Re: [PATCH bpf-next v4 2/8] bpf: Introduce global percpu data

[Leon Hwang <leon.hwang@linux.dev>]

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On Mon, 20 Apr 2026 18:59:39 -0700, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> On Mon, Apr 20, 2026 at 6:43 PM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> On Mon, 20 Apr 2026 07:58:06 -0700, "Alexei Starovoitov" <alexei.starovoitov@gmail.com> wrote:
>>> On Sun Apr 19, 2026 at 10:24 PM PDT, Leon Hwang wrote:
>>>>
>>>> int xdp_prog(struct xdp_md * ctx):
>>>> ; cnt++;
>>>> 0: (18) r6 = map[id:28][0]+0
>>>> 2: (bf) r6 = &(void __percpu *)(r6)
>>>
>>> well. that insn was inserted by the verifier and it shows up in xlated.
>>> That was expected.
>>> The point about 'bogus xlated' was about offset translation.
>>> map_direct_value_meta() should recover proper insns[i + 1].imm = off;
>>> In your example it's zero, so not an interesting test.
>>>
>>
>> The original insns[i + 1] wasn't broken. See the key change of this
>> patch.
>
> You're missing the point insns[i + 1].imm = off is how
> off returned from map_direct_value_meta() is used in updating _this_
> ldimm64 insn.

After looking into bpf_insn_prepare_dump(), agreed with
"not an interesting test".

Then, updated the selftest with non-zero offset. And updated
disasm_helpers.c::disasm_insn() to dump such ldimm64 insn as map info by
the way of bpftool/xlated_dumper.c::print_imm().

+static volatile const char fmt2[] SEC(".percpu.fmt") = "data %d\n";
+
+SEC("?kprobe")
+__success
+__xlated("r{{[0-9]+}} = map[id:{{[0-9]+}}][0]+9")
+__xlated("r{{[0-9]+}} = &(void __percpu *)(r{{[0-9]+}})")
+__xlated("r{{[0-9]+}} = *(u8 *)(r{{[0-9]+}} +4)")
+int verifier_percpu_read(void *ctx)
+{
+	char c = fmt2[4];
+
+	return c == ' ';
+}

It would verify the following xlated insns.

XLATED:
=============
0: r1 = map[id:7][0]+9
2: r1 = &(void __percpu *)(r1)
3: r1 = *(u8 *)(r1 +4)
4: w0 = 1
5: if w1 == 0x20 goto pc+1
6: w0 = 0
7: exit

The selftest looks better now.

Thanks,
Leon