Re: [PATCH] x86_64: uninline TASK_SIZE

Andy Lutomirski
2019-04-22T14:30:46Z

> On Apr 22, 2019, at 3:34 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * 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.
>

Are there really enough TASK_SIZE users to justify any of this?