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Re: User-space probes: Plan B+

jkenisto wrote:

> [...]
> > > 1. Instrumentation can be coded entirely as a user-space app...
> > 
> > I am not aware of this being a *requirement*.  To my mind, it is an
> > unfortunate implementation artifact.  Running in kernel space but in
> > the traced processes' user context would be better.
> [...] Our interpretation of feedback from LKML and other sources was
> that uprobes would not be accepted without a reasonably friendly
> user-mode API.

That may be, but we would not be obligated to use such a thing.  To
what extent can this work be layered on top of the kernel-side utrace

> A few things, such as stack backtraces with symbol info, cannot be
> accomplished entirely in the kernel, so you need user-mode
> cooperation at some point anyway.

That's sort of a separate issue.  We're not contemplating running live
elfutils-style dwarf searches during a probe handler to evaluate
backtraces.  It is more likely that information required for a precise
backtrace would be encoded into the probing module in the form of an
extract of the dwarf unwind tables, and searched in situ.

> > > [...]  3. A user-mode tracer can invoke a previously registered
> > > kernel-mode handler, so we have simple and efficient communication
> > > between user- and kernel-mode instrumentation. [...]
> > 
> > How is this registration aspect supposed to work?
> Kernel-mode handlers can be registered in a kernel module. Registration
> requires the handler address, a unique name, and permission info (e.g.,
> only root can call it, or only a process that has permission to run
> uprobe_register() on the traced process can call it).

> Handlers that we expect to have widespread use (e.g., collect
> stack-trace addresses, report register contents, report memory
> contents) could even be registered by the kernel.

FWIW, such designated "widespread-use" handlers are not in good match
with systemtap's preference for programmable handlers.  It may be
helpful for specialized purposes such as LKET (say, by some sort of
clever handler body matching or new syntax), but not for general
systemtap usage.

> > Is something
> > keeping us from compiling & registering this instrumentation
> > dynamically, the same way we do normal kprobes handlers?
> No.  I envision SystemTap generating both the user-space tracer app
> and the module that registers any needed ad hoc kernel handlers (as
> well as the usual kernel-probing stuff).

Would this approach (compiling user-space probe handlers into
kernel-space registered but custom-built utrace handlers) make the
user-space tracer almost a content-free stub?

- FChE

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