[PATCH v1] tracing/kprobes: expose maxactive for kretprobe in kprobe_events

[Alban Crequy <alban.crequy@gmail.com>]

When a kretprobe is installed on a kernel function, there is a maximum
limit of how many calls in parallel it can catch (aka "maxactive"). A
kernel module could call register_kretprobe() and initialize maxactive
(see example in samples/kprobes/kretprobe_example.c).

But that is not exposed to userspace and it is currently not possible to
choose maxactive when writing to /sys/kernel/debug/tracing/kprobe_events

The default maxactive can be as low as 1 on single-core with a
non-preemptive kernel. This is too low and we need to increase it not
only for recursive functions, but for functions that sleep or resched.

This patch updates the format of the command that can be written to
kprobe_events so that maxactive can be optionally specified.

I need this for a bpf program attached to the kretprobe of
inet_csk_accept, which can sleep for a long time.

BugLink: https://github.com/iovisor/bcc/issues/1072
Signed-off-by: Alban Crequy <alban@kinvolk.io>
---
 Documentation/trace/kprobetrace.txt |  4 +++-
 kernel/trace/trace_kprobe.c         | 34 +++++++++++++++++++++++++++++-----
 2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
index 41ef9d8..655ca7e 100644
--- a/Documentation/trace/kprobetrace.txt
+++ b/Documentation/trace/kprobetrace.txt
@@ -23,7 +23,7 @@ current_tracer. Instead of that, add probe points via
 Synopsis of kprobe_events
 -------------------------
   p[:[GRP/]EVENT] [MOD:]SYM[+offs]|MEMADDR [FETCHARGS]	: Set a probe
-  r[:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]		: Set a return probe
+  r[MAXACTIVE][:[GRP/]EVENT] [MOD:]SYM[+0] [FETCHARGS]	: Set a return probe
   -:[GRP/]EVENT						: Clear a probe
 
  GRP		: Group name. If omitted, use "kprobes" for it.
@@ -32,6 +32,8 @@ Synopsis of kprobe_events
  MOD		: Module name which has given SYM.
  SYM[+offs]	: Symbol+offset where the probe is inserted.
  MEMADDR	: Address where the probe is inserted.
+ MAXACTIVE	: Maximum number of instances of the specified function that
+		  can be probed simultaneously, or 0 for the default.(*)
 
  FETCHARGS	: Arguments. Each probe can have up to 128 args.
   %REG		: Fetch register REG
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 5f688cc..807e01c 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -282,6 +282,7 @@ static struct trace_kprobe *alloc_trace_kprobe(const char *group,
 					     void *addr,
 					     const char *symbol,
 					     unsigned long offs,
+					     int maxactive,
 					     int nargs, bool is_return)
 {
 	struct trace_kprobe *tk;
@@ -309,6 +310,8 @@ static struct trace_kprobe *alloc_trace_kprobe(const char *group,
 	else
 		tk->rp.kp.pre_handler = kprobe_dispatcher;
 
+	tk->rp.maxactive = maxactive;
+
 	if (!event || !is_good_name(event)) {
 		ret = -EINVAL;
 		goto error;
@@ -598,8 +601,10 @@ static int create_trace_kprobe(int argc, char **argv)
 {
 	/*
 	 * Argument syntax:
-	 *  - Add kprobe: p[:[GRP/]EVENT] [MOD:]KSYM[+OFFS]|KADDR [FETCHARGS]
-	 *  - Add kretprobe: r[:[GRP/]EVENT] [MOD:]KSYM[+0] [FETCHARGS]
+	 *  - Add kprobe:
+	 *      p[:[GRP/]EVENT] [MOD:]KSYM[+OFFS]|KADDR [FETCHARGS]
+	 *  - Add kretprobe:
+	 *      r[MAXACTIVE][:[GRP/]EVENT] [MOD:]KSYM[+0] [FETCHARGS]
 	 * Fetch args:
 	 *  $retval	: fetch return value
 	 *  $stack	: fetch stack address
@@ -619,6 +624,7 @@ static int create_trace_kprobe(int argc, char **argv)
 	int i, ret = 0;
 	bool is_return = false, is_delete = false;
 	char *symbol = NULL, *event = NULL, *group = NULL;
+	int maxactive = 0;
 	char *arg;
 	unsigned long offset = 0;
 	void *addr = NULL;
@@ -637,8 +643,26 @@ static int create_trace_kprobe(int argc, char **argv)
 		return -EINVAL;
 	}
 
-	if (argv[0][1] == ':') {
+	if (is_return && isdigit(argv[0][1]) && strchr(&argv[0][1], ':')) {
+		event = strchr(&argv[0][1], ':') + 1;
+		event[-1] = '\0';
+		ret = kstrtouint(&argv[0][1], 0, &maxactive);
+		if (ret) {
+			pr_info("Failed to parse maxactive.\n");
+			return ret;
+		}
+		/* kretprobes instances are iterated over via a list. The
+		 * maximum should stay reasonable.
+		 */
+		if (maxactive > 1024) {
+			pr_info("Maxactive is too big.\n");
+			return -EINVAL;
+		}
+	} else if (argv[0][1] == ':') {
 		event = &argv[0][2];
+	}
+
+	if (event) {
 		if (strchr(event, '/')) {
 			group = event;
 			event = strchr(group, '/') + 1;
@@ -718,8 +742,8 @@ static int create_trace_kprobe(int argc, char **argv)
 				 is_return ? 'r' : 'p', addr);
 		event = buf;
 	}
-	tk = alloc_trace_kprobe(group, event, addr, symbol, offset, argc,
-			       is_return);
+	tk = alloc_trace_kprobe(group, event, addr, symbol, offset, maxactive,
+			       argc, is_return);
 	if (IS_ERR(tk)) {
 		pr_info("Failed to allocate trace_probe.(%d)\n",
 			(int)PTR_ERR(tk));
-- 
2.7.4