bpf, x64: Add predicate for bpf2bpf with tailcalls support in JIT

The BPF core/verifier is hard-coded to permit mixing bpf2bpf and tail
calls for only x86-64. Change the logic to instead rely on a new weak
function 'bool bpf_jit_supports_subprog_tailcalls(void)', which a capable
JIT backend can override.

Update the x86-64 eBPF JIT to reflect this.
Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
[jakub: drop MIPS bits and tweak patch subject]
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220617105735.733938-2-jakub@cloudflare.com

arch/x86/net/bpf_jit_comp.c

@@ -2491,3 +2491,9 @@ void *bpf_arch_text_copy(void *dst, void *src, size_t len)
 		return ERR_PTR(-EINVAL);
 	return dst;
 }
+
+/* Indicate the JIT backend supports mixing bpf2bpf and tailcalls. */
+bool bpf_jit_supports_subprog_tailcalls(void)
+{
+	return true;
+}

include/linux/filter.h

@@ -914,6 +914,7 @@ u64 __bpf_call_base(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
 struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog);
 void bpf_jit_compile(struct bpf_prog *prog);
 bool bpf_jit_needs_zext(void);
+bool bpf_jit_supports_subprog_tailcalls(void);
 bool bpf_jit_supports_kfunc_call(void);
 bool bpf_helper_changes_pkt_data(void *func);
 

kernel/bpf/core.c

@@ -2729,6 +2729,12 @@ bool __weak bpf_jit_needs_zext(void)
 	return false;
 }
 
+/* Return TRUE if the JIT backend supports mixing bpf2bpf and tailcalls. */
+bool __weak bpf_jit_supports_subprog_tailcalls(void)
+{
+	return false;
+}
+
 bool __weak bpf_jit_supports_kfunc_call(void)
 {
 	return false;

kernel/bpf/verifier.c

@@ -6154,7 +6154,8 @@ static bool may_update_sockmap(struct bpf_verifier_env *env, int func_id)
 
 static bool allow_tail_call_in_subprogs(struct bpf_verifier_env *env)
 {
-	return env->prog->jit_requested && IS_ENABLED(CONFIG_X86_64);
+	return env->prog->jit_requested &&
+	       bpf_jit_supports_subprog_tailcalls();
 }
 
 static int check_map_func_compatibility(struct bpf_verifier_env *env,