RE: [PATCH] bpf/arm64: support packet data load instructions

Marat Khalili
2026-03-18T16:16:59Z

> > In Classic BPF, negative "k" has special meaning for both BPF_ABS and BPF_IND.
> > So we should consider it invalid for both cases.
> >
> > That prevents applications from using it the way you describe.
> > And it will allow us to add BPF library support for Linux-compatible special meanings later, without
> breaking the ABI.
> >
>
> Aren't these invalid offsets already taken care during the syntax
> check when we validate the BPF program ?
> in bpf_validate.c +1499:
> /* load absolute instructions */
> [(BPF_LD | BPF_ABS | BPF_B)] = {
> .mask = {. dreg = ZERO_REG, .sreg = ZERO_REG},
> .off = { .min = 0, .max = 0},
> .imm = { .min = 0, .max = INT32_MAX},
> .eval = eval_ld_mbuf,
> },
>
> IIUC, as __rte_bpf_validate fails when we cal rte_bpf_load ( in
> bpf_load.c +113), we can't even interpret the cBPF program.

Good point, we can probably consider BPF_ABS case covered by this.

For BPF_IND however it does not seem to exclude any values, and even if it did
we don't know what's in the register. Speaking of which, I just noticed that
we're truncating it.

I suggest the following logic in pseudo-code:

static void
emit_ld_mbuf(struct a64_jit_ctx *ctx, uint32_t op, uint8_t tmp1, uint8_t tmp2,
uint8_t src, int32_t imm)
{
// ...

/* r1 = off: for ABS use imm, for IND use src + imm */
if (mode == BPF_ABS) {
assert imm >= 0, "verified by verifier"
emit MOV W1, #<imm>
} else {
/* add signed imm to the source register */
emit(s) X1 = src + #<imm>
/* verify dynamically that offset is within the domain of __rte_pktmbuf_read */
emit(s) jump_to_epilogue if X1 <s 0 || X1 > INT32_MAX
}

// ...
}