Merge branch 'fixes-for-tailcall-with-subprog-and-callback'

Pu Lehui says:

====================
Fixes for tailcall with subprog and callback

From: Pu Lehui <pulehui@huawei.com>

This series fix two issues related to tailcall:

patch 1: Fix tail_call_reachable state leak across call branches.
patch 2: Reject callback subprogs invoke tailcall.
patch 3: testcases for patch 1 and 2.

v3:
- Fix tail_call_reachable state leak across call branches. (Sashiko)
- Add testcase for the above issue.

v2:
https://lore.kernel.org/bpf/20260714024459.420075-1-pulehui@huaweicloud.com
- Remove redantant testcase.
- Add ACK tag by Eduard.

v1:
https://lore.kernel.org/bpf/20260711104727.4023420-1-pulehui@huaweicloud.com
====================

Link: https://patch.msgid.link/20260716120157.835937-1-pulehui@huaweicloud.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

kernel/bpf/verifier.c

@@ -5237,10 +5237,6 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
 		if (verifier_bug_if(sidx < 0, env, "callee not found at insn %d", next_insn))
 			return -EFAULT;
 		if (subprog[sidx].is_async_cb) {
-			if (subprog[sidx].has_tail_call) {
-				verifier_bug(env, "subprog has tail_call and async cb");
-				return -EFAULT;
-			}
 			/* async callbacks don't increase bpf prog stack size unless called directly */
 			if (!bpf_pseudo_call(insn + i))
 				continue;
@@ -5263,8 +5259,8 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
 		if (!priv_stack_supported)
 			subprog[idx].priv_stack_mode = NO_PRIV_STACK;
 
-		if (subprog[idx].has_tail_call)
-			tail_call_reachable = true;
+		/* sync tail_call_reachable with callee state on entry */
+		tail_call_reachable = subprog[idx].has_tail_call;
 
 		frame = bpf_subprog_is_global(env, idx) ? 0 : frame + 1;
 		if (frame >= MAX_CALL_FRAMES) {
@@ -5281,8 +5277,8 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
 	 */
 	if (tail_call_reachable) {
 		for (tmp = idx; tmp >= 0; tmp = dinfo[tmp].caller) {
-			if (subprog[tmp].is_exception_cb) {
-				verbose(env, "cannot tail call within exception cb\n");
+			if (subprog[tmp].is_cb) {
+				verbose(env, "cannot tail call within callback\n");
 				return -EINVAL;
 			}
 			if (subprog[tmp].stack_arg_cnt) {

tools/testing/selftests/bpf/prog_tests/tailcalls.c

@@ -12,6 +12,7 @@
 #include "tailcall_cgrp_storage_no_storage.skel.h"
 #include "tailcall_cgrp_storage.skel.h"
 #include "tailcall_sleepable.skel.h"
+#include "tailcall_callback.skel.h"
 
 /* test_tailcall_1 checks basic functionality by patching multiple locations
  * in a single program for a single tail call slot with nop->jmp, jmp->nop
@@ -1901,6 +1902,11 @@ static void test_tailcall_sleepable(void)
 	tailcall_sleepable__destroy(skel);
 }
 
+static void test_tailcall_callback(void)
+{
+	RUN_TESTS(tailcall_callback);
+}
+
 void test_tailcalls(void)
 {
 	if (test__start_subtest("tailcall_1"))
@@ -1967,4 +1973,5 @@ void test_tailcalls(void)
 		test_tailcall_cgrp_storage_no_storage_leaf();
 	if (test__start_subtest("tailcall_cgrp_storage_no_storage_bridge"))
 		test_tailcall_cgrp_storage_no_storage_bridge();
+	test_tailcall_callback();
 }

tools/testing/selftests/bpf/progs/tailcall_callback.c

@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_test_utils.h"
+
+int classifier_0(struct __sk_buff *skb);
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(max_entries, 1);
+	__uint(key_size, sizeof(__u32));
+	__array(values, void (void));
+} jmp_table SEC(".maps") = {
+	.values = {
+		[0] = (void *) &classifier_0,
+	},
+};
+
+__auxiliary
+SEC("tc")
+int classifier_0(struct __sk_buff *skb)
+{
+	return 0;
+}
+
+static __noinline
+int subprog_tail0(struct __sk_buff *skb)
+{
+	int ret = 0;
+
+	bpf_tail_call_static(skb, &jmp_table, 0);
+	barrier_var(ret);
+	return ret;
+}
+
+static __noinline
+int callback_loop(int index, void **cb_ctx)
+{
+	int ret;
+
+	ret = subprog_tail0(*cb_ctx);
+	barrier_var(ret);
+	return ret ? 1 : 0;
+}
+
+static __noinline
+int callback_empty(int index, void *data)
+{
+	return 0;
+}
+
+/* callback involving subprog with tail call is rejected */
+SEC("tc")
+__failure __msg("cannot tail call within callback")
+int tailcall_callback_1(struct __sk_buff *skb)
+{
+	clobber_regs_stack();
+
+	bpf_loop(1, callback_loop, &skb, 0);
+	return 0;
+}
+
+/* subprogs with tailcall do not affect no-tailcall callback */
+SEC("tc")
+__success
+__retval(0)
+int tailcall_callback_2(struct __sk_buff *skb)
+{
+	int ret;
+
+	clobber_regs_stack();
+
+	ret = subprog_tail0(skb);
+	__sink(ret);
+
+	bpf_loop(1, callback_empty, NULL, 0);
+	return 0;
+}
+
+char __license[] SEC("license") = "GPL";