netdev - Re: [PATCH bpf-next 2/3] libbpf: cleanup after partial failure in bpf_object_

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Date:   Wed, 7 Nov 2018 15:38:25 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, shuah@...nel.org,
        quentin.monnet@...ronome.com, guro@...com,
        jiong.wang@...ronome.com, bhole_prashant_q7@....ntt.co.jp,
        john.fastabend@...il.com, jbenc@...hat.com,
        treeze.taeung@...il.com, yhs@...com, osk@...com,
        sandipan@...ux.vnet.ibm.com
Subject: Re: [PATCH bpf-next 2/3] libbpf: cleanup after partial failure in
 bpf_object__pin

On Wed, 7 Nov 2018 15:25:16 -0800, Stanislav Fomichev wrote:
> On 11/07, Jakub Kicinski wrote:
> > On Wed, 7 Nov 2018 15:00:21 -0800, Stanislav Fomichev wrote:  
> > > > > +err_unpin_programs:
> > > > > +	bpf_object__for_each_program(prog, obj) {
> > > > > +		char buf[PATH_MAX];
> > > > > +		int len;
> > > > > +
> > > > > +		len = snprintf(buf, PATH_MAX, "%s/%s", path,
> > > > > +			       prog->section_name);
> > > > > +		if (len < 0)
> > > > > +			continue;
> > > > > +		else if (len >= PATH_MAX)
> > > > > +			continue;
> > > > > +
> > > > > +		unlink(buf);    
> > > > 
> > > > I think that's no bueno, if pin failed because the file already exists
> > > > you'll now remove that already existing file.    
> > >
> > > How about we check beforehand and bail early if we are going to
> > > overwrite something?  
> > 
> > Possible, although the most common way to handle situation like this in
> > the kernel is to "continue the iteration in reverse" over the list.
> > I.e. walk the list back.  I think the objects are on a double linked
> > list.  You may need to add the appropriate foreach macro and helper..  
>
> That sounds more complicated than just ensuring that the top directory
> for the pins doesn't exist and then rm -rf it on failure.

Why would we require that the directory does not exist?  We can
check if it exists and then either create or just pin all in an existing
one.

I don't think it should be that much effort to write a reverse for
loop - it could actually be less LoC than that rm_rf function :)

> I'm thinking about copy-pasting rm_rf from perf
> (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/util.c#n119).
> Thoughts?
>
> Btw, current patch won't work because of those /0 added by bpf_program__pin.