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user-space probes -- plan B from outer space
- From: fche at redhat dot com (Frank Ch. Eigler)
- To: systemtap at sources dot redhat dot com
- Date: 06 Jun 2006 15:07:39 -0400
- Subject: user-space probes -- plan B from outer space
Hi -
Here is an outline of how systemtap might support user-space probes,
even in the absence of kernel-based user kprobes. This is a "plan B"
only, a desperate stopgap until lkml sees the light. Maybe "plan Z"
is more appropriate, considering the limitations I'm about to outline.
The idea is to support limited systemtap scripts that refer only to
user-space probe targets such as existing processes. These scripts
would be translated to a user-space probe program instead of a kernel
probe module.
Probes would be specified with a probe point syntax such as:
user.process(233).statement(0xfeedface)
user("fche").process("/bin/vi").function("*init*")
Instead of kprobes of a probe module, this probe program would use
ptrace to insert breakpoints into any target processes, perhaps using
code from RDA or GDB. Given the process-id or process name, systemtap
should be able to locate the necessary debugging information at
translation time. When probes are hit, the probe process would run
the compiled probe handlers in much the same way as now. Access to
$target vars should be possible. The runtime code would have to have
a new variant to use some user-level facility (plain pipes?) to
communicate with the front-end.
Q: Wouldn't this be slow?
A: Oh yes, quite. Several ptrace context-switch round-trips per
probe hit. Lots more if we want to pull out target-side
state like $variables or stack backtraces.
Q: What about concurrency?
A: You mean like probes concurrently hit in several target processes,
like SMP kprobes? If there was any indication that this was
worthwhile, then we could make the systemtap-generated probe
process be multi-threaded (one probe thread per target thread).
Q: Any other limitations?
A: Because of ptrace, any process can be supervised by only one
process at a time. So if you run systemtap on a user process,
you won't be able to run gdb or another systemtap session on it.
Q: What about probing the kernel and user space together?
A: Maybe this scheme would work if kernel-space systemtap probes
run concurrently, and arrange to share systemtap globals with
userspace somehow (mmap?). Shared variables like this would
likely cause many more locking timeouts (=> skipped probes)
than now. There are also additional security concerns.
Q: What about probing shared libraries?
A: Because of the way ptrace works, we'd have to turn these into
process-level probes, including probes that just sit around
monitoring the threads and all their children to dlopen/mmap
the named libraries.
Q: Is it worth it to try? Is there a better way?
A: You tell me.
- FChE