bpf: implement sleepable uprobes by chaining gps

uprobes work by raising a trap, setting a task flag from within the
interrupt handler, and processing the actual work for the uprobe on the
way back to userspace. As a result, uprobe handlers already execute in a
might_fault/_sleep context. The primary obstacle to sleepable bpf uprobe
programs is therefore on the bpf side.

Namely, the bpf_prog_array attached to the uprobe is protected by normal
rcu. In order for uprobe bpf programs to become sleepable, it has to be
protected by the tasks_trace rcu flavor instead (and kfree() called after
a corresponding grace period).

Therefore, the free path for bpf_prog_array now chains a tasks_trace and
normal grace periods one after the other.

Users who iterate under tasks_trace read section would
be safe, as would users who iterate under normal read sections (from
non-sleepable locations).

The downside is that the tasks_trace latency affects all perf_event-attached
bpf programs (and not just uprobe ones). This is deemed safe given the
possible attach rates for kprobe/uprobe/tp programs.

Separately, non-sleepable programs need access to dynamically sized
rcu-protected maps, so bpf_run_prog_array_sleepables now conditionally takes
an rcu read section, in addition to the overarching tasks_trace section.

Signed-off-by: Delyan Kratunov <delyank@fb.com>
Link: https://lore.kernel.org/r/ce844d62a2fd0443b08c5ab02e95bc7149f9aeb1.1655248076.git.delyank@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 2 insertions, 3 deletions

diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index 9711589273cd..0282c119b1b2 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -16,6 +16,7 @@
 #include <linux/namei.h>
 #include <linux/string.h>
 #include <linux/rculist.h>
+#include <linux/filter.h>
 
 #include "trace_dynevent.h"
 #include "trace_probe.h"
@@ -1346,9 +1347,7 @@ static void __uprobe_perf_func(struct trace_uprobe *tu,
 	if (bpf_prog_array_valid(call)) {
 		u32 ret;
 
-		preempt_disable();
-		ret = trace_call_bpf(call, regs);
-		preempt_enable();
+		ret = bpf_prog_run_array_sleepable(call->prog_array, regs, bpf_prog_run);
 		if (!ret)
 			return;
 	}