Messages in this thread | | | Date | Thu, 29 Nov 2007 10:16:22 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task |
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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.
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