On 2026/1/14 11:27 Menglong Dong [off-list ref] write:

On 2026/1/14 09:25 Andrii Nakryiko [off-list ref] write:

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On Sat, Jan 10, 2026 at 6:12 AM Menglong Dong [off-list ref] wrote:

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[...]

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+       if (bpf_fsession_cnt(tlinks)) {
+               /* clear all the session cookies' value */
+               for (int i = 0; i < cookie_cnt; i++)
+                       emit_store_stack_imm64(&prog, cookie_off - 8 * i, 0);
+               /* clear the return value to make sure fentry always get 0 */
+               emit_store_stack_imm64(&prog, 8, 0);
+       }
+       func_meta = nr_regs + (((cookie_off - regs_off) / 8) << BPF_TRAMP_M_COOKIE);

func_meta conceptually is a collection of bit fields, so using +/-
feels weird, use | and &, more in line with working with bits?

It's not only for bit fields. For nr_args and cookie offset, they are
byte fields. Especially for cookie offset, arithmetic operation is performed
too. So I think it make sense here, right?

Oh, I see what you mean now. It's OK to use "&" instead of "+"
here. I were explaining the decreasing of func_meta in invoke_bpf().
That can use "&/|" too, but use "-/+" can make the code much
simpler.

Thanks!
Menglong Dong

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(also you defined that BPF_TRAMP_M_NR_ARGS but you are not using it
consistently...)

I'm not sure if we should define it. As we use the least significant byte for
the nr_args, the shift for it is always 0. If we use it in the inline, unnecessary
instruction will be generated, which is the bit shift instruction.


I defined it here for better code reading. Maybe we can do some comment
in the inline of bpf_get_func_arg(), instead of defining such a unused
macro?


Thanks!
Menglong Dong

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+
        if (fentry->nr_links) {
                if (invoke_bpf(m, &prog, fentry, regs_off, run_ctx_off,
-                              flags & BPF_TRAMP_F_RET_FENTRY_RET, image, rw_image))
+                              flags & BPF_TRAMP_F_RET_FENTRY_RET, image, rw_image,
+                              func_meta))
                        return -EINVAL;
        }

@@ -3445,9 +3467,14 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
                }
        }

+       /* set the "is_return" flag for fsession */
+       func_meta += (1 << BPF_TRAMP_M_IS_RETURN);
+       if (bpf_fsession_cnt(tlinks))
+               emit_store_stack_imm64(&prog, nregs_off, func_meta);
+
        if (fexit->nr_links) {
                if (invoke_bpf(m, &prog, fexit, regs_off, run_ctx_off,
-                              false, image, rw_image)) {
+                              false, image, rw_image, func_meta)) {
                        ret = -EINVAL;
                        goto cleanup;
                }
--

2.52.0