Function sunrise_libuser::syscalls::set_thread_area

pub unsafe fn set_thread_area(address: usize) -> Result<(), KernelError>

Set thread local area pointer.

Akin to set_thread_area on Linux, this syscall sets the gs segment selector's base address to the address passed as argument.

The user will likely want to make it point to its elf thread local storage, as gs:0 is expected to contain the thread pointer tp.

Unlike linux, you only have one user controlled segment, found in gs, and you can only set its address.

The limit will always be set to 0xFFFFFFFF, and adding this offset to a non-zero base address means that the resulting address will "wrap around" the address space, and end-up under the base address. You can use this property to implement thread local storage variant II - gnu model, as thread local variable are expected to be found "below" gs:0, with "negative" offset such as gs:0xFFFFFFFC.

x86_64

fs is used instead of gs, because reasons.

Safety

address should point to a valid TLS image, unique to the current thread. Setting gs to random data, malformed image, or shared image is UB.

Errors

• The whole initial design of TLS on x86 should be considered an error.
• No returned error otherwise.