linux-kernel - Re: [PATCH bpf-next v2 04/13] resolve_btfids: Introduce finalize

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Message-ID: <45b3df2a1b83a90c134b265ecd713731173556f0.camel@gmail.com>
Date: Tue, 20 Jan 2026 10:40:03 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Ihor Solodrai <ihor.solodrai@...ux.dev>, Alexei Starovoitov
 <ast@...nel.org>,  Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Mykyta Yatsenko <yatsenko@...a.com>, Tejun Heo <tj@...nel.org>, Alan
 Maguire <alan.maguire@...cle.com>, Benjamin Tissoires <bentiss@...nel.org>,
 Jiri Kosina	 <jikos@...nel.org>, Amery Hung <ameryhung@...il.com>,
 bpf@...r.kernel.org, 	linux-kernel@...r.kernel.org,
 linux-input@...r.kernel.org, 	sched-ext@...ts.linux.dev
Subject: Re: [PATCH bpf-next v2 04/13] resolve_btfids: Introduce
 finalize_btf() step

On Tue, 2026-01-20 at 10:35 -0800, Ihor Solodrai wrote:
> On 1/20/26 10:19 AM, Eduard Zingerman wrote:
> > On Tue, 2026-01-20 at 10:11 -0800, Ihor Solodrai wrote:
> > 
> > [...]
> > 
> > > > > @@ -1099,12 +1116,22 @@ int main(int argc, const char **argv)
> > > > >  	if (obj.efile.idlist_shndx == -1 ||
> > > > >  	    obj.efile.symbols_shndx == -1) {
> > > > >  		pr_debug("Cannot find .BTF_ids or symbols sections, skip symbols resolution\n");
> > > > > -		goto dump_btf;
> > > > > +		resolve_btfids = false;
> > > > >  	}
> > > > >  
> > > > > -	if (symbols_collect(&obj))
> > > > > +	if (resolve_btfids)
> > > > > +		if (symbols_collect(&obj))
> > > > > +			goto out;
> > > > 
> > > > Nit: check obj.efile.idlist_shndx and obj.efile.symbols_shndx inside symbols_collect()?
> > > >      To avoid resolve_btfids flag and the `goto dump_btf;` below.
> > > 
> > > Hi Eduard, thank you for review.
> > > 
> > > The issue is that in case of .BTF_ids section absent we have to skip
> > > some of the steps, specifically:
> > >   - symbols_collect()
> > >   - sequence between symbols_resolve() and dump_raw_btf_ids()
> > 
> > > It's not an exit condition, we still have to do load/dump of the BTF.
> > > 
> > > I tried in symbols_collect():
> > > 
> > > 	if (obj.efile.idlist_shndx == -1 || obj.efile.symbols_shndx == -1)
> > > 		return 0;
> > > 
> > > But then, we either have to do the same check in symbols_resolve() and
> > > co, or maybe store a flag in the struct object.  So I decided it's
> > > better to have an explicit flag in the main control flow, instead of
> > > hiding it.
> > 
> > For symbols_resolve() is any special logic necessary?
> > I think that `id = btf_id__find(root, str);` will just return NULL for
> > every type, thus the whole function would be a noop passing through
> > BTF types once.
> > 
> > symbols_patch() will be a noop, as it will attempt traversing empty roots.
> > dump_raw_btf_ids() already returns if there are no .BTF_ids.
> 
> Hm... Looks like you're right, those would be noops.
> 
> Still, I think it's clearer what steps are skipped with a toplevel
> flag.  Otherwise to figure out that those are noops you need to check
> every subroutine (as you just did), and a future change may
> unintentionally break the expectation of noop creating an unnecessary
> debugging session.

I'd argue that resolve_btfids is written in a clear read data /
process data manner, so it should behave correctly even if collected
data is empty. Is there a difference between no .BTF_ids and empty
.BTF_ids?

> And re symbols_resolve(), if we don't like allocating unnecessary
> memory, why are we ok with traversing the BTF with noops? Seems
> a bit inconsistent to me.

He-he, I never heard about consistency :)
Just don't like flags, they make me think more.