linux-kernel - Re: [PATCH v5 6/7] btf: Add lazy sorting validation for binary search

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Message-ID: <CAADnVQKbgno=yGjshJpo+fwRDMTfXXVPWq0eh7avBj154dCq_g@mail.gmail.com>
Date: Fri, 7 Nov 2025 11:01:41 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Donglin Peng <dolinux.peng@...il.com>, bot+bpf-ci@...nel.org, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii.nakryiko@...il.com>, zhangxiaoqin@...omi.com, 
	LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Alan Maguire <alan.maguire@...cle.com>, Song Liu <song@...nel.org>, 
	pengdonglin <pengdonglin@...omi.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, Chris Mason <clm@...a.com>, 
	Ihor Solodrai <ihor.solodrai@...ux.dev>
Subject: Re: [PATCH v5 6/7] btf: Add lazy sorting validation for binary search

On Fri, Nov 7, 2025 at 10:58 AM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Fri, 2025-11-07 at 10:54 -0800, Alexei Starovoitov wrote:
>
> [...]
>
> > > > > > @@ -610,7 +674,7 @@ s32 btf_find_by_name_kind(const struct
> > > > > > btf
> > > > > > *btf, const char *name, u8 kind)
> > > > > >                       goto out;
> > > > > >       }
> > > > > >
> > > > > > -     if (btf->nr_sorted_types != BTF_NEED_SORT_CHECK) {
> > > > > > +     if (btf_check_sorted((struct btf *)btf)) {
> > > > >                                   ^
> > > > >
> > > > > The const cast here enables the concurrent writes discussed
> > > > > above.
> > > > > Is
> > > > > there a reason to mark the btf parameter as const if we're
> > > > > modifying it?
> > > >
> > > > Hi team, is casting away const an acceptable approach for our
> > > > codebase?
> > >
> > > Casting away const is undefined behaviour, e.g. see paragraph
> > > 6.7.3.6
> > > N1570 ISO/IEC 9899:201x Programming languages — C.
> > >
> > > Both of the problems above can be avoided if kernel will do sorted
> > > check non-lazily. But Andrii and Alexei seem to like that property.
> >
> > Ihor is going to move BTF manipulations into resolve_btfid.
> > Sorting of BTF should be in resolve_btfid as well.
> > This way the build process will guarantee that BTF is sorted
> > to the kernel liking. So the kernel doesn't even need to check
> > that BTF is sorted.
>
> This would be great.
> Does this imply that module BTFs are sorted too?

Yes. The module build is supposed to use the kernel build tree where
kernel BTF expectations will match resolve_btfid actions.
Just like compiler and config flags should be the same.