CODEWITHSUNDEEP
linuxlinux-kernelarm
zbarni
zbarni

Reputation: 69

Where does an ARM Linux store the current kernel SP while in user mode?

I have the following situation: a Linux OS (ARM) runs on a virtual platform, and a simple program is started within the OS with a breakpoint on a certain symbol. When this breakpoint is hit, we are in the user space.

My question is: having a full overview of the entire virtual memory of the (currently running) process (i.e., kernel + user space), can I somehow find out the address of the task_struct or thread_info structures located on the (bottom) of the kernel stack? I'm aware that in the kernel mode it's possible to use a mask on the SP to get the bottom of the kernel stack but this won't work if the SP points to something in the user space. Essentially I'm trying to do something similar to the current / current_thread_info() macros in the kernel.

The reason I think this is hard / impossible is because the kernel stack address is randomized and thus changes with each new process.

Things I need to avoid:

  1. system calls : the OS, kernel and process are 'frozen' and we have a static view of the memory
  2. call any functions that require a running OS

Upvotes: 0

Views: 552

Answers (1)

caf
caf

Reputation: 239041

To determine the supervisor mode SP while the target is in userspace, you need to examine the R13_svc register.

If you take this value and set the 13 least significant bits to zero, you should have the virtual address for the struct thread_info of the current thread.

Upvotes: 3

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