argv:
{{"foo bar"}, {"--pid"}, {"1234"}}

Which I think handles what you are looking for.

> I did not defend Ubuntu's usage of environment variables, on the
> contrary. Using the standard macros is more explicit and elegant, and
> I welcome that change. I just pointed out the reason why we chose the
> environment variable approach initially.
> 
Ah, my bad.  We're on the same page then, and I just misunderstood what you were
referring to when you said macro expansion. I thought you wanted to have
core_pattern translate things like $HOME and soforth, which can more esaily be
passed to things like apport via environment inheritence, which we can look at
at a later date.

> I just wanted to mention this little problem for the sake of
> correctness.
> 
> Thank you, and have a nice weekend!
> 
> Martin
> 


Thank you for clearing me up on this.  So it would seem we're ok with what we
have now, correct?  We just have a potential corner case to address, which I can
reasonably handle with a modification to split_argv, that I have a todo on next
week.

Thanks & Regards
Neil

> -- 
> Martin Pitt        http://www.piware.de
> Ubuntu Developer   http://www.ubuntu.com
> Debian Developer   http://www.debian.org



-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *Red Hat, Inc.
 *nhorman@tuxdriver.com
 *gpg keyid: 1024D / 0x92A74FA1
 *http://pgp.mit.edu
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