libbpf: add ksyscall/kretsyscall sections support for syscall kprobes

Add SEC("ksyscall")/SEC("ksyscall/<syscall_name>") and corresponding
kretsyscall variants (for return kprobes) to allow users to kprobe
syscall functions in kernel. These special sections allow to ignore
complexities and differences between kernel versions and host
architectures when it comes to syscall wrapper and corresponding
__<arch>_sys_<syscall> vs __se_sys_<syscall> differences, depending on
whether host kernel has CONFIG_ARCH_HAS_SYSCALL_WRAPPER (though libbpf
itself doesn't rely on /proc/config.gz for detecting this, see
BPF_KSYSCALL patch for how it's done internally).

Combined with the use of BPF_KSYSCALL() macro, this allows to just
specify intended syscall name and expected input arguments and leave
dealing with all the variations to libbpf.

In addition to SEC("ksyscall+") and SEC("kretsyscall+") add
bpf_program__attach_ksyscall() API which allows to specify syscall name
at runtime and provide associated BPF cookie value.

At the moment SEC("ksyscall") and bpf_program__attach_ksyscall() do not
handle all the calling convention quirks for mmap(), clone() and compat
syscalls. It also only attaches to "native" syscall interfaces. If host
system supports compat syscalls or defines 32-bit syscalls in 64-bit
kernel, such syscall interfaces won't be attached to by libbpf.

These limitations may or may not change in the future. Therefore it is
recommended to use SEC("kprobe") for these syscalls or if working with
compat and 32-bit interfaces is required.

Tested-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220714070755.3235561-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -4670,6 +4670,8 @@ static int probe_kern_btf_enum64(void)
 					     strs, sizeof(strs)));
 }
 
+static int probe_kern_syscall_wrapper(void);
+
 enum kern_feature_result {
 	FEAT_UNKNOWN = 0,
 	FEAT_SUPPORTED = 1,
@@ -4738,6 +4740,9 @@ static struct kern_feature_desc {
 	[FEAT_BTF_ENUM64] = {
 		"BTF_KIND_ENUM64 support", probe_kern_btf_enum64,
 	},
+	[FEAT_SYSCALL_WRAPPER] = {
+		"Kernel using syscall wrapper", probe_kern_syscall_wrapper,
+	},
 };
 
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
@@ -8421,6 +8426,7 @@ int bpf_program__set_log_buf(struct bpf_program *prog, char *log_buf, size_t log
 
 static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf_link **link);
 static int attach_uprobe(const struct bpf_program *prog, long cookie, struct bpf_link **link);
+static int attach_ksyscall(const struct bpf_program *prog, long cookie, struct bpf_link **link);
 static int attach_usdt(const struct bpf_program *prog, long cookie, struct bpf_link **link);
 static int attach_tp(const struct bpf_program *prog, long cookie, struct bpf_link **link);
 static int attach_raw_tp(const struct bpf_program *prog, long cookie, struct bpf_link **link);
@@ -8441,6 +8447,8 @@ static const struct bpf_sec_def section_defs[] = {
 	SEC_DEF("uretprobe.s+",		KPROBE, 0, SEC_SLEEPABLE, attach_uprobe),
 	SEC_DEF("kprobe.multi+",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
 	SEC_DEF("kretprobe.multi+",	KPROBE,	BPF_TRACE_KPROBE_MULTI, SEC_NONE, attach_kprobe_multi),
+	SEC_DEF("ksyscall+",		KPROBE,	0, SEC_NONE, attach_ksyscall),
+	SEC_DEF("kretsyscall+",		KPROBE, 0, SEC_NONE, attach_ksyscall),
 	SEC_DEF("usdt+",		KPROBE,	0, SEC_NONE, attach_usdt),
 	SEC_DEF("tc",			SCHED_CLS, 0, SEC_NONE),
 	SEC_DEF("classifier",		SCHED_CLS, 0, SEC_NONE),
@@ -9797,7 +9805,7 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
 {
 	struct perf_event_attr attr = {};
 	char errmsg[STRERR_BUFSIZE];
-	int type, pfd, err;
+	int type, pfd;
 
 	if (ref_ctr_off >= (1ULL << PERF_UPROBE_REF_CTR_OFFSET_BITS))
 		return -EINVAL;
@@ -9833,14 +9841,7 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
 		      pid < 0 ? -1 : pid /* pid */,
 		      pid == -1 ? 0 : -1 /* cpu */,
 		      -1 /* group_fd */, PERF_FLAG_FD_CLOEXEC);
-	if (pfd < 0) {
-		err = -errno;
-		pr_warn("%s perf_event_open() failed: %s\n",
-			uprobe ? "uprobe" : "kprobe",
-			libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
-		return err;
-	}
-	return pfd;
+	return pfd >= 0 ? pfd : -errno;
 }
 
 static int append_to_file(const char *file, const char *fmt, ...)
@@ -9945,6 +9946,60 @@ err_clean_legacy:
 	return err;
 }
 
+static const char *arch_specific_syscall_pfx(void)
+{
+#if defined(__x86_64__)
+	return "x64";
+#elif defined(__i386__)
+	return "ia32";
+#elif defined(__s390x__)
+	return "s390x";
+#elif defined(__s390__)
+	return "s390";
+#elif defined(__arm__)
+	return "arm";
+#elif defined(__aarch64__)
+	return "arm64";
+#elif defined(__mips__)
+	return "mips";
+#elif defined(__riscv)
+	return "riscv";
+#else
+	return NULL;
+#endif
+}
+
+static int probe_kern_syscall_wrapper(void)
+{
+	char syscall_name[64];
+	const char *ksys_pfx;
+
+	ksys_pfx = arch_specific_syscall_pfx();
+	if (!ksys_pfx)
+		return 0;
+
+	snprintf(syscall_name, sizeof(syscall_name), "__%s_sys_bpf", ksys_pfx);
+
+	if (determine_kprobe_perf_type() >= 0) {
+		int pfd;
+
+		pfd = perf_event_open_probe(false, false, syscall_name, 0, getpid(), 0);
+		if (pfd >= 0)
+			close(pfd);
+
+		return pfd >= 0 ? 1 : 0;
+	} else { /* legacy mode */
+		char probe_name[128];
+
+		gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name), syscall_name, 0);
+		if (add_kprobe_event_legacy(probe_name, false, syscall_name, 0) < 0)
+			return 0;
+
+		(void)remove_kprobe_event_legacy(probe_name, false);
+		return 1;
+	}
+}
+
 struct bpf_link *
 bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 				const char *func_name,
@@ -10030,6 +10085,29 @@ struct bpf_link *bpf_program__attach_kprobe(const struct bpf_program *prog,
 	return bpf_program__attach_kprobe_opts(prog, func_name, &opts);
 }
 
+struct bpf_link *bpf_program__attach_ksyscall(const struct bpf_program *prog,
+					      const char *syscall_name,
+					      const struct bpf_ksyscall_opts *opts)
+{
+	LIBBPF_OPTS(bpf_kprobe_opts, kprobe_opts);
+	char func_name[128];
+
+	if (!OPTS_VALID(opts, bpf_ksyscall_opts))
+		return libbpf_err_ptr(-EINVAL);
+
+	if (kernel_supports(prog->obj, FEAT_SYSCALL_WRAPPER)) {
+		snprintf(func_name, sizeof(func_name), "__%s_sys_%s",
+			 arch_specific_syscall_pfx(), syscall_name);
+	} else {
+		snprintf(func_name, sizeof(func_name), "__se_sys_%s", syscall_name);
+	}
+
+	kprobe_opts.retprobe = OPTS_GET(opts, retprobe, false);
+	kprobe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
+
+	return bpf_program__attach_kprobe_opts(prog, func_name, &kprobe_opts);
+}
+
 /* Adapted from perf/util/string.c */
 static bool glob_match(const char *str, const char *pat)
 {
@@ -10200,6 +10278,27 @@ static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf
 	return libbpf_get_error(*link);
 }
 
+static int attach_ksyscall(const struct bpf_program *prog, long cookie, struct bpf_link **link)
+{
+	LIBBPF_OPTS(bpf_ksyscall_opts, opts);
+	const char *syscall_name;
+
+	*link = NULL;
+
+	/* no auto-attach for SEC("ksyscall") and SEC("kretsyscall") */
+	if (strcmp(prog->sec_name, "ksyscall") == 0 || strcmp(prog->sec_name, "kretsyscall") == 0)
+		return 0;
+
+	opts.retprobe = str_has_pfx(prog->sec_name, "kretsyscall/");
+	if (opts.retprobe)
+		syscall_name = prog->sec_name + sizeof("kretsyscall/") - 1;
+	else
+		syscall_name = prog->sec_name + sizeof("ksyscall/") - 1;
+
+	*link = bpf_program__attach_ksyscall(prog, syscall_name, &opts);
+	return *link ? 0 : -errno;
+}
+
 static int attach_kprobe_multi(const struct bpf_program *prog, long cookie, struct bpf_link **link)
 {
 	LIBBPF_OPTS(bpf_kprobe_multi_opts, opts);

tools/lib/bpf/libbpf.h

@@ -457,6 +457,52 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
 				      const char *pattern,
 				      const struct bpf_kprobe_multi_opts *opts);
 
+struct bpf_ksyscall_opts {
+	/* size of this struct, for forward/backward compatiblity */
+	size_t sz;
+	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
+	__u64 bpf_cookie;
+	/* attach as return probe? */
+	bool retprobe;
+	size_t :0;
+};
+#define bpf_ksyscall_opts__last_field retprobe
+
+/**
+ * @brief **bpf_program__attach_ksyscall()** attaches a BPF program
+ * to kernel syscall handler of a specified syscall. Optionally it's possible
+ * to request to install retprobe that will be triggered at syscall exit. It's
+ * also possible to associate BPF cookie (though options).
+ *
+ * Libbpf automatically will determine correct full kernel function name,
+ * which depending on system architecture and kernel version/configuration
+ * could be of the form __<arch>_sys_<syscall> or __se_sys_<syscall>, and will
+ * attach specified program using kprobe/kretprobe mechanism.
+ *
+ * **bpf_program__attach_ksyscall()** is an API counterpart of declarative
+ * **SEC("ksyscall/<syscall>")** annotation of BPF programs.
+ *
+ * At the moment **SEC("ksyscall")** and **bpf_program__attach_ksyscall()** do
+ * not handle all the calling convention quirks for mmap(), clone() and compat
+ * syscalls. It also only attaches to "native" syscall interfaces. If host
+ * system supports compat syscalls or defines 32-bit syscalls in 64-bit
+ * kernel, such syscall interfaces won't be attached to by libbpf.
+ *
+ * These limitations may or may not change in the future. Therefore it is
+ * recommended to use SEC("kprobe") for these syscalls or if working with
+ * compat and 32-bit interfaces is required.
+ *
+ * @param prog BPF program to attach
+ * @param syscall_name Symbolic name of the syscall (e.g., "bpf")
+ * @param opts Additional options (see **struct bpf_ksyscall_opts**)
+ * @return Reference to the newly created BPF link; or NULL is returned on
+ * error, error code is stored in errno
+ */
+LIBBPF_API struct bpf_link *
+bpf_program__attach_ksyscall(const struct bpf_program *prog,
+			     const char *syscall_name,
+			     const struct bpf_ksyscall_opts *opts);
+
 struct bpf_uprobe_opts {
 	/* size of this struct, for forward/backward compatiblity */
 	size_t sz;

tools/lib/bpf/libbpf.map

@@ -356,6 +356,7 @@ LIBBPF_0.8.0 {
 LIBBPF_1.0.0 {
 	global:
 		bpf_prog_query_opts;
+		bpf_program__attach_ksyscall;
 		btf__add_enum64;
 		btf__add_enum64_value;
 		libbpf_bpf_attach_type_str;

tools/lib/bpf/libbpf_internal.h

@@ -352,6 +352,8 @@ enum kern_feature_id {
 	FEAT_BPF_COOKIE,
 	/* BTF_KIND_ENUM64 support and BTF_KIND_ENUM kflag support */
 	FEAT_BTF_ENUM64,
+	/* Kernel uses syscall wrapper (CONFIG_ARCH_HAS_SYSCALL_WRAPPER) */
+	FEAT_SYSCALL_WRAPPER,
 	__FEAT_CNT,
 };