[PATCH bpf-next v4 3/4] bpf, libbpf: Capture error message of freplace attachment failure

Leon Hwang
2025-02-24T15:34:53Z

To improve debugging, this patch captures the log of
bpf_check_attach_target() when a freplace program fails to attach. Users
can supply a buffer to receive the log.

Changes:

* Extended bpf_attr and bpf_link_create_opts to include a log buffer for
tracing.
* Updated bpf_link_create() to handle log buffer properly.
* Add bpf_program__attach_freplace_opts() to use users' supplied log
buffer.

This helps diagnose freplace attachment failures more efficiently.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
tools/include/uapi/linux/bpf.h | 2 ++
tools/lib/bpf/bpf.c | 6 +++++-
tools/lib/bpf/bpf.h | 2 ++
tools/lib/bpf/libbpf.c | 29 +++++++++++++++++++++++++----
tools/lib/bpf/libbpf.h | 14 ++++++++++++++
tools/lib/bpf/libbpf.map | 1 +
6 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index fff6cdb8d11a2..bea4d802d4463 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1759,6 +1759,8 @@ union bpf_attr {
* accessible through bpf_get_attach_cookie() BPF helper
*/
__u64 cookie;
+ __aligned_u64 log_buf; /* user supplied buffer */
+ __u32 log_size; /* size of user buffer */
} tracing;
struct {
__u32 pf;
diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index 359f73ead6137..cd422ecd53ae2 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c
@@ -741,7 +741,7 @@ int bpf_link_create(int prog_fd, int target_fd,
if (iter_info_len || target_btf_id) {
if (iter_info_len && target_btf_id)
return libbpf_err(-EINVAL);
- if (!OPTS_ZEROED(opts, target_btf_id))
+ if (!OPTS_ZEROED(opts, tracing))
return libbpf_err(-EINVAL);
}

@@ -753,6 +753,8 @@ int bpf_link_create(int prog_fd, int target_fd,

if (target_btf_id) {
attr.link_create.target_btf_id = target_btf_id;
+ attr.link_create.tracing.log_buf = ptr_to_u64(OPTS_GET(opts, tracing.log_buf, 0));
+ attr.link_create.tracing.log_size = OPTS_GET(opts, tracing.log_size, 0);
goto proceed;
}

@@ -794,6 +796,8 @@ int bpf_link_create(int prog_fd, int target_fd,
case BPF_MODIFY_RETURN:
case BPF_LSM_MAC:
attr.link_create.tracing.cookie = OPTS_GET(opts, tracing.cookie, 0);
+ attr.link_create.tracing.log_buf = 0;
+ attr.link_create.tracing.log_size = 0;
if (!OPTS_ZEROED(opts, tracing))
return libbpf_err(-EINVAL);
break;
diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
index 435da95d20589..c76222617c18c 100644
--- a/tools/lib/bpf/bpf.h
+++ b/tools/lib/bpf/bpf.h
@@ -421,6 +421,8 @@ struct bpf_link_create_opts {
} uprobe_multi;
struct {
__u64 cookie;
+ const char *log_buf;
+ size_t log_size;
} tracing;
struct {
__u32 pf;
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6df258912e1ec..f0c42669ec1e1 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -12837,10 +12837,11 @@ bpf_program__attach_netkit(const struct bpf_program *prog, int ifindex,
return bpf_program_attach_fd(prog, ifindex, "netkit", &link_create_opts);
}

-struct bpf_link *bpf_program__attach_freplace(const struct bpf_program *prog,
- int target_fd,
- const char *attach_func_name)
+static struct bpf_link *bpf_program_attach_freplace(const struct bpf_freplace_opts *opts)
{
+ const char *attach_func_name = opts->attach_func_name;
+ const struct bpf_program *prog = opts->prog;
+ int target_fd = opts->target_prog_fd;
int btf_id;

if (!!target_fd != !!attach_func_name) {
@@ -12863,7 +12864,8 @@ struct bpf_link *bpf_program__attach_freplace(const struct bpf_program *prog,
return libbpf_err_ptr(btf_id);

target_opts.target_btf_id = btf_id;
-
+ target_opts.tracing.log_buf = opts->log_buf;
+ target_opts.tracing.log_size = opts->log_buf_size;
return bpf_program_attach_fd(prog, target_fd, "freplace",
&target_opts);
} else {
@@ -12874,6 +12876,25 @@ struct bpf_link *bpf_program__attach_freplace(const struct bpf_program *prog,
}
}

+struct bpf_link *bpf_program__attach_freplace(const struct bpf_program *prog,
+ int target_fd,
+ const char *attach_func_name)
+{
+ LIBBPF_OPTS(bpf_freplace_opts, opts,
+ .prog = prog,
+ .target_prog_fd = target_fd,
+ .attach_func_name = attach_func_name,
+ );
+
+ return bpf_program_attach_freplace(&opts);
+}
+
+struct bpf_link *
+bpf_program__attach_freplace_opts(const struct bpf_freplace_opts *opts)
+{
+ return bpf_program_attach_freplace(opts);
+}
+
struct bpf_link *
bpf_program__attach_iter(const struct bpf_program *prog,
const struct bpf_iter_attach_opts *opts)
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3020ee45303a0..8de595da7e7d7 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -819,6 +819,20 @@ LIBBPF_API struct bpf_link *
bpf_program__attach_freplace(const struct bpf_program *prog,
int target_fd, const char *attach_func_name);

+struct bpf_freplace_opts {
+ size_t sz; /* size of this struct for forward/backward compatibility */
+
+ /* freplace bpf prog */
+ const struct bpf_program *prog;
+ int target_prog_fd;
+ const char *attach_func_name;
+ /* buffer to receive error message when fail to bpf_check_attach_target */
+ const char *log_buf;
+ size_t log_buf_size;
+};
+LIBBPF_API struct bpf_link *
+bpf_program__attach_freplace_opts(const struct bpf_freplace_opts *opts);
+
struct bpf_netfilter_opts {
/* size of this struct, for forward/backward compatibility */
size_t sz;
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b5a838de6f47c..5088f8af51dd6 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -438,4 +438,5 @@ LIBBPF_1.6.0 {
bpf_linker__new_fd;
btf__add_decl_attr;
btf__add_type_attr;
+ bpf_program__attach_freplace_opts;
} LIBBPF_1.5.0;
--
2.47.1