selftests/bpf: Test gen_pro/epilogue that generate kfuncs

Test gen_prologue and gen_epilogue that generate kfuncs that have not
been seen in the main program.

The main bpf program and return value checks are identical to
pro_epilogue.c introduced in commit 47e69431b57a ("selftests/bpf: Test
gen_prologue and gen_epilogue"). However, now when bpf_testmod_st_ops
detects a program name with prefix "test_kfunc_", it generates slightly
different prologue and epilogue: They still add 1000 to args->a in
prologue, add 10000 to args->a and set r0 to 2 * args->a in epilogue,
but involve kfuncs.

At high level, the alternative version of prologue and epilogue look
like this:

  cgrp = bpf_cgroup_from_id(0);
  if (cgrp)
          bpf_cgroup_release(cgrp);
  else
          /* Perform what original bpf_testmod_st_ops prologue or
           * epilogue does
           */

Since 0 is never a valid cgroup id, the original prologue or epilogue
logic will be performed. As a result, the __retval check should expect
the exact same return value.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20250225233545.285481-2-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 12426e10baf3..519e3f5e9c10 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -606,6 +606,7 @@ s32 bpf_find_btf_id(const char *name, u32 kind, struct btf **btf_p)
 	spin_unlock_bh(&btf_idr_lock);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(bpf_find_btf_id);
 
 const struct btf_type *btf_type_skip_modifiers(const struct btf *btf,
 					       u32 id, u32 *res_id)