[Bug uprobes/10836] uprobes-provided pt_regs* are unreliable

["srikar at linux dot vnet dot ibm dot com" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From srikar at linux dot vnet dot ibm dot com  2009-10-27 11:44 -------
I am not sure if I understand the problem completely.

> Several registers appearing in a utrace-oriented pt_regs do not accurately
> represent the state of the user-space task. 

Is this concern about instruction pointer pointing past the breakpoint?
or do we have additional concerns? If yes do we have specific registers in mind?
                                        
uprobes passes the pt_regs it gets from utrace's report_signal callback as is to
the handler.

> As per bug #10601, this breaks
> some dwarf-based $context variables.

This bug refers to two other bugs which point to problems in user space markers.
 So is this problem only seen on user space markers? or can we see this problem
on plain uprobes probe points too.

> It seems to me that either uprobes should not pass pt_regs at all to its
> callbacks, or else populate it with synthetic (but accurate) values pulled
> out of utrace regset calls.  (The latter would be slower and require per-arch
> code, but would make it more convenient for the clients.  The former would
> be faster, and still require per-arch code at the client, but would be less
> convenient.)

Is there any reason why this synthesis should be done at the uprobes end and not
at the client end?  Do you see all uprobe clients facing this problem?
If its a problem faced by all uprobe clients, then is it worth checking if
utrace should send the synthesized pt_regs as a parameter to report_signal.


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http://sourceware.org/bugzilla/show_bug.cgi?id=10836

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