Linus Torvalds wrote:
But this one is correct:
case offsetof(struct user32, regs.gs):
*val = child->thread.gsindex;
+ if (child == current)
+ asm("movl %%gs,%0" : "=r" (*val));
Won't this return the kernel's GS instead of the user's?
No, %gs is untouched by the kernel, so it contains user space version, and
getting the value directly from %gs looks correct.
Brief summary/reminder:
The kernel uses %fs in 32-bit mode and %gs in 64-bit mode.
User space TLS uses %gs in 32-bit mode and %fs in 64-bit mode.
The 64-bit kernel has to use %gs in order for SWAPGS to be available to
it (by which time the 32-bit ABI was already fixed.) It is advantageous
for user space to use the register the kernel typically won't, in order
to speed up system call entry/exit.
-hpa
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