bpf: add kfunc bpf_fsession_cookie for TRACING SESSION

Add the kfunc bpf_fsession_cookie(), which is similar to
bpf_session_cookie() and return the address of the session cookie. The
address of the session cookie is stored after session flags, which means
ctx[nr_args + 2].

Inline this kfunc in the verifier too.

Signed-off-by: Menglong Dong <[email protected]>

Author: image-dragon

include/linux/bpf.h

@@ -1736,6 +1736,7 @@ 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 */

kernel/bpf/verifier.c

@@ -12294,6 +12294,7 @@ enum special_kfunc_type {
 	KF_bpf_task_work_schedule_signal,
 	KF_bpf_task_work_schedule_resume,
 	KF_bpf_tracing_is_exit,
+	KF_bpf_fsession_cookie,
 };
 
 BTF_ID_LIST(special_kfunc_list)
@@ -12367,6 +12368,7 @@ BTF_ID(func, __bpf_trap)
 BTF_ID(func, bpf_task_work_schedule_signal)
 BTF_ID(func, bpf_task_work_schedule_resume)
 BTF_ID(func, bpf_tracing_is_exit)
+BTF_ID(func, bpf_fsession_cookie)
 
 static bool is_task_work_add_kfunc(u32 func_id)
 {
@@ -12422,7 +12424,8 @@ 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_tracing_is_exit])
+	    meta->func_id == special_kfunc_list[KF_bpf_tracing_is_exit] ||
+	    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,
@@ -13915,7 +13918,8 @@ 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;
 	}
@@ -14196,6 +14200,9 @@ 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;
 }
 
@@ -22025,6 +22032,15 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
 		insn_buf[5] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
 		*cnt = 6;
+	} 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);
+		/* add rax, 2 */
+		insn_buf[1] = BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 2);
+		insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
+		insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
+		insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
+		*cnt = 5;
 	}
 
 	if (env->insn_aux_data[insn_idx].arg_prog) {

kernel/trace/bpf_trace.c

@@ -3370,10 +3370,20 @@ __bpf_kfunc bool bpf_tracing_is_exit(void *ctx)
 	return ((u64 *)ctx)[nr_args + 1] & 1;
 }
 
+__bpf_kfunc u64 *bpf_fsession_cookie(void *ctx)
+{
+	/* This helper call is inlined by verifier. */
+	u64 nr_args = ((u64 *)ctx)[-1];
+
+	/* ctx[nr_args + 2] is the session cookie address */
+	return (u64 *)((u64 *)ctx)[nr_args + 2];
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(tracing_kfunc_set_ids)
 BTF_ID_FLAGS(func, bpf_tracing_is_exit, 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)