On Sun, Nov 27, 2011 at 4:14 AM, Philip Guenther <[email protected]> wrote: > On Sat, Nov 26, 2011 at 10:25 PM, Luis Useche <[email protected]> wrote: >> On Sat, Nov 26, 2011 at 7:31 PM, Ted Unangst <[email protected]> wrote: >>> On Sat, Nov 26, 2011, Luis Useche wrote: >>>> For a project I need to single step a user space process while >>>> executing the fault handler. I was thinking that probably ddb single >>>> stepping can be reused but this is probably for kernel single stepping >>>> only. How about ptrace? >>> >>> I am officially confused because the fault handler runs in the kernel. >>> If you want to step through a process then ptrace is what you want, or >>> it's slightly easier to use interface, gdb. >> >> The interfaces you are talking about are to single step a user level >> process from another user level process. However, I need to single >> step in the fault handler the (user level) instruction that just >> faulted. >> >> My final goal is to extract the data that the application was trying >> to store at the unmapped page by mapping a temporary page. > > That won't always work if you're on a platform like i386 where the > kernel memory appears in the address space of processes, as you > *can't* just map in a temporary page if the process tries to access an > address in the kernel's range.
The idea is to create this temporary mapping after we make sure the access was valid but the page was not in memory. > Anyway, you don't need to trace the fault handler to do what you want. > You trace the process with ptrace(), then use ptrace() to examine > the state of the process when it gets a signal when it tries to access > the page in a non-permitted way. For example, gdb reports the signal > that was sent after examining the registers with ptrace(PT_GETREGS) > and then uses ptrace(PT_IO) to get the instruction that faulted. > Disassemble that to figure out what instruction it was and work out > the exact address and data involved. I am not proposing to trace the fault handler. My idea is to single step the last user-space instruction of the process in the fault handler to obtain the data that it was trying to write and return so the process can continue. Example: map_pte(cr2, tmp_page) single_step(cur) unmap_pte(cr2) Luis
