[PATCH bpf-next v3 1/2] libbpf: clarify raw-address single kprobe attach behavior

[Hoyeon Lee <hoyeon.lee@suse.com>]

bpf_program__attach_kprobe_opts() documents single-kprobe attach
through func_name, with an optional offset. For the PMU-based
non-legacy path, func_name = NULL with an absolute address in offset
already works as well, but that is not described in the API.

This commit clarifies this existing non-legacy behavior. For PMU-based
attach, callers can use func_name = NULL with an absolute address in
offset as the raw-address form. For legacy tracefs/debugfs kprobes,
reject this form explicitly.

Signed-off-by: Hoyeon Lee <hoyeon.lee@suse.com>
---
 tools/lib/bpf/libbpf.c | 33 ++++++++++++++++++++++++---------
 tools/lib/bpf/libbpf.h |  3 ++-
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 9ea41f40dc82..8cf90a073455 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -28,6 +28,7 @@
 #include <asm/unistd.h>
 #include <linux/err.h>
 #include <linux/kernel.h>
+#include <linux/kallsyms.h>
 #include <linux/bpf.h>
 #include <linux/btf.h>
 #include <linux/filter.h>
@@ -11523,7 +11524,8 @@ static int determine_uprobe_retprobe_bit(void)
 #define PERF_UPROBE_REF_CTR_OFFSET_SHIFT 32
 
 static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
-				 uint64_t offset, int pid, size_t ref_ctr_off)
+				 uint64_t offset_or_addr, int pid,
+				 size_t ref_ctr_off)
 {
 	const size_t attr_sz = sizeof(struct perf_event_attr);
 	struct perf_event_attr attr;
@@ -11558,7 +11560,7 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
 	attr.type = type;
 	attr.config |= (__u64)ref_ctr_off << PERF_UPROBE_REF_CTR_OFFSET_SHIFT;
 	attr.config1 = ptr_to_u64(name); /* kprobe_func or uprobe_path */
-	attr.config2 = offset;		 /* kprobe_addr or probe_offset */
+	attr.config2 = offset_or_addr;	 /* kprobe_addr or probe_offset */
 
 	/* pid filter is meaningful only for uprobes */
 	pfd = syscall(__NR_perf_event_open, &attr,
@@ -11648,6 +11650,15 @@ static void gen_probe_legacy_event_name(char *buf, size_t buf_sz,
 	}
 }
 
+static void gen_kprobe_target(char *buf, size_t buf_sz, const char *name,
+			      uint64_t offset_or_addr)
+{
+	if (name)
+		snprintf(buf, buf_sz, "%s+0x%" PRIx64, name, offset_or_addr);
+	else
+		snprintf(buf, buf_sz, "0x%" PRIx64, offset_or_addr);
+}
+
 static int add_kprobe_event_legacy(const char *probe_name, bool retprobe,
 				   const char *kfunc_name, size_t offset)
 {
@@ -11781,6 +11792,7 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 				const struct bpf_kprobe_opts *opts)
 {
 	DECLARE_LIBBPF_OPTS(bpf_perf_event_opts, pe_opts);
+	char probe_target[KSYM_NAME_LEN + 32];
 	enum probe_attach_mode attach_mode;
 	char *legacy_probe = NULL;
 	struct bpf_link *link;
@@ -11816,6 +11828,11 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 	default:
 		return libbpf_err_ptr(-EINVAL);
 	}
+	if (!func_name && legacy)
+		return libbpf_err_ptr(-ENOTSUP);
+
+	gen_kprobe_target(probe_target, sizeof(probe_target),
+			  func_name, offset);
 
 	if (!legacy) {
 		pfd = perf_event_open_probe(false /* uprobe */, retprobe,
@@ -11835,21 +11852,19 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 						    offset, -1 /* pid */);
 	}
 	if (pfd < 0) {
-		err = -errno;
-		pr_warn("prog '%s': failed to create %s '%s+0x%zx' perf event: %s\n",
+		err = pfd;
+		pr_warn("prog '%s': failed to create %s '%s' perf event: %s\n",
 			prog->name, retprobe ? "kretprobe" : "kprobe",
-			func_name, offset,
-			errstr(err));
+			probe_target, errstr(err));
 		goto err_out;
 	}
 	link = bpf_program__attach_perf_event_opts(prog, pfd, &pe_opts);
 	err = libbpf_get_error(link);
 	if (err) {
 		close(pfd);
-		pr_warn("prog '%s': failed to attach to %s '%s+0x%zx': %s\n",
+		pr_warn("prog '%s': failed to attach to %s '%s': %s\n",
 			prog->name, retprobe ? "kretprobe" : "kprobe",
-			func_name, offset,
-			errstr(err));
+			probe_target, errstr(err));
 		goto err_clean_legacy;
 	}
 	if (legacy) {
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 0be34852350f..f75f3ab0f20c 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -557,7 +557,7 @@ struct bpf_kprobe_opts {
 	size_t sz;
 	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
 	__u64 bpf_cookie;
-	/* function's offset to install kprobe to */
+	/* function offset, or raw address if func_name == NULL (non-legacy) */
 	size_t offset;
 	/* kprobe is return probe */
 	bool retprobe;
@@ -565,6 +565,7 @@ struct bpf_kprobe_opts {
 	enum probe_attach_mode attach_mode;
 	size_t :0;
 };
+
 #define bpf_kprobe_opts__last_field attach_mode
 
 LIBBPF_API struct bpf_link *
-- 
2.52.0