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

[Xu Kuohai]

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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > ---
>
> 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.

> [...]