Re: [RFC PATCH bpf-next 00/12] bpf: Introduce static-defined tracing probe for BPF

From: Xu Kuohai

Date: Mon Jun 29 2026 - 07:37:42 EST

On 6/29/2026 6:55 PM, Leon Hwang wrote:
On 29/6/26 15:51, Xu Kuohai wrote:

With the elimination, the callsite of 'subprog' has been eliminated.

However, as you mentioned above, the argument preparation insns are kept.


I see, the CALL insntruction is removed, not patched to NOP.

So the call site is either a call to subprog, or totally removed. The
fentry
tracing only works when the CALL instruction and the subprog are not
eliminated.
This is exactly what SDT tries to solve.

So, why not add a macro for the stub following the subprog+fentry idea?

#define DEFINE_TRACING_STUB(__stub, ...) \
static __noinline void __stub(...) \
{ \
... \
}

DEFINE_TRACING_STUB(my_trace, int, len, int, ret);

SEC("xdp")
int xdp_prog(struct xdp_md *ctx)
{
...
my_trace(len, ret);
...
}

Then, add a libbpf API bpf_program__remove_stub(prog, my_stub) to drop
the stub by the way like my demo.


Since the stub prog is dropped in libbpf, where is the fentry prog attached?

Thanks,
Leon