netdev - Re: [PATCH bpf-next 0/4] bpf: tailcall: Eliminate max_entries and bpf

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aWd9z8GVYO12YsaH@krava>
Date: Wed, 14 Jan 2026 12:28:15 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Leon Hwang <leon.hwang@...ux.dev>, bpf <bpf@...r.kernel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Puranjay Mohan <puranjay@...nel.org>,
	Xu Kuohai <xukuohai@...weicloud.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	"David S . Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	kernel-patches-bot@...com
Subject: Re: [PATCH bpf-next 0/4] bpf: tailcall: Eliminate max_entries and
 bpf_func access at runtime

On Fri, Jan 02, 2026 at 04:10:01PM -0800, Alexei Starovoitov wrote:
> On Fri, Jan 2, 2026 at 7:01 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
> >
> > This patch series optimizes BPF tail calls on x86_64 and arm64 by
> > eliminating runtime memory accesses for max_entries and 'prog->bpf_func'
> > when the prog array map is known at verification time.
> >
> > Currently, every tail call requires:
> >   1. Loading max_entries from the prog array map
> >   2. Dereferencing 'prog->bpf_func' to get the target address
> >
> > This series introduces a mechanism to precompute and cache the tail call
> > target addresses (bpf_func + prologue_offset) in the prog array itself:
> >   array->ptrs[max_entries + index] = prog->bpf_func + prologue_offset
> >
> > When a program is added to or removed from the prog array, the cached
> > target is atomically updated via xchg().
> >
> > The verifier now encodes additional information in the tail call
> > instruction's imm field:
> >   - bits 0-7:   map index in used_maps[]
> >   - bits 8-15:  dynamic array flag (1 if map pointer is poisoned)
> >   - bits 16-31: poke table index + 1 for direct tail calls
> >
> > For static tail calls (map known at verification time):
> >   - max_entries is embedded as an immediate in the comparison instruction
> >   - The cached target from array->ptrs[max_entries + index] is used
> >     directly, avoiding the 'prog->bpf_func' dereference
> >
> > For dynamic tail calls (map pointer poisoned):
> >   - Fall back to runtime lookup of max_entries and prog->bpf_func
> >
> > This reduces cache misses and improves tail call performance for the
> > common case where the prog array is statically known.
> 
> Sorry, I don't like this. tail_calls are complex enough and
> I'd rather let them be as-is and deprecate their usage altogether
> instead of trying to optimize them in certain conditions.
> We have indirect jumps now. The next step is indirect calls.
> When it lands there will be no need to use tail_calls.
> Consider tail_calls to be legacy. No reason to improve them.

hi,
I'd like to make tail calls available in sleepable programs. I still
need to check if there's technical reason we don't have that, but seeing
this answer I wonder you'd be against that anyway ?

fyi I briefly discussed that with Andrii indicating that it might not
be worth the effort at this stage.

thanks,
jirka