Re: [PATCH][RFC] kprobes: Add user entry-handler in kretprobes

Abhishek Sagar wrote:
On Nov 13, 2007 12:09 AM, Abhishek Sagar <sagar.abhishek@xxxxxxxxx> wrote:
Whoops...sry for the repeated email..emailer trouble.

Expecting this one to makes it to the list. Summary again:

This patch introduces a provision to specify a user-defined callback
to run at function entry to complement the return handler in
kretprobes. Currently, whenever a kretprobe is registered, a user can
specify a callback (return-handler) to be run each time the target
function returns. This is also not guaranteed and is limited by the
number of concurrently pending return instances of the target function
in the current process's context.

This patch will now allow registration of another user defined handler
which is guaranteed to run each time the current return instance is
allocated and the return handler is set-up. Conversely, if the
entry-handler returns an error, it'll cause the current return
instance to be dropped and the return handler will also not run. The
purpose is to provide flexibility to do certain kinds of function
level profiling using kretprobes. By being able to register function
entry and return handlers, kretprobes will now be able to reduce an
extra probe registration (and associated race) for scenarios where an
entry handler is required to capture the function call/entry event
along with the corresponding function exit event.


If I understand your intentions(to capture information on function call/entry and corresponding function exit) cleary, I have few concerns on this.

1) How do you map the entry_handler(which gets executed when a process enters the function) with each instance of return probe handler.
I accept that entry_handler() will execute each time process enters the function, but to calculate time, one needs to know corresponding instance of return probe handler(there should be a map for each return handler).

Let me explain briefly.
Suppose in a SMP system, 4 processes enter the same function at almost sametime(by executing entry_hanlder()) and returns from 4 different locations by executing the return handler. Now how do I match entry_handler() with corresponding instance of return handler for calculating time.

Sometime back, Even I was interested in calculating the function execution time, but I used approach a) .

Now What I think is, there could be 2 solutions to these problem.

a) Collect the entry time and exit time and put it in that kretprobe_instance structure and fetch it before freeing that instance.

b) Or pass ri(kretprobe_instance address to entry_handler) and match it with return probe handler.
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