[PATCH bpf-next v7 05/11] bpf: support fsession for bpf_session_cookie

From: Menglong Dong <hidden>
Date: 2026-01-07 06:45:00
Also in: bpf, lkml
Subsystem: bpf [core], bpf [general] (safe dynamic programs and tools), bpf [security & lsm] (security audit and enforcement using bpf), bpf [tracing], the rest, tracing · Maintainers: Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, KP Singh, Matt Bobrowski, Song Liu, Linus Torvalds, Steven Rostedt, Masami Hiramatsu

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 <redacted>
---
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

`h`	back out one level
`j`	next message in thread
`k`	previous message in thread
`l`	drill in
`Esc`	close help / fold thread tree
`?`	toggle this help