Message-ID: <20260107064352.291069-6-dongml2@chinatelecom.cn>
Date: Wed,  7 Jan 2026 14:43:46 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: ast@...nel.org,
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Cc: daniel@...earbox.net,
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Subject: [PATCH bpf-next v7 05/11] bpf: support fsession for bpf_session_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 verifier too. The calling to
bpf_session_cookie() will be changed to bpf_fsession_cookie() in verifier
for fsession.

Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
---
v7:
- reuse bpf_session_cookie() instead of introduce new kfunc

v5:
- remove "cookie_cnt" in struct bpf_trampoline

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  | 13 +++++++++++--
 kernel/bpf/verifier.c    | 19 ++++++++++++++++++-
 kernel/trace/bpf_trace.c |  8 ++++++++
 4 files changed, 53 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index d996dd390681..31e03886c864 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1231,6 +1231,7 @@ 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];
@@ -1783,6 +1784,7 @@ 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_session_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 */
@@ -2191,6 +2193,19 @@ 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);
 
@@ -3949,5 +3964,6 @@ static inline int bpf_map_check_op_flags(struct bpf_map *map, u64 flags, u64 all
 }
 
 bool bpf_fsession_is_return(void *ctx);
+u64 *bpf_fsession_cookie(void *ctx);
 
 #endif /* _LINUX_BPF_H */
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 11e043049d68..29b4e00d860c 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)
@@ -600,7 +602,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 	struct bpf_tramp_link *link_exiting;
 	struct bpf_fsession_link *fslink;
 	struct hlist_head *prog_list;
-	int err = 0;
+	int err = 0, cookie_cnt = 0;
 	int cnt = 0, i;
 
 	kind = bpf_attach_type_to_tramp(link->link.prog);
@@ -637,11 +639,18 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 		/* prog already linked */
 		return -EBUSY;
 	hlist_for_each_entry(link_exiting, prog_list, tramp_hlist) {
-		if (link_exiting->link.prog != link->link.prog)
+		if (link_exiting->link.prog != link->link.prog) {
+			if (kind == BPF_TRAMP_FSESSION &&
+			    link_exiting->link.prog->call_session_cookie)
+				cookie_cnt++;
 			continue;
+		}
 		/* prog already linked */
 		return -EBUSY;
 	}
+	if (link->link.prog->call_session_cookie &&
+	    cookie_cnt >= BPF_TRAMP_MAX_COOKIES)
+		return -E2BIG;
 
 	hlist_add_head(&link->tramp_hlist, prog_list);
 	if (kind == BPF_TRAMP_FSESSION) {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d3709edd0e51..210af94e3957 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12508,7 +12508,8 @@ 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_session_is_return])
+	    meta->func_id == special_kfunc_list[KF_bpf_session_is_return] ||
+	    meta->func_id == special_kfunc_list[KF_bpf_session_cookie])
 		return KF_ARG_PTR_TO_CTX;
 
 	if (argno + 1 < nargs &&
@@ -14294,6 +14295,9 @@ 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_session_cookie])
+		env->prog->call_session_cookie = true;
+
 	return 0;
 }
 
@@ -22446,6 +22450,9 @@ static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc
 	} else if (func_id == special_kfunc_list[KF_bpf_session_is_return]) {
 		if (prog->expected_attach_type == BPF_TRACE_FSESSION)
 			addr = (unsigned long)bpf_fsession_is_return;
+	} else if (func_id == special_kfunc_list[KF_bpf_session_cookie]) {
+		if (prog->expected_attach_type == BPF_TRACE_FSESSION)
+			addr = (unsigned long)bpf_fsession_cookie;
 	}
 	desc->addr = addr;
 	return 0;
@@ -22571,6 +22578,16 @@ 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_session_cookie] &&
+		   env->prog->expected_attach_type == BPF_TRACE_FSESSION) {
+		/* 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 9d3bf3bbe8f6..4960bb69cb30 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3320,6 +3320,14 @@ bool bpf_fsession_is_return(void *ctx)
 	return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
 }
 
+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_start_defs();
 
 __bpf_kfunc bool bpf_session_is_return(void *ctx)
-- 
2.52.0

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