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Date:   Mon, 22 Apr 2019 12:34:49 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     hpa@...or.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] x86_64: uninline TASK_SIZE


* Alexey Dobriyan <adobriyan@...il.com> wrote:

> > >> +++ b/arch/x86/kernel/task_size_64.c
> > >> @@ -0,0 +1,9 @@
> > >> +#include <linux/export.h>
> > >> +#include <linux/sched.h>
> > >> +#include <linux/thread_info.h>
> > >> +
> > >> +unsigned long _task_size(void)
> > >> +{
> > >> +	return test_thread_flag(TIF_ADDR32) ? IA32_PAGE_OFFSET :
> > >TASK_SIZE_MAX;
> > >> +}
> > >> +EXPORT_SYMBOL(_task_size);
> > >
> > >Good idea - but instead of adding yet another compilation unit, why not
> > >
> > >stick _task_size() into arch/x86/kernel/process_64.c, which is the 
> > >canonical place for process management related arch functions?
> > >
> > >Thanks,
> > >
> > >	Ingo
> > 
> > Better yet... since TIF_ADDR32 isn't something that changes randomly, 
> > perhaps this should be a separate variable?
> 
> Maybe. I only thought about putting every 32-bit related flag under 
> CONFIG_COMPAT to further eradicate bloat (and force everyone else to 
> keep an eye on it, ha-ha).

Basically TIF_ADDR32 is only set for a task if set_personality_ia32() is 
called, which function is called in the following circumstances:

 - arch/x86/ia32/ia32_aout.c:load_aout_binary()

   This is in exec(), when a new binary is loaded for the current task, 
   via search_binary_handler() and exec_binprm(). Ordering is 
   synchronous, AFAICS there can be no race between TASK_SIZE users and 
   the set_personality_ia32() call which is always for the current task.

 - in COMPAT_SET_PERSONALITY(), which through macro detours ends up being 
   in SET_PERSONALITY2(), which is used in fs/compat_binfmt_elf.c's 
   load_elf_binary(), used in a similar fashion in exec() as the AOUT 
   case above. One particular macro detour of note is that 
   fs/compat_binfmt_elf.c #includes fs/binfmt_elf.c and re-defines the 
   personality setting method to map to set_personality_ia32().

When set_personality_ia32() is called then TIF_ADDR32 is set 
unconditionally, without any Kconfig variations.

TIF_ADDR32 is cleared:

 - In set_personality_64bit(), when a 64-bit binary is loaded via 
   fs/binfmt_elf.c.

 - It also defaults to clear in the init task, which is inherited by the 
   initial kernel threads and any user-space task they might end up 
   executing.

So the conclusion is that IMO we can safely put TASK_SIZE into a new 
thread_info()->task_size field, and:

 - change ->task_size to the 32-bit address space in 
   set_personality_ia32()

 - change ->task_size to teh 64-bit address space in the init task and in 
   set_personality_64bit().

This should cover it I think, unless I missed something.

Thanks,

	Ingo

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