Message-ID: <32bd1c9d-ef1a-4e97-80b5-a069ce28125f@huaweicloud.com>
Date: Mon, 19 Jan 2026 10:35:02 +0800
From: Xu Kuohai <xukuohai@...weicloud.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: Eduard Zingerman <eddyz87@...il.com>, bpf <bpf@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
 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 1/19/2026 1:20 AM, Alexei Starovoitov wrote:
> On Wed, Jan 14, 2026 at 11:47 PM Xu Kuohai <xukuohai@...weicloud.com> wrote:
>>
>> 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.
> 
> iirc we struggled with lack of env/insn_aux in JIT earlier.
> 
> func[i]->aux->used_maps = env->used_maps;
> is one such example.
> 
> Let's move bpf_prog_select_runtime() into bpf_check() and
> consistently pass 'env' into bpf_int_jit_compile() while
> env is still valid.
> Close to jit_subprogs().
> Or remove bpf_prog_select_runtime() and make jit_subprogs()
> do the whole thing. tbd.
> 
> This way we can remove used_maps workaround and don't need to do
> this insn_aux copy.
> Errors during JIT can be printed into the verifier log too.
>

Sounds great. Using jit_subprogs for the whole thing seems cleaner. I'll
try this approach first.

> Kumar,
> what do you think about it from modularization pov ?

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