From: Eduard Zingerman <eddyz87@gmail.com>
To: Yonghong Song <yonghong.song@linux.dev>, bpf@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
kernel-team@fb.com
Subject: Re: [PATCH bpf-next v5 07/21] bpf: Explore the landing pads no call site reaches
Date: Wed, 23 Sep 2026 09:26:19 -0700 [thread overview]
Message-ID: <df24934c5ecbd70bc35b528b8e58ae668ce02051.camel@gmail.com> (raw)
In-Reply-To: <20260923045922.2417689-1-yonghong.song@linux.dev>
On Tue, 2026-09-22 at 21:59 -0700, Yonghong Song wrote:
> A cleanup record need not cover a call an exception can unwind out of: a
> frontend is free to emit a region around a helper or an ordinary kfunc,
> both nounwind here. Nothing marks a call site then, and that record's
> landing pad is reached by nothing at all -- leaving bpf_check_cfg() to
> refuse the program over code its own frontend had no way not to emit:
>
> 0: call bpf_preempt_disable
> 1: call bpf_preempt_enable record = { begin = 1, end = 2, pad = 4 }
> 2: r0 = 0
> 3: exit
> 4: r1 = pads_ran ll landing pad
> 6: r2 = *(u64 *)(r1 + 0)
> 7: r2 |= RAN_NOUNWIND_REC
> 8: *(u64 *)(r1 + 0) = r2
> 9: call bpf_unwind_resume
> 10: exit
>
> The range [1,2) holds one call, and it is a kfunc, so an exception cannot
> come out of it. mark_call_sites() marks nothing, nothing pushes an edge to
> 4, and 4 through 10 are reachable from nothing: "unreachable insn 4". The
> pad is dead, which is correct -- no exception can ever arrive at it -- but
> the program is fine and has to load.
>
> Walk every pad the table names that the edges did not reach, the way the
> walk is already re-seeded at an exception callback. From there the pad is
> code like any other: do_check() never enters it, because no call site
> dispatches to it, so the dead code sweep removes it along with everything
> else that was not reached.
>
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
Yonghong, in v4 you said that rustc does not generate dead landing pads.
Why do we need this patch?
...
next prev parent reply other threads:[~2026-09-23 16:26 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-23 4:58 [PATCH bpf-next v5 00/21] bpf: Run exception cleanup landing pads when bpf_throw() unwinds Yonghong Song
2026-09-23 4:58 ` [PATCH bpf-next v5 01/21] bpf: Pack bpf_insn_aux_data flags into bit fields Yonghong Song
2026-09-23 5:40 ` bot+bpf-ci
2026-09-23 19:16 ` Eduard Zingerman
2026-09-23 4:58 ` [PATCH bpf-next v5 02/21] bpf: Accept the compiler's exception cleanup table at program load Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 03/21] bpf: Add the bpf_unwind_resume() kfunc Yonghong Song
2026-09-23 6:13 ` Kumar Kartikeya Dwivedi
2026-09-23 4:59 ` [PATCH bpf-next v5 04/21] bpf: Add lookups for exception cleanup resumes and landing pads Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 05/21] bpf: Prepare for an exception cleanup table before the CFG walk Yonghong Song
2026-09-23 5:40 ` bot+bpf-ci
2026-09-23 4:59 ` [PATCH bpf-next v5 06/21] bpf: Make exception landing pads reachable in the CFG Yonghong Song
2026-09-23 5:51 ` bot+bpf-ci
2026-09-23 19:25 ` Eduard Zingerman
2026-09-23 4:59 ` [PATCH bpf-next v5 07/21] bpf: Explore the landing pads no call site reaches Yonghong Song
2026-09-23 16:26 ` Eduard Zingerman [this message]
2026-09-23 17:27 ` Yonghong Song
2026-09-23 18:06 ` Eduard Zingerman
2026-09-23 18:09 ` Yonghong Song
2026-09-23 21:28 ` Alexei Starovoitov
2026-09-23 22:19 ` Eduard Zingerman
2026-09-23 23:20 ` Alexei Starovoitov
2026-09-24 0:13 ` Kumar Kartikeya Dwivedi
2026-09-24 1:53 ` Alexei Starovoitov
2026-09-24 6:12 ` Kumar Kartikeya Dwivedi
2026-09-23 23:09 ` Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 08/21] bpf: Walk the exception unwind in the verifier Yonghong Song
2026-09-23 16:34 ` Eduard Zingerman
2026-09-23 18:01 ` Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 09/21] bpf: Refuse exception cleanup shapes bpf_throw() cannot dispatch Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 10/21] bpf: Refuse a private stack for a program with an exception cleanup table Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 11/21] bpf: Dispatch exception cleanup pads from bpf_throw() Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 12/21] bpf, x86: Dispatch exception cleanup pads at run time Yonghong Song
2026-09-23 6:45 ` Eduard Zingerman
2026-09-23 20:45 ` Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 13/21] bpf, arm64: " Yonghong Song
2026-09-23 4:59 ` [PATCH bpf-next v5 14/21] libbpf: Resolve the compiler's _Unwind_Resume to the kernel's kfunc Yonghong Song
2026-09-23 8:36 ` Kumar Kartikeya Dwivedi
2026-09-23 5:00 ` [PATCH bpf-next v5 15/21] libbpf: Add cleanup_info to bpf_prog_load_opts Yonghong Song
2026-09-23 5:00 ` [PATCH bpf-next v5 16/21] libbpf: Collect .bpf_cleanup records and pass them to the kernel Yonghong Song
2026-09-23 5:52 ` Kumar Kartikeya Dwivedi
2026-09-23 5:00 ` [PATCH bpf-next v5 17/21] libbpf: Carry the exception cleanup table through the light skeleton Yonghong Song
2026-09-23 5:00 ` [PATCH bpf-next v5 18/21] libbpf: Let the static linker carry .bpf_cleanup relocations Yonghong Song
2026-09-23 5:40 ` bot+bpf-ci
2026-09-23 5:00 ` [PATCH bpf-next v5 19/21] selftests/bpf: Add an end-to-end .bpf_cleanup exception test Yonghong Song
2026-09-23 5:51 ` bot+bpf-ci
2026-09-23 5:00 ` [PATCH bpf-next v5 20/21] selftests/bpf: Cover the exception cleanup shapes the chain does not reach Yonghong Song
2026-09-23 5:51 ` bot+bpf-ci
2026-09-23 7:50 ` Eduard Zingerman
2026-09-23 5:00 ` [PATCH bpf-next v5 21/21] selftests/bpf: Load an exception cleanup program from a light skeleton Yonghong Song
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