bpf: support bpf_fastcall patterns for kfuncs

Recognize bpf_fastcall patterns around kfunc calls.
For example, suppose bpf_cast_to_kern_ctx() follows bpf_fastcall
contract (which it does), in such a case allow verifier to rewrite BPF
program below:

  r2 = 1;
  *(u64 *)(r10 - 32) = r2;
  call %[bpf_cast_to_kern_ctx];
  r2 = *(u64 *)(r10 - 32);
  r0 = r2;

By removing the spill/fill pair:

  r2 = 1;
  call %[bpf_cast_to_kern_ctx];
  r0 = r2;
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240822084112.3257995-4-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -16125,7 +16125,7 @@ static int visit_func_call_insn(int t, struct bpf_insn *insns,
  */
 static u32 helper_fastcall_clobber_mask(const struct bpf_func_proto *fn)
 {
-	u8 mask;
+	u32 mask;
 	int i;
 
 	mask = 0;
@@ -16153,6 +16153,26 @@ static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
 	}
 }
 
+/* Same as helper_fastcall_clobber_mask() but for kfuncs, see comment above */
+static u32 kfunc_fastcall_clobber_mask(struct bpf_kfunc_call_arg_meta *meta)
+{
+	u32 vlen, i, mask;
+
+	vlen = btf_type_vlen(meta->func_proto);
+	mask = 0;
+	if (!btf_type_is_void(btf_type_by_id(meta->btf, meta->func_proto->type)))
+		mask |= BIT(BPF_REG_0);
+	for (i = 0; i < vlen; ++i)
+		mask |= BIT(BPF_REG_1 + i);
+	return mask;
+}
+
+/* Same as verifier_inlines_helper_call() but for kfuncs, see comment above */
+static bool is_fastcall_kfunc_call(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return false;
+}
+
 /* LLVM define a bpf_fastcall function attribute.
  * This attribute means that function scratches only some of
  * the caller saved registers defined by ABI.
@@ -16250,6 +16270,19 @@ static void mark_fastcall_pattern_for_call(struct bpf_verifier_env *env,
 				  bpf_jit_inlines_helper_call(call->imm));
 	}
 
+	if (bpf_pseudo_kfunc_call(call)) {
+		struct bpf_kfunc_call_arg_meta meta;
+		int err;
+
+		err = fetch_kfunc_meta(env, call, &meta, NULL);
+		if (err < 0)
+			/* error would be reported later */
+			return;
+
+		clobbered_regs_mask = kfunc_fastcall_clobber_mask(&meta);
+		can_be_inlined = is_fastcall_kfunc_call(&meta);
+	}
+
 	if (clobbered_regs_mask == ALL_CALLER_SAVED_REGS)
 		return;