linux-kernel - Re: [PATCH bpf-next v4 2/4] bpf: Add helper to detect indirect jump targets

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Message-ID: <21aec5e1-4152-4d51-ad25-91524c544b66@huaweicloud.com>
Date: Thu, 15 Jan 2026 15:47:25 +0800
From: Xu Kuohai <xukuohai@...weicloud.com>
To: Eduard Zingerman <eddyz87@...il.com>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>,
 Yonghong Song <yonghong.song@...ux.dev>, Puranjay Mohan
 <puranjay@...nel.org>, Anton Protopopov <a.s.protopopov@...il.com>
Subject: Re: [PATCH bpf-next v4 2/4] bpf: Add helper to detect indirect jump
 targets

On 1/15/2026 4:46 AM, Eduard Zingerman wrote:
> On Wed, 2026-01-14 at 17:39 +0800, Xu Kuohai wrote:
>> From: Xu Kuohai <xukuohai@...wei.com>
>>
>> Introduce helper bpf_insn_is_indirect_target to determine whether a BPF
>> instruction is an indirect jump target. This helper will be used by
>> follow-up patches to decide where to emit indirect landing pad instructions.
>>
>> Add a new flag to struct bpf_insn_aux_data to mark instructions that are
>> indirect jump targets. The BPF verifier sets this flag, and the helper
>> checks it to determine whether an instruction is an indirect jump target.
>>
>> Since bpf_insn_aux_data is only available before JIT stage, add a new
>> field to struct bpf_prog_aux to store a pointer to the bpf_insn_aux_data
>> array, making it accessible to the JIT.
>>
>> For programs with multiple subprogs, each subprog uses its own private
>> copy of insn_aux_data, since subprogs may insert additional instructions
>> during JIT and need to update the array. For non-subprog, the verifier's
>> insn_aux_data array is used directly to avoid unnecessary copying.
>>
>> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
>> ---
> 
> Hm, I've missed the fact insn_aux_data is not currently available to jit.
> Is it really necessary to copy this array for each subprogram?
> Given that we still want to free insn_aux_data after program load,
> I'd expect that it should be possible just to pass a pointer with an
> offset pointing to a start of specific subprogram. Wdyt?
>

I think it requires an additional field in struct bpf_prog to record the length
of the global insn_aux_data array. If a subprog inserts new instructions during
JIT (e.g., due to constant blinding), all entries in the array, including those
of the subsequent subprogs, would need to be adjusted. With per-subprog copying,
only the local insn_aux_data needs to be updated, reducing the amount of copying.

However, if you prefer a global array, I’m happy to switch to it.

> [...]