linux-kernel - Re: [RFC PATCH v2 2/5] btf: sort BTF types by kind and name to enable binary search

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Message-ID: <CAEf4BzYLyi6=Fyz9ziOAwkFOjUPyJmTj4c6g247XBwgwJ8m-qw@mail.gmail.com>
Date: Thu, 23 Oct 2025 12:39:57 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Donglin Peng <dolinux.peng@...il.com>, Eduard Zingerman <eddyz87@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Alan Maguire <alan.maguire@...cle.com>, 
	LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Song Liu <song@...nel.org>, pengdonglin <pengdonglin@...omi.com>
Subject: Re: [RFC PATCH v2 2/5] btf: sort BTF types by kind and name to enable
 binary search

On Thu, Oct 23, 2025 at 11:37 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Oct 23, 2025 at 9:28 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> >
> > Speaking of flags, though. I think adding BTF_F_SORTED flag to
> > btf_header->flags seems useful, as that would allow libbpf (and user
> > space apps working with BTF in general) to use more optimal
> > find_by_name implementation. The only gotcha is that old kernels
> > enforce this btf_header->flags to be zero, so pahole would need to
> > know not to emit this when building BTF for old kernels (or, rather,
> > we'll just teach pahole_flags in kernel build scripts to add this
> > going forward). This is not very important for kernel, because kernel
> > has to validate all this anyways, but would allow saving time for user
> > space.
>
> Thinking more about it... I don't think it's worth it.
> It's an operational headache. I'd rather have newer pahole sort it
> without on/off flags and detection, so that people can upgrade
> pahole and build older kernels.
> Also BTF_F_SORTED doesn't spell out the way it's sorted.
> Things may change and we will need a new flag and so on.
> I think it's easier to check in the kernel and libbpf whether
> BTF is sorted the way they want it.
> The check is simple, fast and done once. Then both (kernel and libbpf) can
> set an internal flag and use different functions to search
> within a given BTF.

I guess that's fine. libbpf can do this check lazily on the first
btf__find_by_name() to avoid unnecessary overhead. Agreed.