Merge branch 'bpf: Add helpers to access traced function arguments'

Jiri Olsa says:

====================
Add new helpers to access traced function arguments that
came out of the trampoline batch changes [1].

  Get n-th argument of the traced function:
    long bpf_get_func_arg(void *ctx, u32 n, u64 *value)

  Get return value of the traced function:
    long bpf_get_func_ret(void *ctx, u64 *value)

  Get arguments count of the traced funtion:
    long bpf_get_func_arg_cnt(void *ctx)

v2 changes:
  - added acks
  - updated stack diagram
  - return -EOPNOTSUPP instead of -EINVAL in bpf_get_func_ret
  - removed gpl_only for all helpers
  - added verifier fix to allow proper arguments checks,
    Andrii asked for checking also 'int *b' argument in
    bpf_modify_return_test programs and it turned out that it's currently
    not supported by verifier - we can't read argument that is int pointer,
    so I had to add verifier change to allow that + adding verifier selftest
  - checking all arguments in bpf_modify_return_test test programs
  - moved helpers proto gets in tracing_prog_func_proto with attach type check

thanks,
jirka

[1] https://lore.kernel.org/bpf/20211118112455.475349-1-jolsa@kernel.org/
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/x86/net/bpf_jit_comp.c

@@ -1941,7 +1941,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 				void *orig_call)
 {
 	int ret, i, nr_args = m->nr_args;
-	int stack_size = nr_args * 8;
+	int regs_off, ip_off, args_off, stack_size = nr_args * 8;
 	struct bpf_tramp_progs *fentry = &tprogs[BPF_TRAMP_FENTRY];
 	struct bpf_tramp_progs *fexit = &tprogs[BPF_TRAMP_FEXIT];
 	struct bpf_tramp_progs *fmod_ret = &tprogs[BPF_TRAMP_MODIFY_RETURN];
@@ -1956,14 +1956,39 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	if (!is_valid_bpf_tramp_flags(flags))
 		return -EINVAL;
 
+	/* Generated trampoline stack layout:
+	 *
+	 * RBP + 8         [ return address  ]
+	 * RBP + 0         [ RBP             ]
+	 *
+	 * RBP - 8         [ return value    ]  BPF_TRAMP_F_CALL_ORIG or
+	 *                                      BPF_TRAMP_F_RET_FENTRY_RET flags
+	 *
+	 *                 [ reg_argN        ]  always
+	 *                 [ ...             ]
+	 * RBP - regs_off  [ reg_arg1        ]  program's ctx pointer
+	 *
+	 * RBP - args_off  [ args count      ]  always
+	 *
+	 * RBP - ip_off    [ traced function ]  BPF_TRAMP_F_IP_ARG flag
+	 */
+
 	/* room for return value of orig_call or fentry prog */
 	save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
 	if (save_ret)
 		stack_size += 8;
 
+	regs_off = stack_size;
+
+	/* args count  */
+	stack_size += 8;
+	args_off = stack_size;
+
 	if (flags & BPF_TRAMP_F_IP_ARG)
 		stack_size += 8; /* room for IP address argument */
 
+	ip_off = stack_size;
+
 	if (flags & BPF_TRAMP_F_SKIP_FRAME)
 		/* skip patched call instruction and point orig_call to actual
 		 * body of the kernel function.
@@ -1977,23 +2002,25 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
 	EMIT1(0x53);		 /* push rbx */
 
+	/* Store number of arguments of the traced function:
+	 *   mov rax, nr_args
+	 *   mov QWORD PTR [rbp - args_off], rax
+	 */
+	emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_args);
+	emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -args_off);
+
 	if (flags & BPF_TRAMP_F_IP_ARG) {
 		/* Store IP address of the traced function:
 		 * mov rax, QWORD PTR [rbp + 8]
 		 * sub rax, X86_PATCH_SIZE
-		 * mov QWORD PTR [rbp - stack_size], rax
+		 * mov QWORD PTR [rbp - ip_off], rax
 		 */
 		emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
 		EMIT4(0x48, 0x83, 0xe8, X86_PATCH_SIZE);
-		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -stack_size);
-
-		/* Continue with stack_size for regs storage, stack will
-		 * be correctly restored with 'leave' instruction.
-		 */
-		stack_size -= 8;
+		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ip_off);
 	}
 
-	save_regs(m, &prog, nr_args, stack_size);
+	save_regs(m, &prog, nr_args, regs_off);
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		/* arg1: mov rdi, im */
@@ -2005,7 +2032,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	}
 
 	if (fentry->nr_progs)
-		if (invoke_bpf(m, &prog, fentry, stack_size,
+		if (invoke_bpf(m, &prog, fentry, regs_off,
 			       flags & BPF_TRAMP_F_RET_FENTRY_RET))
 			return -EINVAL;
 
@@ -2015,7 +2042,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 		if (!branches)
 			return -ENOMEM;
 
-		if (invoke_bpf_mod_ret(m, &prog, fmod_ret, stack_size,
+		if (invoke_bpf_mod_ret(m, &prog, fmod_ret, regs_off,
 				       branches)) {
 			ret = -EINVAL;
 			goto cleanup;
@@ -2023,7 +2050,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	}
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
-		restore_regs(m, &prog, nr_args, stack_size);
+		restore_regs(m, &prog, nr_args, regs_off);
 
 		/* call original function */
 		if (emit_call(&prog, orig_call, prog)) {
@@ -2053,13 +2080,13 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	}
 
 	if (fexit->nr_progs)
-		if (invoke_bpf(m, &prog, fexit, stack_size, false)) {
+		if (invoke_bpf(m, &prog, fexit, regs_off, false)) {
 			ret = -EINVAL;
 			goto cleanup;
 		}
 
 	if (flags & BPF_TRAMP_F_RESTORE_REGS)
-		restore_regs(m, &prog, nr_args, stack_size);
+		restore_regs(m, &prog, nr_args, regs_off);
 
 	/* This needs to be done regardless. If there were fmod_ret programs,
 	 * the return value is only updated on the stack and still needs to be

include/linux/bpf.h

@@ -777,6 +777,7 @@ void bpf_ksym_add(struct bpf_ksym *ksym);
 void bpf_ksym_del(struct bpf_ksym *ksym);
 int bpf_jit_charge_modmem(u32 pages);
 void bpf_jit_uncharge_modmem(u32 pages);
+bool bpf_prog_has_trampoline(const struct bpf_prog *prog);
 #else
 static inline int bpf_trampoline_link_prog(struct bpf_prog *prog,
 					   struct bpf_trampoline *tr)
@@ -805,6 +806,10 @@ static inline bool is_bpf_image_address(unsigned long address)
 {
 	return false;
 }
+static inline bool bpf_prog_has_trampoline(const struct bpf_prog *prog)
+{
+	return false;
+}
 #endif
 
 struct bpf_func_info_aux {

include/uapi/linux/bpf.h

@@ -4993,6 +4993,31 @@ union bpf_attr {
  *		An integer less than, equal to, or greater than zero
  *		if the first **s1_sz** bytes of **s1** is found to be
  *		less than, to match, or be greater than **s2**.
+ *
+ * long bpf_get_func_arg(void *ctx, u32 n, u64 *value)
+ *	Description
+ *		Get **n**-th argument (zero based) of the traced function (for tracing programs)
+ *		returned in **value**.
+ *
+ *	Return
+ *		0 on success.
+ *		**-EINVAL** if n >= arguments count of traced function.
+ *
+ * long bpf_get_func_ret(void *ctx, u64 *value)
+ *	Description
+ *		Get return value of the traced function (for tracing programs)
+ *		in **value**.
+ *
+ *	Return
+ *		0 on success.
+ *		**-EOPNOTSUPP** for tracing programs other than BPF_TRACE_FEXIT or BPF_MODIFY_RETURN.
+ *
+ * long bpf_get_func_arg_cnt(void *ctx)
+ *	Description
+ *		Get number of arguments of the traced function (for tracing programs).
+ *
+ *	Return
+ *		The number of arguments of the traced function.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5178,6 +5203,9 @@ union bpf_attr {
 	FN(find_vma),			\
 	FN(loop),			\
 	FN(strncmp),			\
+	FN(get_func_arg),		\
+	FN(get_func_ret),		\
+	FN(get_func_arg_cnt),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

kernel/bpf/btf.c

@@ -4826,7 +4826,7 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 		return prog->aux->attach_btf;
 }
 
-static bool is_string_ptr(struct btf *btf, const struct btf_type *t)
+static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
 {
 	/* t comes in already as a pointer */
 	t = btf_type_by_id(btf, t->type);
@@ -4835,8 +4835,7 @@ static bool is_string_ptr(struct btf *btf, const struct btf_type *t)
 	if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
 		t = btf_type_by_id(btf, t->type);
 
-	/* char, signed char, unsigned char */
-	return btf_type_is_int(t) && t->size == 1;
+	return btf_type_is_int(t);
 }
 
 bool btf_ctx_access(int off, int size, enum bpf_access_type type,
@@ -4957,7 +4956,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		 */
 		return true;
 
-	if (is_string_ptr(btf, t))
+	if (is_int_ptr(btf, t))
 		return true;
 
 	/* this is a pointer to another type */

kernel/bpf/trampoline.c

@@ -27,6 +27,14 @@ static struct hlist_head trampoline_table[TRAMPOLINE_TABLE_SIZE];
 /* serializes access to trampoline_table */
 static DEFINE_MUTEX(trampoline_mutex);
 
+bool bpf_prog_has_trampoline(const struct bpf_prog *prog)
+{
+	enum bpf_attach_type eatype = prog->expected_attach_type;
+
+	return eatype == BPF_TRACE_FENTRY || eatype == BPF_TRACE_FEXIT ||
+	       eatype == BPF_MODIFY_RETURN;
+}
+
 void *bpf_jit_alloc_exec_page(void)
 {
 	void *image;

kernel/bpf/verifier.c

@@ -6395,13 +6395,11 @@ static int check_bpf_snprintf_call(struct bpf_verifier_env *env,
 
 static int check_get_func_ip(struct bpf_verifier_env *env)
 {
-	enum bpf_attach_type eatype = env->prog->expected_attach_type;
 	enum bpf_prog_type type = resolve_prog_type(env->prog);
 	int func_id = BPF_FUNC_get_func_ip;
 
 	if (type == BPF_PROG_TYPE_TRACING) {
-		if (eatype != BPF_TRACE_FENTRY && eatype != BPF_TRACE_FEXIT &&
-		    eatype != BPF_MODIFY_RETURN) {
+		if (!bpf_prog_has_trampoline(env->prog)) {
 			verbose(env, "func %s#%d supported only for fentry/fexit/fmod_ret programs\n",
 				func_id_name(func_id), func_id);
 			return -ENOTSUPP;
@@ -12997,6 +12995,7 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env,
 static int do_misc_fixups(struct bpf_verifier_env *env)
 {
 	struct bpf_prog *prog = env->prog;
+	enum bpf_attach_type eatype = prog->expected_attach_type;
 	bool expect_blinding = bpf_jit_blinding_enabled(prog);
 	enum bpf_prog_type prog_type = resolve_prog_type(prog);
 	struct bpf_insn *insn = prog->insnsi;
@@ -13367,11 +13366,79 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			continue;
 		}
 
+		/* Implement bpf_get_func_arg inline. */
+		if (prog_type == BPF_PROG_TYPE_TRACING &&
+		    insn->imm == BPF_FUNC_get_func_arg) {
+			/* 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_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6);
+			insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3);
+			insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
+			insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
+			insn_buf[5] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
+			insn_buf[6] = BPF_MOV64_IMM(BPF_REG_0, 0);
+			insn_buf[7] = BPF_JMP_A(1);
+			insn_buf[8] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
+			cnt = 9;
+
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta    += cnt - 1;
+			env->prog = prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
+			continue;
+		}
+
+		/* Implement bpf_get_func_ret inline. */
+		if (prog_type == BPF_PROG_TYPE_TRACING &&
+		    insn->imm == BPF_FUNC_get_func_ret) {
+			if (eatype == BPF_TRACE_FEXIT ||
+			    eatype == BPF_MODIFY_RETURN) {
+				/* 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_LSH, BPF_REG_0, 3);
+				insn_buf[2] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
+				insn_buf[3] = BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
+				insn_buf[4] = BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_3, 0);
+				insn_buf[5] = BPF_MOV64_IMM(BPF_REG_0, 0);
+				cnt = 6;
+			} else {
+				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, -EOPNOTSUPP);
+				cnt = 1;
+			}
+
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta    += cnt - 1;
+			env->prog = prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
+			continue;
+		}
+
+		/* Implement get_func_arg_cnt inline. */
+		if (prog_type == BPF_PROG_TYPE_TRACING &&
+		    insn->imm == BPF_FUNC_get_func_arg_cnt) {
+			/* Load nr_args from ctx - 8 */
+			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
+
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
+			if (!new_prog)
+				return -ENOMEM;
+
+			env->prog = prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
+			continue;
+		}
+
 		/* Implement bpf_get_func_ip inline. */
 		if (prog_type == BPF_PROG_TYPE_TRACING &&
 		    insn->imm == BPF_FUNC_get_func_ip) {
-			/* Load IP address from ctx - 8 */
-			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
+			/* Load IP address from ctx - 16 */
+			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -16);
 
 			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
 			if (!new_prog)

kernel/trace/bpf_trace.c

@@ -1012,7 +1012,7 @@ const struct bpf_func_proto bpf_snprintf_btf_proto = {
 BPF_CALL_1(bpf_get_func_ip_tracing, void *, ctx)
 {
 	/* This helper call is inlined by verifier. */
-	return ((u64 *)ctx)[-1];
+	return ((u64 *)ctx)[-2];
 }
 
 static const struct bpf_func_proto bpf_get_func_ip_proto_tracing = {
@@ -1091,6 +1091,53 @@ static const struct bpf_func_proto bpf_get_branch_snapshot_proto = {
 	.arg2_type	= ARG_CONST_SIZE_OR_ZERO,
 };
 
+BPF_CALL_3(get_func_arg, void *, ctx, u32, n, u64 *, value)
+{
+	/* This helper call is inlined by verifier. */
+	u64 nr_args = ((u64 *)ctx)[-1];
+
+	if ((u64) n >= nr_args)
+		return -EINVAL;
+	*value = ((u64 *)ctx)[n];
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_get_func_arg_proto = {
+	.func		= get_func_arg,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_ANYTHING,
+	.arg3_type	= ARG_PTR_TO_LONG,
+};
+
+BPF_CALL_2(get_func_ret, void *, ctx, u64 *, value)
+{
+	/* This helper call is inlined by verifier. */
+	u64 nr_args = ((u64 *)ctx)[-1];
+
+	*value = ((u64 *)ctx)[nr_args];
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_get_func_ret_proto = {
+	.func		= get_func_ret,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_PTR_TO_LONG,
+};
+
+BPF_CALL_1(get_func_arg_cnt, void *, ctx)
+{
+	/* This helper call is inlined by verifier. */
+	return ((u64 *)ctx)[-1];
+}
+
+static const struct bpf_func_proto bpf_get_func_arg_cnt_proto = {
+	.func		= get_func_arg_cnt,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+};
+
 static const struct bpf_func_proto *
 bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -1629,6 +1676,12 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		       NULL;
 	case BPF_FUNC_d_path:
 		return &bpf_d_path_proto;
+	case BPF_FUNC_get_func_arg:
+		return bpf_prog_has_trampoline(prog) ? &bpf_get_func_arg_proto : NULL;
+	case BPF_FUNC_get_func_ret:
+		return bpf_prog_has_trampoline(prog) ? &bpf_get_func_ret_proto : NULL;
+	case BPF_FUNC_get_func_arg_cnt:
+		return bpf_prog_has_trampoline(prog) ? &bpf_get_func_arg_cnt_proto : NULL;
 	default:
 		fn = raw_tp_prog_func_proto(func_id, prog);
 		if (!fn && prog->expected_attach_type == BPF_TRACE_ITER)

tools/include/uapi/linux/bpf.h

@@ -4993,6 +4993,31 @@ union bpf_attr {
  *		An integer less than, equal to, or greater than zero
  *		if the first **s1_sz** bytes of **s1** is found to be
  *		less than, to match, or be greater than **s2**.
+ *
+ * long bpf_get_func_arg(void *ctx, u32 n, u64 *value)
+ *	Description
+ *		Get **n**-th argument (zero based) of the traced function (for tracing programs)
+ *		returned in **value**.
+ *
+ *	Return
+ *		0 on success.
+ *		**-EINVAL** if n >= arguments count of traced function.
+ *
+ * long bpf_get_func_ret(void *ctx, u64 *value)
+ *	Description
+ *		Get return value of the traced function (for tracing programs)
+ *		in **value**.
+ *
+ *	Return
+ *		0 on success.
+ *		**-EOPNOTSUPP** for tracing programs other than BPF_TRACE_FEXIT or BPF_MODIFY_RETURN.
+ *
+ * long bpf_get_func_arg_cnt(void *ctx)
+ *	Description
+ *		Get number of arguments of the traced function (for tracing programs).
+ *
+ *	Return
+ *		The number of arguments of the traced function.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5178,6 +5203,9 @@ union bpf_attr {
 	FN(find_vma),			\
 	FN(loop),			\
 	FN(strncmp),			\
+	FN(get_func_arg),		\
+	FN(get_func_ret),		\
+	FN(get_func_arg_cnt),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "get_func_args_test.skel.h"
+
+void test_get_func_args_test(void)
+{
+	struct get_func_args_test *skel = NULL;
+	__u32 duration = 0, retval;
+	int err, prog_fd;
+
+	skel = get_func_args_test__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "get_func_args_test__open_and_load"))
+		return;
+
+	err = get_func_args_test__attach(skel);
+	if (!ASSERT_OK(err, "get_func_args_test__attach"))
+		goto cleanup;
+
+	/* This runs bpf_fentry_test* functions and triggers
+	 * fentry/fexit programs.
+	 */
+	prog_fd = bpf_program__fd(skel->progs.test1);
+	err = bpf_prog_test_run(prog_fd, 1, NULL, 0,
+				NULL, NULL, &retval, &duration);
+	ASSERT_OK(err, "test_run");
+	ASSERT_EQ(retval, 0, "test_run");
+
+	/* This runs bpf_modify_return_test function and triggers
+	 * fmod_ret_test and fexit_test programs.
+	 */
+	prog_fd = bpf_program__fd(skel->progs.fmod_ret_test);
+	err = bpf_prog_test_run(prog_fd, 1, NULL, 0,
+				NULL, NULL, &retval, &duration);
+	ASSERT_OK(err, "test_run");
+	ASSERT_EQ(retval, 1234, "test_run");
+
+	ASSERT_EQ(skel->bss->test1_result, 1, "test1_result");
+	ASSERT_EQ(skel->bss->test2_result, 1, "test2_result");
+	ASSERT_EQ(skel->bss->test3_result, 1, "test3_result");
+	ASSERT_EQ(skel->bss->test4_result, 1, "test4_result");
+
+cleanup:
+	get_func_args_test__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <errno.h>
+
+char _license[] SEC("license") = "GPL";
+
+__u64 test1_result = 0;
+SEC("fentry/bpf_fentry_test1")
+int BPF_PROG(test1)
+{
+	__u64 cnt = bpf_get_func_arg_cnt(ctx);
+	__u64 a = 0, z = 0, ret = 0;
+	__s64 err;
+
+	test1_result = cnt == 1;
+
+	/* valid arguments */
+	err = bpf_get_func_arg(ctx, 0, &a);
+
+	/* We need to cast access to traced function argument values with
+	 * proper type cast, because trampoline uses type specific instruction
+	 * to save it, like for 'int a' with 32-bit mov like:
+	 *
+	 *   mov %edi,-0x8(%rbp)
+	 *
+	 * so the upper 4 bytes are not zeroed.
+	 */
+	test1_result &= err == 0 && ((int) a == 1);
+
+	/* not valid argument */
+	err = bpf_get_func_arg(ctx, 1, &z);
+	test1_result &= err == -EINVAL;
+
+	/* return value fails in fentry */
+	err = bpf_get_func_ret(ctx, &ret);
+	test1_result &= err == -EOPNOTSUPP;
+	return 0;
+}
+
+__u64 test2_result = 0;
+SEC("fexit/bpf_fentry_test2")
+int BPF_PROG(test2)
+{
+	__u64 cnt = bpf_get_func_arg_cnt(ctx);
+	__u64 a = 0, b = 0, z = 0, ret = 0;
+	__s64 err;
+
+	test2_result = cnt == 2;
+
+	/* valid arguments */
+	err = bpf_get_func_arg(ctx, 0, &a);
+	test2_result &= err == 0 && (int) a == 2;
+
+	err = bpf_get_func_arg(ctx, 1, &b);
+	test2_result &= err == 0 && b == 3;
+
+	/* not valid argument */
+	err = bpf_get_func_arg(ctx, 2, &z);
+	test2_result &= err == -EINVAL;
+
+	/* return value */
+	err = bpf_get_func_ret(ctx, &ret);
+	test2_result &= err == 0 && ret == 5;
+	return 0;
+}
+
+__u64 test3_result = 0;
+SEC("fmod_ret/bpf_modify_return_test")
+int BPF_PROG(fmod_ret_test, int _a, int *_b, int _ret)
+{
+	__u64 cnt = bpf_get_func_arg_cnt(ctx);
+	__u64 a = 0, b = 0, z = 0, ret = 0;
+	__s64 err;
+
+	test3_result = cnt == 2;
+
+	/* valid arguments */
+	err = bpf_get_func_arg(ctx, 0, &a);
+	test3_result &= err == 0 && ((int) a == 1);
+
+	err = bpf_get_func_arg(ctx, 1, &b);
+	test3_result &= err == 0 && ((int *) b == _b);
+
+	/* not valid argument */
+	err = bpf_get_func_arg(ctx, 2, &z);
+	test3_result &= err == -EINVAL;
+
+	/* return value */
+	err = bpf_get_func_ret(ctx, &ret);
+	test3_result &= err == 0 && ret == 0;
+
+	/* change return value, it's checked in fexit_test program */
+	return 1234;
+}
+
+__u64 test4_result = 0;
+SEC("fexit/bpf_modify_return_test")
+int BPF_PROG(fexit_test, int _a, int *_b, int _ret)
+{
+	__u64 cnt = bpf_get_func_arg_cnt(ctx);
+	__u64 a = 0, b = 0, z = 0, ret = 0;
+	__s64 err;
+
+	test4_result = cnt == 2;
+
+	/* valid arguments */
+	err = bpf_get_func_arg(ctx, 0, &a);
+	test4_result &= err == 0 && ((int) a == 1);
+
+	err = bpf_get_func_arg(ctx, 1, &b);
+	test4_result &= err == 0 && ((int *) b == _b);
+
+	/* not valid argument */
+	err = bpf_get_func_arg(ctx, 2, &z);
+	test4_result &= err == -EINVAL;
+
+	/* return value */
+	err = bpf_get_func_ret(ctx, &ret);
+	test4_result &= err == 0 && ret == 1234;
+	return 0;
+}

tools/testing/selftests/bpf/verifier/btf_ctx_access.c

+{
+	"btf_ctx_access accept",
+	.insns = {
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 8),	/* load 2nd argument value (int pointer) */
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.prog_type = BPF_PROG_TYPE_TRACING,
+	.expected_attach_type = BPF_TRACE_FENTRY,
+	.kfunc = "bpf_modify_return_test",
+},