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Date: Sat, 30 Mar 2019 07:11:41 -0400 (EDT)
From: Frediano Ziglio <fziglio@...hat.com>
To: musl@...ts.openwall.com
Subject: Re: Does TD point to itself intentionally?

> 
> Hi all,
> 
> I was looking over my old C experiments and saw an old file, trying to
> use clang's address_space attribute to access something like a thread
> pointer. That made me wonder how it is implemented in musl.
> 
> In most architectures, the thread pointer is just stored in a register,
> and __pthread_self() will just grab it out of there. For x86_64,
> something slightly similar happens: The thread pointer is stored in
> FS.base, which is an MSR the kernel has to set for us, but we can read
> it with FS-relative addressing.
> 
> Incidentally: Is there any interest in using the "wrfsbase" instruction
> for that, where available? From a cursory first glance, it looks like
> that would mean that musl would have to do the entire CPUID dance on
> AMD64 and i386, and in the latter case the dance would be a bit longer
> since the ID bit dance would have to preceed it.
> 
> Back to setting the thread pointer: The relevant code is in __init_tp(),
> which is always called with the return value from __copy_tls(), which
> points to the new thread descriptor. __init_tp() will then call
> __set_thread_area() with the adjusted thread pointer, and on AMD64, this
> will just call arch_prctl(SET_FS, p). Though I don't know why that
> function has to be in assembly.
> 
> OK, got it. After this, FS.base will point directly at the TD, so we can
> just load FS.base into any register and have a thread pointer, right?
> Enter __pthread_self():
> 
> static inline struct pthread *__pthread_self()
> {
> 	struct pthread *self;
> 	__asm__ ("mov %%fs:0,%0" : "=r" (self) );
> 	return self;
> }
> 
> But that is not the same thing! This will load FS.base, and then
> dereference it and load the qword it is pointing at into a register. So
> how did this ever work? Well, the answer is back in __init_tp():
> 
> 	td->self = td;
> 
> And of course, "self" is the first member of struct pthread.
> 
> So, now the question I've been building up to: Is that intentional? Is
> there a reason for there to be a pointer pointing to itself, other than
> the "mov" in __pthread_self()? Could that mov not be replaced with a
> "lea" and save one useless memory access?
> 
> Ciao,
> Markus
> 

But "lea" how? It would be a rdfsbase instruction as "standard" registers
are used for other purposes. But as you said you cannot assume rdfsbase would
work so it's hard to inline it. Doing that way you can inline that single
assembly instruction easily.

Frediano

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