add helper to get value of Thread Local Storage base register

add helper to get value of Thread Local Storage base register

[chen.yunxing]

Hi, 

I want to discuss the requirement of add bpf helper:

for the purpose of accessing user space variable of TLS(Thread Local Storage)，we need get the TLS register of current Thread/Task.

then the TLS variable can be accessed via add offset to this register (as the base)

for example at arm:

mrs x0, tpidr_el0

Re: add helper to get value of Thread Local Storage base register

[Andrii Nakryiko]

On Mon, May 15, 2023 at 8:25 PM <chen.yunxing@icloud.com> wrote:
>
> Hi,
>
> I want to discuss the requirement of add bpf helper:
>
> for the purpose of accessing user space variable of TLS(Thread Local Storage)，we need get the TLS register of current Thread/Task.
>
> then the TLS variable can be accessed via add offset to this register (as the base)
>
> for example at arm:
>
> mrs x0, tpidr_el0

thread-local storage is only meaningful for user-space, so you don't
need to access actual register state, you can get it from struct
task_struct:

- for x86-64: task->thread.fsbase
- for aarm64: task->thread.uw.tp_value

Each architecture should have a similar field storing thread-local
storage base pointer.