Identifying whether a user-process or kernel-thread execution takes place by looking at CPU registers

[Mulyadi Santosa]

Hi....

On Thu, May 12, 2011 at 17:55, limp <johnkyr83 at hotmail.com> wrote:
> I was wondering if any other CPU register (apart from CR3) can indicate if a
> user-process or a kernel thread under it (and which one) is executed. Is it
> possible to know such a thing *only* by looking at CPU registers?

one thing you can use is by looking at so called CPL (Current
Privilege level) and check it whether it is 0. According to
http://en.wikipedia.org/wiki/X86_memory_segmentation, CPL is the lower
2 bits in CS.

However, you need to watch it continously, because user space apps
could switch to CPL=0 (which denotes kernel mode, where CPL=3 denotes
user mode) in the case of system call etc.

perhaps better is by looking at the address of mm. However, to do
this, you need to check starting from its task_struct, which is mapped
in its kernel stack in x86 AFAIK. In other arch such as ARM, AFAIK
task_struct could be simply derived from certain register.
-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
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