For now, the offset of the return value in trampoline is fixed 8-bytes.
In this commit, we introduce the variable "ret_off" to represent the
offset of the return value. For now, the "ret_off" is just 8. And in the
following patch, we will make it something else to use the room after it.

Signed-off-by: Menglong Dong <[email protected]>

Add BPF_TRACE_SESSION supporting to x86_64. invoke_bpf_session_entry and
invoke_bpf_session_exit is introduced for this purpose.

In invoke_bpf_session_entry(), we will check if the return value of the
fentry is 0, and set the corresponding session flag if not. And in
invoke_bpf_session_exit(), we will check if the corresponding flag is
set. If set, the fexit will be skipped.

As designed, the session flags and session cookie address is stored after
the return value, and the stack look like this:

  cookie ptr	-> 8 bytes
  session flags	-> 8 bytes
  return value	-> 8 bytes
  argN		-> 8 bytes
  ...
  arg1		-> 8 bytes
  nr_args	-> 8 bytes
  ...
  cookieN	-> 8 bytes
  cookie1	-> 8 bytes

In the entry of the session, we will clear the return value, so the fentry
will always get 0 with ctx[nr_args] or bpf_get_func_ret().

Before the execution of the BPF prog, the "cookie ptr" will be filled with
the corresponding cookie address, which is done in
invoke_bpf_session_entry() and invoke_bpf_session_exit().

Signed-off-by: Menglong Dong <[email protected]>
Co-developed-by: Leon Hwang <[email protected]>
Signed-off-by: Leon Hwang <[email protected]>

Add BPF_TRACE_SESSION to libbpf and bpftool.

Signed-off-by: Menglong Dong <[email protected]>

Add testcases for BPF_TRACE_SESSION. The function arguments and return
value are tested both in the entry and exit. And the kfunc
bpf_tracing_is_exit() is also tested.

As the layout of the stack changed for fsession, so we also test
bpf_get_func_ip() for it.

Session cookie for fsession is also tested. Multiple fsession BPF progs is
attached to bpf_fentry_test1() and session cookie is read and write in
the testcase.

Signed-off-by: Menglong Dong <[email protected]>

Test the fsession when it is used together with fentry, fexit.

Signed-off-by: Menglong Dong <[email protected]>