Extending l4sys by reading/writing register values of a preempted thread

Christian Ludwig christian_ludwig at genua.de
Tue Mar 14 13:11:24 CET 2017


> I want to implement a new syscall for getting/setting stored register
> values of a specific thread (identified by its cap).

Why not use ex_regs() on the thread to trigger an exception. That way, the 
attached userland exception handler can introspect the thread's complete 
register set. It can also alter the registers, so that the thread continues to 
run with the modified register set.

Is there anything more you need?

> Before implementing the syscall, I need to better understand the
> kernel-part of Fiasco.OC. Can you help me answer the following questions
> and point me to the source code regarding each question:
> * Where (in the source code) are threads preempted and their register
> state stored?

A thread's userland registers are stored on every entry to the kernel. The 
class in Fiasco is called Entry_frame.

The userland exception handler mentioned above modifies that Entry_frame.

> * Where is the register state of a thread loaded to the CPU and the
> thread started (e.g. after a preempt)?

When the scheduler selects a thread, its userland state (Entry_frame) is 
restored on exit from kernel mode.

> * How can I introspect the stored thread register of a thread in the
> kernel debugger JDB?
> ** My thoughts: Is it K<kobj_ptr> with kobj_ptr of the thread and the 3
> lines beginning with PC=...?
> e.g.
> "
> PC=010579cc USP=200ffdb0                smlatbeq	r9, r0, r2, r5
> [0] 00000003 000010c0 00218003 00000000 tsteq000r5, ip, rrx
> [8] 200ffdb0 200ffe40 01086ef0 200ffe98 [c] 200ffef8 010579cc fffffff8
> 20000010
> "

Yes, that PC line is the userland program counter. the USP is the userland 
stack pointer. All userland registers are located at the top of kernel stack 
for a particular thread.
> I also have another, short, off-topic question:
> * How can I introspect the capability space of a task inside the kernel
> debugger?

In short mode, press 's' for a list of all tasks. Select the task in question 
and the press 'o' to see the capability space for that task.

Hope that helps,

 - Christian

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