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

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Message-ID: <20181107150605.59a8ac1c@cakuba.netronome.com>
Date:   Wed, 7 Nov 2018 15:06:05 -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 15:00:21 -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..