Message-ID: <20251217095445.218428-5-dongml2@chinatelecom.cn>
Date: Wed, 17 Dec 2025 17:54:40 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: ast@...nel.org,
	andrii@...nel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	daniel@...earbox.net,
	martin.lau@...ux.dev,
	eddyz87@...il.com,
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	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next v4 4/9] bpf: add the kfunc bpf_fsession_cookie

Implement session cookie for fsession. In order to limit the stack usage,
we make 4 as the maximum of the cookie count.

The offset of the current cookie is stored in the
"(ctx[-1] >> BPF_TRAMP_M_COOKIE) & 0xFF". Therefore, we can get the
session cookie with ctx[-offset].

The stack will look like this:

  return value	-> 8 bytes
  argN		-> 8 bytes
  ...
  arg1		-> 8 bytes
  nr_args	-> 8 bytes
  ip(optional)	-> 8 bytes
  cookie2	-> 8 bytes
  cookie1	-> 8 bytes

Inline the bpf_fsession_cookie() in the verifer too.

Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
---
v4:
- limit the maximum of the cookie count to 4
- store the session cookies before nr_regs in stack
---
 include/linux/bpf.h      | 16 ++++++++++++++++
 kernel/bpf/trampoline.c  | 14 +++++++++++++-
 kernel/bpf/verifier.c    | 20 ++++++++++++++++++--
 kernel/trace/bpf_trace.c |  9 +++++++++
 4 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index d165ace5cc9b..0f35c6ab538c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1215,6 +1215,7 @@ enum {
 
 #define BPF_TRAMP_M_NR_ARGS	0
 #define BPF_TRAMP_M_IS_RETURN	8
+#define BPF_TRAMP_M_COOKIE	9
 
 struct bpf_tramp_links {
 	struct bpf_tramp_link *links[BPF_MAX_TRAMP_LINKS];
@@ -1318,6 +1319,7 @@ struct bpf_trampoline {
 	struct mutex mutex;
 	refcount_t refcnt;
 	u32 flags;
+	int cookie_cnt;
 	u64 key;
 	struct {
 		struct btf_func_model model;
@@ -1762,6 +1764,7 @@ struct bpf_prog {
 				enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */
 				call_get_stack:1, /* Do we call bpf_get_stack() or bpf_get_stackid() */
 				call_get_func_ip:1, /* Do we call get_func_ip() */
+				call_session_cookie:1, /* Do we call bpf_fsession_cookie() */
 				tstamp_type_access:1, /* Accessed __sk_buff->tstamp_type */
 				sleepable:1;	/* BPF program is sleepable */
 	enum bpf_prog_type	type;		/* Type of BPF program */
@@ -2137,6 +2140,19 @@ static inline int bpf_fsession_cnt(struct bpf_tramp_links *links)
 	return cnt;
 }
 
+static inline int bpf_fsession_cookie_cnt(struct bpf_tramp_links *links)
+{
+	struct bpf_tramp_links fentries = links[BPF_TRAMP_FENTRY];
+	int cnt = 0;
+
+	for (int i = 0; i < links[BPF_TRAMP_FENTRY].nr_links; i++) {
+		if (fentries.links[i]->link.prog->call_session_cookie)
+			cnt++;
+	}
+
+	return cnt;
+}
+
 int bpf_prog_ctx_arg_info_init(struct bpf_prog *prog,
 			       const struct bpf_ctx_arg_aux *info, u32 cnt);
 
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 3b9fc99e1e89..68f3d73a94a9 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -592,6 +592,8 @@ static int bpf_freplace_check_tgt_prog(struct bpf_prog *tgt_prog)
 	return 0;
 }
 
+#define BPF_TRAMP_MAX_COOKIES 4
+
 static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 				      struct bpf_trampoline *tr,
 				      struct bpf_prog *tgt_prog)
@@ -599,7 +601,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 	enum bpf_tramp_prog_type kind, okind;
 	struct bpf_tramp_link *link_existing;
 	struct bpf_fsession_link *fslink;
-	int err = 0;
+	int err = 0, cookie_cnt;
 	int cnt = 0, i;
 
 	okind = kind = bpf_attach_type_to_tramp(link->link.prog);
@@ -640,6 +642,12 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 		/* prog already linked */
 		return -EBUSY;
 	}
+	cookie_cnt = tr->cookie_cnt;
+	if (link->link.prog->call_session_cookie) {
+		if (cookie_cnt >= BPF_TRAMP_MAX_COOKIES)
+			return -E2BIG;
+		cookie_cnt++;
+	}
 
 	hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
 	tr->progs_cnt[kind]++;
@@ -657,6 +665,8 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 		}
 		hlist_del_init(&link->tramp_hlist);
 		tr->progs_cnt[kind]--;
+	} else {
+		tr->cookie_cnt = cookie_cnt;
 	}
 	return err;
 }
@@ -696,6 +706,8 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
 		hlist_del_init(&fslink->fexit.tramp_hlist);
 		tr->progs_cnt[BPF_TRAMP_FEXIT]--;
 		kind = BPF_TRAMP_FENTRY;
+		if (link->link.prog->call_session_cookie)
+			tr->cookie_cnt--;
 	}
 	hlist_del_init(&link->tramp_hlist);
 	tr->progs_cnt[kind]--;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b0dcd715150f..da8335cf7c46 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12381,6 +12381,7 @@ enum special_kfunc_type {
 	KF_bpf_task_work_schedule_signal_impl,
 	KF_bpf_task_work_schedule_resume_impl,
 	KF_bpf_fsession_is_return,
+	KF_bpf_fsession_cookie,
 };
 
 BTF_ID_LIST(special_kfunc_list)
@@ -12456,6 +12457,7 @@ BTF_ID(func, __bpf_trap)
 BTF_ID(func, bpf_task_work_schedule_signal_impl)
 BTF_ID(func, bpf_task_work_schedule_resume_impl)
 BTF_ID(func, bpf_fsession_is_return)
+BTF_ID(func, bpf_fsession_cookie)
 
 static bool is_task_work_add_kfunc(u32 func_id)
 {
@@ -12511,7 +12513,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	bool arg_mem_size = false;
 
 	if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
-	    meta->func_id == special_kfunc_list[KF_bpf_fsession_is_return])
+	    meta->func_id == special_kfunc_list[KF_bpf_fsession_is_return] ||
+	    meta->func_id == special_kfunc_list[KF_bpf_fsession_cookie])
 		return KF_ARG_PTR_TO_CTX;
 
 	/* In this function, we verify the kfunc's BTF as per the argument type,
@@ -14009,7 +14012,8 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		}
 	}
 
-	if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie]) {
+	if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie] ||
+	    meta.func_id == special_kfunc_list[KF_bpf_fsession_cookie]) {
 		meta.r0_size = sizeof(u64);
 		meta.r0_rdonly = false;
 	}
@@ -14293,6 +14297,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			return err;
 	}
 
+	if (meta.func_id == special_kfunc_list[KF_bpf_fsession_cookie])
+		env->prog->call_session_cookie = true;
+
 	return 0;
 }
 
@@ -22565,6 +22572,15 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		insn_buf[1] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, BPF_TRAMP_M_IS_RETURN);
 		insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
 		*cnt = 3;
+	} else if (desc->func_id == special_kfunc_list[KF_bpf_fsession_cookie]) {
+		/* Load nr_args from ctx - 8 */
+		insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
+		insn_buf[1] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, BPF_TRAMP_M_COOKIE);
+		insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
+		insn_buf[3] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
+		insn_buf[4] = BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1);
+		insn_buf[5] = BPF_ALU64_IMM(BPF_NEG, BPF_REG_0, 0);
+		*cnt = 6;
 	}
 
 	if (env->insn_aux_data[insn_idx].arg_prog) {
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 0857d77eb34c..ae7866d8ea62 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3364,10 +3364,19 @@ __bpf_kfunc bool bpf_fsession_is_return(void *ctx)
 	return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
 }
 
+__bpf_kfunc u64 *bpf_fsession_cookie(void *ctx)
+{
+	/* This helper call is inlined by verifier. */
+	u64 off = (((u64 *)ctx)[-1] >> BPF_TRAMP_M_COOKIE) & 0xFF;
+
+	return &((u64 *)ctx)[-off];
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(tracing_kfunc_set_ids)
 BTF_ID_FLAGS(func, bpf_fsession_is_return, KF_FASTCALL)
+BTF_ID_FLAGS(func, bpf_fsession_cookie, KF_FASTCALL)
 BTF_KFUNCS_END(tracing_kfunc_set_ids)
 
 static int bpf_tracing_filter(const struct bpf_prog *prog, u32 kfunc_id)
-- 
2.52.0

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