Re: [PATCH] x86_64: uninline TASK_SIZE

Ingo Molnar
2019-04-22T10:34:57Z

* Alexey Dobriyan <adobriyan@gmail.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