From: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
To: "Eduard Zingerman" <eddyz87@gmail.com>,
"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 23:20:59 +0000 [thread overview]
Message-ID: <DLN2XYLSE1PL.3HCQB74CH54GY@gmail.com> (raw)
In-Reply-To: <9d794d45da75370d010dd0234cbba5dea61c19ba.camel@gmail.com>
On Wed Sep 23, 2026 at 10:19 PM UTC, Eduard Zingerman wrote:
> On Wed, 2026-09-23 at 21:28 +0000, Alexei Starovoitov wrote:
>
> ...
>
>> imo the following is cleaner:
>> - do not repurpose bpf_throw() for this new thing.
>> Introduce new kfunc that will do the unwind, let's name it bpf_unwind() ?
>> - replace all 'call bpf_unwind_resume' with NOP by the verifier (or may with bpf_exit. tbd)
>> Technically rustc or llvm can do that too, but it's cleaner to do in the verifier.
>> - In bpf_unwind() walk all exception tables and replace return addresses
>> in corresponding frames to landing_pad_ip-s.
>> - just return from bpf_unwind().
>> restoring callee saved registers will happen automatically by corresponding
>> frames and there is no need to search exception tables at each step.
>> That's what typical eh unwinder does, but it's doing it due to C++ logic
>> that is more complex that Rust. For Rust unwinds we don't need all that.
>> The above algorithm will do.
>
> +1, makes sense.
> After return address rewrite the unwind would have to jump to it's epilogue, right?
yes. I feel there will be no need in asm() tricks in such kfunc.
unwind will rewrite its own return address, and will return through normal C
and will restore callee saved (whatever it needed),
then immediate callee in bpf prog should have had a landing pad for this kfunc,
since this kfunc is throwing.
so bpf_unwind() will jump to whatever is necessary to unwind in that bpf prog.
And so on till the end.
> Why not reusing bpf_throw() though, is it because of the exception cb mechanics?
yes. bpf_throw() is not seen as throwing from compiler pov.
I believe all kfuncs are not throwing. (that was a bug in this patch I alluded earlier).
So we need a new kfunc and mark it attr(may_throw) or whatever the attr is called,
so that compiler will generate exception tables for that bpf prog.
Last time I checked there were bpf_cleanup section for rust-c in that case,
but that was back in April.
There is also a case of 'catching' abort in rust. iirc it only stops unwind and
rust-c generates something else instead of 'call bpf_unwind_resume'.
Memory is vague. I think that frame will continue as normal after processing
landing pad drop()s.
next prev parent reply other threads:[~2026-09-23 23:21 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
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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=DLN2XYLSE1PL.3HCQB74CH54GY@gmail.com \
--to=alexei.starovoitov@gmail.com \
--cc=andrii@kernel.org \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=eddyz87@gmail.com \
--cc=kernel-team@fb.com \
--cc=yonghong.song@linux.dev \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox