2022-04-22 19:53:58

by Connor O'Brien

Subject: [PATCH bpf-next] libbpf: also check /sys/kernel/tracing for tracefs files

libbpf looks for tracefs files only under debugfs, but tracefs may be
mounted even if debugfs is not. When /sys/kernel/debug/tracing is
absent, try looking under /sys/kernel/tracing instead.

Signed-off-by: Connor O'Brien <[email protected]>
---
tools/lib/bpf/libbpf.c | 26 +++++++++++++++++++-------
1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 68cc134d070d..6ef587329eb2 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10140,10 +10140,16 @@ static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
__sync_fetch_and_add(&index, 1));
}

+static bool debugfs_available(void)
+{
+ return !access("/sys/kernel/debug/tracing", F_OK);
+}
+
static int add_kprobe_event_legacy(const char *probe_name, bool retprobe,
const char *kfunc_name, size_t offset)
{
- const char *file = "/sys/kernel/debug/tracing/kprobe_events";
+ const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/kprobe_events" :
+ "/sys/kernel/tracing/kprobe_events";

return append_to_file(file, "%c:%s/%s %s+0x%zx",
retprobe ? 'r' : 'p',
@@ -10153,7 +10159,8 @@ static int add_kprobe_event_legacy(const char *probe_name, bool retprobe,

static int remove_kprobe_event_legacy(const char *probe_name, bool retprobe)
{
- const char *file = "/sys/kernel/debug/tracing/kprobe_events";
+ const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/kprobe_events" :
+ "/sys/kernel/tracing/kprobe_events";

return append_to_file(file, "-:%s/%s", retprobe ? "kretprobes" : "kprobes", probe_name);
}
@@ -10163,7 +10170,8 @@ static int determine_kprobe_perf_type_legacy(const char *probe_name, bool retpro
char file[256];

snprintf(file, sizeof(file),
- "/sys/kernel/debug/tracing/events/%s/%s/id",
+ debugfs_available() ? "/sys/kernel/debug/tracing/events/%s/%s/id" :
+ "/sys/kernel/tracing/events/%s/%s/id",
retprobe ? "kretprobes" : "kprobes", probe_name);

return parse_uint_from_file(file, "%d\n");
@@ -10493,7 +10501,8 @@ static void gen_uprobe_legacy_event_name(char *buf, size_t buf_sz,
static inline int add_uprobe_event_legacy(const char *probe_name, bool retprobe,
const char *binary_path, size_t offset)
{
- const char *file = "/sys/kernel/debug/tracing/uprobe_events";
+ const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/uprobe_events" :
+ "/sys/kernel/tracing/uprobe_events";

return append_to_file(file, "%c:%s/%s %s:0x%zx",
retprobe ? 'r' : 'p',
@@ -10503,7 +10512,8 @@ static inline int add_uprobe_event_legacy(const char *probe_name, bool retprobe,

static inline int remove_uprobe_event_legacy(const char *probe_name, bool retprobe)
{
- const char *file = "/sys/kernel/debug/tracing/uprobe_events";
+ const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/uprobe_events" :
+ "/sys/kernel/tracing/uprobe_events";

return append_to_file(file, "-:%s/%s", retprobe ? "uretprobes" : "uprobes", probe_name);
}
@@ -10513,7 +10523,8 @@ static int determine_uprobe_perf_type_legacy(const char *probe_name, bool retpro
char file[512];

snprintf(file, sizeof(file),
- "/sys/kernel/debug/tracing/events/%s/%s/id",
+ debugfs_available() ? "/sys/kernel/debug/tracing/events/%s/%s/id" :
+ "/sys/kernel/tracing/events/%s/%s/id",
retprobe ? "uretprobes" : "uprobes", probe_name);

return parse_uint_from_file(file, "%d\n");
@@ -11071,7 +11082,8 @@ static int determine_tracepoint_id(const char *tp_category,
int ret;

ret = snprintf(file, sizeof(file),
- "/sys/kernel/debug/tracing/events/%s/%s/id",
+ debugfs_available() ? "/sys/kernel/debug/tracing/events/%s/%s/id" :
+ "/sys/kernel/tracing/events/%s/%s/id",
tp_category, tp_name);
if (ret < 0)
return -errno;
--
2.36.0.rc2.479.g8af0fa9b8e-goog

2022-04-22 21:41:28

by Daniel Borkmann

Subject: Re: [PATCH bpf-next] libbpf: also check /sys/kernel/tracing for tracefs files

On 4/21/22 10:11 PM, Connor O'Brien wrote:
> libbpf looks for tracefs files only under debugfs, but tracefs may be
> mounted even if debugfs is not. When /sys/kernel/debug/tracing is
> absent, try looking under /sys/kernel/tracing instead.
>
> Signed-off-by: Connor O'Brien <[email protected]>
> ---
> tools/lib/bpf/libbpf.c | 26 +++++++++++++++++++-------
> 1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 68cc134d070d..6ef587329eb2 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10140,10 +10140,16 @@ static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
> __sync_fetch_and_add(&index, 1));
> }
>
> +static bool debugfs_available(void)
> +{
> + return !access("/sys/kernel/debug/tracing", F_OK);

Should this be a one-time probe, so on subsequent debugfs_available() calls
we return the initially cached result?

> +}
> +
> static int add_kprobe_event_legacy(const char *probe_name, bool retprobe,
> const char *kfunc_name, size_t offset)
> {
> - const char *file = "/sys/kernel/debug/tracing/kprobe_events";
> + const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/kprobe_events" :
> + "/sys/kernel/tracing/kprobe_events";
>
> return append_to_file(file, "%c:%s/%s %s+0x%zx",
> retprobe ? 'r' : 'p',
> @@ -10153,7 +10159,8 @@ static int add_kprobe_event_legacy(const char *probe_name, bool retprobe,
>
> static int remove_kprobe_event_legacy(const char *probe_name, bool retprobe)
> {
> - const char *file = "/sys/kernel/debug/tracing/kprobe_events";
> + const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/kprobe_events" :
> + "/sys/kernel/tracing/kprobe_events";
>
> return append_to_file(file, "-:%s/%s", retprobe ? "kretprobes" : "kprobes", probe_name);
> }
> @@ -10163,7 +10170,8 @@ static int determine_kprobe_perf_type_legacy(const char *probe_name, bool retpro
> char file[256];
>
> snprintf(file, sizeof(file),
> - "/sys/kernel/debug/tracing/events/%s/%s/id",
> + debugfs_available() ? "/sys/kernel/debug/tracing/events/%s/%s/id" :
> + "/sys/kernel/tracing/events/%s/%s/id",
> retprobe ? "kretprobes" : "kprobes", probe_name);
>
> return parse_uint_from_file(file, "%d\n");
> @@ -10493,7 +10501,8 @@ static void gen_uprobe_legacy_event_name(char *buf, size_t buf_sz,
> static inline int add_uprobe_event_legacy(const char *probe_name, bool retprobe,
> const char *binary_path, size_t offset)
> {
> - const char *file = "/sys/kernel/debug/tracing/uprobe_events";
> + const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/uprobe_events" :
> + "/sys/kernel/tracing/uprobe_events";
>
> return append_to_file(file, "%c:%s/%s %s:0x%zx",
> retprobe ? 'r' : 'p',
> @@ -10503,7 +10512,8 @@ static inline int add_uprobe_event_legacy(const char *probe_name, bool retprobe,
>
> static inline int remove_uprobe_event_legacy(const char *probe_name, bool retprobe)
> {
> - const char *file = "/sys/kernel/debug/tracing/uprobe_events";
> + const char *file = debugfs_available() ? "/sys/kernel/debug/tracing/uprobe_events" :
> + "/sys/kernel/tracing/uprobe_events";
>
> return append_to_file(file, "-:%s/%s", retprobe ? "uretprobes" : "uprobes", probe_name);
> }
> @@ -10513,7 +10523,8 @@ static int determine_uprobe_perf_type_legacy(const char *probe_name, bool retpro
> char file[512];
>
> snprintf(file, sizeof(file),
> - "/sys/kernel/debug/tracing/events/%s/%s/id",
> + debugfs_available() ? "/sys/kernel/debug/tracing/events/%s/%s/id" :
> + "/sys/kernel/tracing/events/%s/%s/id",
> retprobe ? "uretprobes" : "uprobes", probe_name);
>
> return parse_uint_from_file(file, "%d\n");
> @@ -11071,7 +11082,8 @@ static int determine_tracepoint_id(const char *tp_category,
> int ret;
>
> ret = snprintf(file, sizeof(file),
> - "/sys/kernel/debug/tracing/events/%s/%s/id",
> + debugfs_available() ? "/sys/kernel/debug/tracing/events/%s/%s/id" :
> + "/sys/kernel/tracing/events/%s/%s/id",
> tp_category, tp_name);
> if (ret < 0)
> return -errno;
>