libbpf: support custom SEC() handlers

Allow registering and unregistering custom handlers for BPF program.
This allows user applications and libraries to plug into libbpf's
declarative SEC() definition handling logic. This allows to offload
complex and intricate custom logic into external libraries, but still
provide a great user experience.

One such example is USDT handling library, which has a lot of code and
complexity which doesn't make sense to put into libbpf directly, but it
would be really great for users to be able to specify BPF programs with
something like SEC("usdt/<path-to-binary>:<usdt_provider>:<usdt_name>")
and have correct BPF program type set (BPF_PROGRAM_TYPE_KPROBE, as it is
uprobe) and even support BPF skeleton's auto-attach logic.

In some cases, it might be even good idea to override libbpf's default
handling, like for SEC("perf_event") programs. With custom library, it's
possible to extend logic to support specifying perf event specification
right there in SEC() definition without burdening libbpf with lots of
custom logic or extra library dependecies (e.g., libpfm4). With current
patch it's possible to override libbpf's SEC("perf_event") handling and
specify a completely custom ones.

Further, it's possible to specify a generic fallback handling for any
SEC() that doesn't match any other custom or standard libbpf handlers.
This allows to accommodate whatever legacy use cases there might be, if
necessary.

See doc comments for libbpf_register_prog_handler() and
libbpf_unregister_prog_handler() for detailed semantics.

This patch also bumps libbpf development version to v0.8 and adds new
APIs there.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>

tools/lib/bpf/libbpf.c

@@ -201,13 +201,6 @@ struct reloc_desc {
 	};
 };
 
-typedef int (*libbpf_prog_setup_fn_t)(struct bpf_program *prog, long cookie);
-typedef int (*libbpf_prog_prepare_load_fn_t)(struct bpf_program *prog,
-					     struct bpf_prog_load_opts *opts, long cookie);
-/* If auto-attach is not supported, callback should return 0 and set link to NULL */
-typedef int (*libbpf_prog_attach_fn_t)(const struct bpf_program *prog, long cookie,
-				       struct bpf_link **link);
-
 /* stored as sec_def->cookie for all libbpf-supported SEC()s */
 enum sec_def_flags {
 	SEC_NONE = 0,
@@ -235,10 +228,11 @@ enum sec_def_flags {
 };
 
 struct bpf_sec_def {
-	const char *sec;
+	char *sec;
 	enum bpf_prog_type prog_type;
 	enum bpf_attach_type expected_attach_type;
 	long cookie;
+	int handler_id;
 
 	libbpf_prog_setup_fn_t prog_setup_fn;
 	libbpf_prog_prepare_load_fn_t prog_prepare_load_fn;
@@ -8584,7 +8578,7 @@ int bpf_program__set_log_buf(struct bpf_program *prog, char *log_buf, size_t log
 }
 
 #define SEC_DEF(sec_pfx, ptype, atype, flags, ...) {			    \
-	.sec = sec_pfx,							    \
+	.sec = (char *)sec_pfx,						    \
 	.prog_type = BPF_PROG_TYPE_##ptype,				    \
 	.expected_attach_type = atype,					    \
 	.cookie = (long)(flags),					    \
@@ -8677,61 +8671,167 @@ static const struct bpf_sec_def section_defs[] = {
 	SEC_DEF("sk_lookup",		SK_LOOKUP, BPF_SK_LOOKUP, SEC_ATTACHABLE | SEC_SLOPPY_PFX),
 };
 
-#define MAX_TYPE_NAME_SIZE 32
+static size_t custom_sec_def_cnt;
+static struct bpf_sec_def *custom_sec_defs;
+static struct bpf_sec_def custom_fallback_def;
+static bool has_custom_fallback_def;
 
-static const struct bpf_sec_def *find_sec_def(const char *sec_name)
+static int last_custom_sec_def_handler_id;
+
+int libbpf_register_prog_handler(const char *sec,
+				 enum bpf_prog_type prog_type,
+				 enum bpf_attach_type exp_attach_type,
+				 const struct libbpf_prog_handler_opts *opts)
 {
-	const struct bpf_sec_def *sec_def;
-	enum sec_def_flags sec_flags;
-	int i, n = ARRAY_SIZE(section_defs), len;
-	bool strict = libbpf_mode & LIBBPF_STRICT_SEC_NAME;
+	struct bpf_sec_def *sec_def;
 
-	for (i = 0; i < n; i++) {
-		sec_def = &section_defs[i];
-		sec_flags = sec_def->cookie;
-		len = strlen(sec_def->sec);
+	if (!OPTS_VALID(opts, libbpf_prog_handler_opts))
+		return libbpf_err(-EINVAL);
 
-		/* "type/" always has to have proper SEC("type/extras") form */
-		if (sec_def->sec[len - 1] == '/') {
-			if (str_has_pfx(sec_name, sec_def->sec))
-				return sec_def;
-			continue;
-		}
+	if (last_custom_sec_def_handler_id == INT_MAX) /* prevent overflow */
+		return libbpf_err(-E2BIG);
 
-		/* "type+" means it can be either exact SEC("type") or
-		 * well-formed SEC("type/extras") with proper '/' separator
-		 */
-		if (sec_def->sec[len - 1] == '+') {
-			len--;
-			/* not even a prefix */
-			if (strncmp(sec_name, sec_def->sec, len) != 0)
-				continue;
-			/* exact match or has '/' separator */
-			if (sec_name[len] == '\0' || sec_name[len] == '/')
-				return sec_def;
-			continue;
-		}
+	if (sec) {
+		sec_def = libbpf_reallocarray(custom_sec_defs, custom_sec_def_cnt + 1,
+					      sizeof(*sec_def));
+		if (!sec_def)
+			return libbpf_err(-ENOMEM);
 
-		/* SEC_SLOPPY_PFX definitions are allowed to be just prefix
-		 * matches, unless strict section name mode
-		 * (LIBBPF_STRICT_SEC_NAME) is enabled, in which case the
-		 * match has to be exact.
-		 */
-		if ((sec_flags & SEC_SLOPPY_PFX) && !strict)  {
-			if (str_has_pfx(sec_name, sec_def->sec))
-				return sec_def;
-			continue;
-		}
+		custom_sec_defs = sec_def;
+		sec_def = &custom_sec_defs[custom_sec_def_cnt];
+	} else {
+		if (has_custom_fallback_def)
+			return libbpf_err(-EBUSY);
 
-		/* Definitions not marked SEC_SLOPPY_PFX (e.g.,
-		 * SEC("syscall")) are exact matches in both modes.
-		 */
-		if (strcmp(sec_name, sec_def->sec) == 0)
+		sec_def = &custom_fallback_def;
+	}
+
+	sec_def->sec = sec ? strdup(sec) : NULL;
+	if (sec && !sec_def->sec)
+		return libbpf_err(-ENOMEM);
+
+	sec_def->prog_type = prog_type;
+	sec_def->expected_attach_type = exp_attach_type;
+	sec_def->cookie = OPTS_GET(opts, cookie, 0);
+
+	sec_def->prog_setup_fn = OPTS_GET(opts, prog_setup_fn, NULL);
+	sec_def->prog_prepare_load_fn = OPTS_GET(opts, prog_prepare_load_fn, NULL);
+	sec_def->prog_attach_fn = OPTS_GET(opts, prog_attach_fn, NULL);
+
+	sec_def->handler_id = ++last_custom_sec_def_handler_id;
+
+	if (sec)
+		custom_sec_def_cnt++;
+	else
+		has_custom_fallback_def = true;
+
+	return sec_def->handler_id;
+}
+
+int libbpf_unregister_prog_handler(int handler_id)
+{
+	struct bpf_sec_def *sec_defs;
+	int i;
+
+	if (handler_id <= 0)
+		return libbpf_err(-EINVAL);
+
+	if (has_custom_fallback_def && custom_fallback_def.handler_id == handler_id) {
+		memset(&custom_fallback_def, 0, sizeof(custom_fallback_def));
+		has_custom_fallback_def = false;
+		return 0;
+	}
+
+	for (i = 0; i < custom_sec_def_cnt; i++) {
+		if (custom_sec_defs[i].handler_id == handler_id)
+			break;
+	}
+
+	if (i == custom_sec_def_cnt)
+		return libbpf_err(-ENOENT);
+
+	free(custom_sec_defs[i].sec);
+	for (i = i + 1; i < custom_sec_def_cnt; i++)
+		custom_sec_defs[i - 1] = custom_sec_defs[i];
+	custom_sec_def_cnt--;
+
+	/* try to shrink the array, but it's ok if we couldn't */
+	sec_defs = libbpf_reallocarray(custom_sec_defs, custom_sec_def_cnt, sizeof(*sec_defs));
+	if (sec_defs)
+		custom_sec_defs = sec_defs;
+
+	return 0;
+}
+
+static bool sec_def_matches(const struct bpf_sec_def *sec_def, const char *sec_name,
+			    bool allow_sloppy)
+{
+	size_t len = strlen(sec_def->sec);
+
+	/* "type/" always has to have proper SEC("type/extras") form */
+	if (sec_def->sec[len - 1] == '/') {
+		if (str_has_pfx(sec_name, sec_def->sec))
+			return true;
+		return false;
+	}
+
+	/* "type+" means it can be either exact SEC("type") or
+	 * well-formed SEC("type/extras") with proper '/' separator
+	 */
+	if (sec_def->sec[len - 1] == '+') {
+		len--;
+		/* not even a prefix */
+		if (strncmp(sec_name, sec_def->sec, len) != 0)
+			return false;
+		/* exact match or has '/' separator */
+		if (sec_name[len] == '\0' || sec_name[len] == '/')
+			return true;
+		return false;
+	}
+
+	/* SEC_SLOPPY_PFX definitions are allowed to be just prefix
+	 * matches, unless strict section name mode
+	 * (LIBBPF_STRICT_SEC_NAME) is enabled, in which case the
+	 * match has to be exact.
+	 */
+	if (allow_sloppy && str_has_pfx(sec_name, sec_def->sec))
+		return true;
+
+	/* Definitions not marked SEC_SLOPPY_PFX (e.g.,
+	 * SEC("syscall")) are exact matches in both modes.
+	 */
+	return strcmp(sec_name, sec_def->sec) == 0;
+}
+
+static const struct bpf_sec_def *find_sec_def(const char *sec_name)
+{
+	const struct bpf_sec_def *sec_def;
+	int i, n;
+	bool strict = libbpf_mode & LIBBPF_STRICT_SEC_NAME, allow_sloppy;
+
+	n = custom_sec_def_cnt;
+	for (i = 0; i < n; i++) {
+		sec_def = &custom_sec_defs[i];
+		if (sec_def_matches(sec_def, sec_name, false))
+			return sec_def;
+	}
+
+	n = ARRAY_SIZE(section_defs);
+	for (i = 0; i < n; i++) {
+		sec_def = &section_defs[i];
+		allow_sloppy = (sec_def->cookie & SEC_SLOPPY_PFX) && !strict;
+		if (sec_def_matches(sec_def, sec_name, allow_sloppy))
 			return sec_def;
 	}
+
+	if (has_custom_fallback_def)
+		return &custom_fallback_def;
+
 	return NULL;
 }
 
+#define MAX_TYPE_NAME_SIZE 32
+
 static char *libbpf_get_type_names(bool attach_type)
 {
 	int i, len = ARRAY_SIZE(section_defs) * MAX_TYPE_NAME_SIZE;

tools/lib/bpf/libbpf.h

@@ -1328,6 +1328,93 @@ LIBBPF_API int bpf_linker__add_file(struct bpf_linker *linker,
 LIBBPF_API int bpf_linker__finalize(struct bpf_linker *linker);
 LIBBPF_API void bpf_linker__free(struct bpf_linker *linker);
 
+/*
+ * Custom handling of BPF program's SEC() definitions
+ */
+
+struct bpf_prog_load_opts; /* defined in bpf.h */
+
+/* Called during bpf_object__open() for each recognized BPF program. Callback
+ * can use various bpf_program__set_*() setters to adjust whatever properties
+ * are necessary.
+ */
+typedef int (*libbpf_prog_setup_fn_t)(struct bpf_program *prog, long cookie);
+
+/* Called right before libbpf performs bpf_prog_load() to load BPF program
+ * into the kernel. Callback can adjust opts as necessary.
+ */
+typedef int (*libbpf_prog_prepare_load_fn_t)(struct bpf_program *prog,
+					     struct bpf_prog_load_opts *opts, long cookie);
+
+/* Called during skeleton attach or through bpf_program__attach(). If
+ * auto-attach is not supported, callback should return 0 and set link to
+ * NULL (it's not considered an error during skeleton attach, but it will be
+ * an error for bpf_program__attach() calls). On error, error should be
+ * returned directly and link set to NULL. On success, return 0 and set link
+ * to a valid struct bpf_link.
+ */
+typedef int (*libbpf_prog_attach_fn_t)(const struct bpf_program *prog, long cookie,
+				       struct bpf_link **link);
+
+struct libbpf_prog_handler_opts {
+	/* size of this struct, for forward/backward compatiblity */
+	size_t sz;
+	/* User-provided cookie passed to each callback */
+	long cookie;
+	/* BPF program initialization callback (see libbpf_prog_setup_fn_t) */
+	libbpf_prog_setup_fn_t prog_setup_fn;
+	/* BPF program loading callback (see libbpf_prog_prepare_load_fn_t) */
+	libbpf_prog_prepare_load_fn_t prog_prepare_load_fn;
+	/* BPF program attach callback (see libbpf_prog_attach_fn_t) */
+	libbpf_prog_attach_fn_t prog_attach_fn;
+};
+#define libbpf_prog_handler_opts__last_field prog_attach_fn
+
+/**
+ * @brief **libbpf_register_prog_handler()** registers a custom BPF program
+ * SEC() handler.
+ * @param sec section prefix for which custom handler is registered
+ * @param prog_type BPF program type associated with specified section
+ * @param exp_attach_type Expected BPF attach type associated with specified section
+ * @param opts optional cookie, callbacks, and other extra options
+ * @return Non-negative handler ID is returned on success. This handler ID has
+ * to be passed to *libbpf_unregister_prog_handler()* to unregister such
+ * custom handler. Negative error code is returned on error.
+ *
+ * *sec* defines which SEC() definitions are handled by this custom handler
+ * registration. *sec* can have few different forms:
+ *   - if *sec* is just a plain string (e.g., "abc"), it will match only
+ *   SEC("abc"). If BPF program specifies SEC("abc/whatever") it will result
+ *   in an error;
+ *   - if *sec* is of the form "abc/", proper SEC() form is
+ *   SEC("abc/something"), where acceptable "something" should be checked by
+ *   *prog_init_fn* callback, if there are additional restrictions;
+ *   - if *sec* is of the form "abc+", it will successfully match both
+ *   SEC("abc") and SEC("abc/whatever") forms;
+ *   - if *sec* is NULL, custom handler is registered for any BPF program that
+ *   doesn't match any of the registered (custom or libbpf's own) SEC()
+ *   handlers. There could be only one such generic custom handler registered
+ *   at any given time.
+ *
+ * All custom handlers (except the one with *sec* == NULL) are processed
+ * before libbpf's own SEC() handlers. It is allowed to "override" libbpf's
+ * SEC() handlers by registering custom ones for the same section prefix
+ * (i.e., it's possible to have custom SEC("perf_event/LLC-load-misses")
+ * handler).
+ */
+LIBBPF_API int libbpf_register_prog_handler(const char *sec,
+					    enum bpf_prog_type prog_type,
+					    enum bpf_attach_type exp_attach_type,
+					    const struct libbpf_prog_handler_opts *opts);
+/**
+ * @brief *libbpf_unregister_prog_handler()* unregisters previously registered
+ * custom BPF program SEC() handler.
+ * @param handler_id handler ID returned by *libbpf_register_prog_handler()*
+ * after successful registration
+ * @return 0 on success, negative error code if handler isn't found
+ */
+LIBBPF_API int libbpf_unregister_prog_handler(int handler_id);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif

tools/lib/bpf/libbpf.map

@@ -439,3 +439,9 @@ LIBBPF_0.7.0 {
 		libbpf_probe_bpf_prog_type;
 		libbpf_set_memlock_rlim_max;
 } LIBBPF_0.6.0;
+
+LIBBPF_0.8.0 {
+	global:
+		libbpf_register_prog_handler;
+		libbpf_unregister_prog_handler;
+} LIBBPF_0.7.0;

tools/lib/bpf/libbpf_version.h

@@ -4,6 +4,6 @@
 #define __LIBBPF_VERSION_H
 
 #define LIBBPF_MAJOR_VERSION 0
-#define LIBBPF_MINOR_VERSION 7
+#define LIBBPF_MINOR_VERSION 8
 
 #endif /* __LIBBPF_VERSION_H */