bpf: Resolve to prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT

The commit 7e40781c ("bpf: verifier: Use target program's type for access verifications")
fixes the verifier checking for BPF_PROG_TYPE_EXT (extension)
prog such that the verifier looks for things based
on the target prog type that it is extending instead of
the BPF_PROG_TYPE_EXT itself.

The current resolve_prog_type() returns the target prog type.
It checks for nullness on prog->aux->dst_prog.  However,
when loading a BPF_PROG_TYPE_TRACING prog and it is tracing another
bpf prog instead of a kernel function, prog->aux->dst_prog is not
NULL also.  In this case, the verifier should still verify as the
BPF_PROG_TYPE_TRACING type instead of the traced prog type in
prog->aux->dst_prog->type.

An oops has been reported when tracing a struct_ops prog.  A NULL
dereference happened in check_return_code() when accessing the
prog->aux->attach_func_proto->type and prog->aux->attach_func_proto
is NULL here because the traced struct_ops prog has the "unreliable" set.

This patch is to change the resolve_prog_type() to only
return the target prog type if the prog being verified is
BPF_PROG_TYPE_EXT.

Fixes: 7e40781c

 ("bpf: verifier: Use target program's type for access verifications")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220330011456.2984509-1-kafai@fb.com