bpf: Fix updating attached freplace prog in prog_array map

The commit f7866c3587 ("bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT")
fixed a NULL pointer dereference panic, but didn't fix the issue that
fails to update attached freplace prog to prog_array map.

Since commit 1c123c567f ("bpf: Resolve fext program type when checking map compatibility"),
freplace prog and its target prog are able to tail call each other.

And the commit 3aac1ead5e ("bpf: Move prog->aux->linked_prog and trampoline into bpf_link on attach")
sets prog->aux->dst_prog as NULL after attaching freplace prog to its
target prog.

After loading freplace the prog_array's owner type is BPF_PROG_TYPE_SCHED_CLS.
Then, after attaching freplace its prog->aux->dst_prog is NULL.
Then, while updating freplace in prog_array the bpf_prog_map_compatible()
incorrectly returns false because resolve_prog_type() returns
BPF_PROG_TYPE_EXT instead of BPF_PROG_TYPE_SCHED_CLS.
After this patch the resolve_prog_type() returns BPF_PROG_TYPE_SCHED_CLS
and update to prog_array can succeed.

Fixes: f7866c3587 ("bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT")
Cc: Toke Høiland-Jørgensen <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Signed-off-by: Leon Hwang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

include/linux/bpf_verifier.h

@@ -856,8 +856,8 @@ static inline u32 type_flag(u32 type)
 /* only use after check_attach_btf_id() */
 static inline enum bpf_prog_type resolve_prog_type(const struct bpf_prog *prog)
 {
-	return (prog->type == BPF_PROG_TYPE_EXT && prog->aux->dst_prog) ?
-		prog->aux->dst_prog->type : prog->type;
+	return (prog->type == BPF_PROG_TYPE_EXT && prog->aux->saved_dst_prog_type) ?
+		prog->aux->saved_dst_prog_type : prog->type;
 }
 
 static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)