Merge branch 'Add libbpf support for USDTs'

Andrii Nakryiko says:

====================

Add libbpf support for USDT (User Statically-Defined Tracing) probes.
USDTs is important part of tracing, and BPF, ecosystem, widely used in
mission-critical production applications for observability, performance
analysis, and debugging.

And while USDTs themselves are pretty complicated abstraction built on top of
uprobes, for end-users USDT is as natural a primitive as uprobes themselves.
And thus it's important for libbpf to provide best possible user experience
when it comes to build tracing applications relying on USDTs.

USDTs historically presented a lot of challenges for libbpf's no
compilation-on-the-fly general approach to BPF tracing. BCC utilizes power of
on-the-fly source code generation and compilation using its embedded Clang
toolchain, which was impractical for more lightweight and thus more rigid
libbpf-based approach. But still, with enough diligence and BPF cookies it's
possible to implement USDT support that feels as natural as tracing any
uprobe.

This patch set is the culmination of such effort to add libbpf USDT support
following the spirit and philosophy of BPF CO-RE (even though it's not
inherently relying on BPF CO-RE much, see patch #1 for some notes regarding
this). Each respective patch has enough details and explanations, so I won't
go into details here.

In the end, I think the overall usability of libbpf's USDT support *exceeds*
the status quo set by BCC due to the elimination of awkward runtime USDT
supporting code generation. It also exceeds BCC's capabilities due to the use
of BPF cookie. This eliminates the need to determine a USDT call site (and
thus specifics about how exactly to fetch arguments) based on its *absolute IP
address*, which is impossible with shared libraries if no PID is specified (as
we then just *can't* know absolute IP at which shared library is loaded,
because it might be different for each process). With BPF cookie this is not
a problem as we record "call site ID" directly in a BPF cookie value. This
makes it possible to do a system-wide tracing of a USDT defined in a shared
library. Think about tracing some USDT in libc across any process in the
system, both running at the time of attachment and all the new processes
started *afterwards*. This is a very powerful capability that allows more
efficient observability and tracing tooling.

Once this functionality lands, the plan is to extend libbpf-bootstrap ([0])
with an USDT example. It will also become possible to start converting BCC
tools that rely on USDTs to their libbpf-based counterparts ([1]).

It's worth noting that preliminary version of this code was currently used and
tested in production code running fleet-wide observability toolkit.

Libbpf functionality is broken down into 5 mostly logically independent parts,
for ease of reviewing:
  - patch #1 adds BPF-side implementation;
  - patch #2 adds user-space APIs and wires bpf_link for USDTs;
  - patch #3 adds the most mundate pieces: handling ELF, parsing USDT notes,
    dealing with memory segments, relative vs absolute addresses, etc;
  - patch #4 adds internal ID allocation and setting up/tearing down of
    BPF-side state (spec and IP-to-ID mapping);
  - patch #5 implements x86/x86-64-specific logic of parsing USDT argument
    specifications;
  - patch #6 adds testing of various basic aspects of handling of USDT;
  - patch #7 extends the set of tests with more combinations of semaphore,
    executable vs shared library, and PID filter options.

  [0] https://github.com/libbpf/libbpf-bootstrap
  [1] https://github.com/iovisor/bcc/tree/master/libbpf-tools

v2->v3:
  - fix typos, leave link to systemtap doc, acks, etc (Dave);
  - include sys/sdt.h to avoid extra system-wide package dependencies;
v1->v2:
  - huge high-level comment describing how all the moving parts fit together
    (Alan, Alexei);
  - switched from `__hidden __weak` to `static inline __noinline` for now, as
    there is a bug in BPF linker breaking final BPF object file due to invalid
    .BTF.ext data; I want to fix it separately at which point I'll switch back
    to __hidden __weak again. The fix isn't trivial, so I don't want to block
    on that. Same for __weak variable lookup bug that Henqi reported.
  - various fixes and improvements, addressing other feedback (Alan, Hengqi);

Cc: Alan Maguire <[email protected]>
Cc: Dave Marchevsky <[email protected]>
Cc: Hengqi Chen <[email protected]>
====================

Signed-off-by: Alexei Starovoitov <[email protected]>

1 files changed, 105 insertions, 10 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 91ce94b61f7f..1111e9d16e01 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -483,6 +483,8 @@ struct elf_state {
 	int st_ops_shndx;
 };
 
+struct usdt_manager;
+
 struct bpf_object {
 	char name[BPF_OBJ_NAME_LEN];
 	char license[64];
@@ -545,6 +547,8 @@ struct bpf_object {
 	size_t fd_array_cap;
 	size_t fd_array_cnt;
 
+	struct usdt_manager *usdt_man;
+
 	char path[];
 };
 
@@ -4678,6 +4682,18 @@ static int probe_perf_link(void)
 	return link_fd < 0 && err == -EBADF;
 }
 
+static int probe_kern_bpf_cookie(void)
+{
+	struct bpf_insn insns[] = {
+		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_get_attach_cookie),
+		BPF_EXIT_INSN(),
+	};
+	int ret, insn_cnt = ARRAY_SIZE(insns);
+
+	ret = bpf_prog_load(BPF_PROG_TYPE_KPROBE, NULL, "GPL", insns, insn_cnt, NULL);
+	return probe_fd(ret);
+}
+
 enum kern_feature_result {
 	FEAT_UNKNOWN = 0,
 	FEAT_SUPPORTED = 1,
@@ -4740,6 +4756,9 @@ static struct kern_feature_desc {
 	[FEAT_MEMCG_ACCOUNT] = {
 		"memcg-based memory accounting", probe_memcg_account,
 	},
+	[FEAT_BPF_COOKIE] = {
+		"BPF cookie support", probe_kern_bpf_cookie,
+	},
 };
 
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
@@ -8200,6 +8219,9 @@ void bpf_object__close(struct bpf_object *obj)
 	if (obj->clear_priv)
 		obj->clear_priv(obj, obj->priv);
 
+	usdt_manager_free(obj->usdt_man);
+	obj->usdt_man = NULL;
+
 	bpf_gen__free(obj->gen_loader);
 	bpf_object__elf_finish(obj);
 	bpf_object_unload(obj);
@@ -8631,6 +8653,7 @@ 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_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);
 static int attach_trace(const struct bpf_program *prog, long cookie, struct bpf_link **link);
@@ -8648,6 +8671,7 @@ static const struct bpf_sec_def section_defs[] = {
 	SEC_DEF("uretprobe+",		KPROBE, 0, SEC_NONE, 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("usdt+",		KPROBE,	0, SEC_NONE, attach_usdt),
 	SEC_DEF("tc",			SCHED_CLS, 0, SEC_NONE),
 	SEC_DEF("classifier",		SCHED_CLS, 0, SEC_NONE | SEC_SLOPPY_PFX | SEC_DEPRECATED),
 	SEC_DEF("action",		SCHED_ACT, 0, SEC_NONE | SEC_SLOPPY_PFX),
@@ -9693,14 +9717,6 @@ int bpf_prog_load_deprecated(const char *file, enum bpf_prog_type type,
 	return bpf_prog_load_xattr2(&attr, pobj, prog_fd);
 }
 
-struct bpf_link {
-	int (*detach)(struct bpf_link *link);
-	void (*dealloc)(struct bpf_link *link);
-	char *pin_path;		/* NULL, if not pinned */
-	int fd;			/* hook FD, -1 if not applicable */
-	bool disconnected;
-};
-
 /* Replace link's underlying BPF program with the new one */
 int bpf_link__update_program(struct bpf_link *link, struct bpf_program *prog)
 {
@@ -10810,8 +10826,8 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
 		err = resolve_full_path(binary_path, full_binary_path,
 					sizeof(full_binary_path));
 		if (err) {
-			pr_warn("prog '%s': failed to resolve full path for '%s'\n",
-				prog->name, binary_path);
+			pr_warn("prog '%s': failed to resolve full path for '%s': %d\n",
+				prog->name, binary_path, err);
 			return libbpf_err_ptr(err);
 		}
 		binary_path = full_binary_path;
@@ -10963,6 +10979,85 @@ struct bpf_link *bpf_program__attach_uprobe(const struct bpf_program *prog,
 	return bpf_program__attach_uprobe_opts(prog, pid, binary_path, func_offset, &opts);
 }
 
+struct bpf_link *bpf_program__attach_usdt(const struct bpf_program *prog,
+					  pid_t pid, const char *binary_path,
+					  const char *usdt_provider, const char *usdt_name,
+					  const struct bpf_usdt_opts *opts)
+{
+	char resolved_path[512];
+	struct bpf_object *obj = prog->obj;
+	struct bpf_link *link;
+	long usdt_cookie;
+	int err;
+
+	if (!OPTS_VALID(opts, bpf_uprobe_opts))
+		return libbpf_err_ptr(-EINVAL);
+
+	if (bpf_program__fd(prog) < 0) {
+		pr_warn("prog '%s': can't attach BPF program w/o FD (did you load it?)\n",
+			prog->name);
+		return libbpf_err_ptr(-EINVAL);
+	}
+
+	if (!strchr(binary_path, '/')) {
+		err = resolve_full_path(binary_path, resolved_path, sizeof(resolved_path));
+		if (err) {
+			pr_warn("prog '%s': failed to resolve full path for '%s': %d\n",
+				prog->name, binary_path, err);
+			return libbpf_err_ptr(err);
+		}
+		binary_path = resolved_path;
+	}
+
+	/* USDT manager is instantiated lazily on first USDT attach. It will
+	 * be destroyed together with BPF object in bpf_object__close().
+	 */
+	if (IS_ERR(obj->usdt_man))
+		return libbpf_ptr(obj->usdt_man);
+	if (!obj->usdt_man) {
+		obj->usdt_man = usdt_manager_new(obj);
+		if (IS_ERR(obj->usdt_man))
+			return libbpf_ptr(obj->usdt_man);
+	}
+
+	usdt_cookie = OPTS_GET(opts, usdt_cookie, 0);
+	link = usdt_manager_attach_usdt(obj->usdt_man, prog, pid, binary_path,
+				        usdt_provider, usdt_name, usdt_cookie);
+	err = libbpf_get_error(link);
+	if (err)
+		return libbpf_err_ptr(err);
+	return link;
+}
+
+static int attach_usdt(const struct bpf_program *prog, long cookie, struct bpf_link **link)
+{
+	char *path = NULL, *provider = NULL, *name = NULL;
+	const char *sec_name;
+	int n, err;
+
+	sec_name = bpf_program__section_name(prog);
+	if (strcmp(sec_name, "usdt") == 0) {
+		/* no auto-attach for just SEC("usdt") */
+		*link = NULL;
+		return 0;
+	}
+
+	n = sscanf(sec_name, "usdt/%m[^:]:%m[^:]:%m[^:]", &path, &provider, &name);
+	if (n != 3) {
+		pr_warn("invalid section '%s', expected SEC(\"usdt/<path>:<provider>:<name>\")\n",
+			sec_name);
+		err = -EINVAL;
+	} else {
+		*link = bpf_program__attach_usdt(prog, -1 /* any process */, path,
+						 provider, name, NULL);
+		err = libbpf_get_error(*link);
+	}
+	free(path);
+	free(provider);
+	free(name);
+	return err;
+}
+
 static int determine_tracepoint_id(const char *tp_category,
 				   const char *tp_name)
 {