Re: [PATCH] futex: Fix fault_in_user_writeable()

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, Aug 17 2021 at 09:53, Huacai Chen wrote:
> On Tue, Aug 17, 2021 at 3:03 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>> That's surely one way to fix that. If that does not work for whatever
>> reason, then we really don't want this find_vma() hack there, but rather
>> something like:
> I don't know why find_vma() is unacceptable here, there is also
> find_vma() in fixup_user_fault().

Wrong. find_extend_vma() != find_vma(). Aside of that fixup_user_fault()
does way more than that.

>>     if (IS_ENABLED(CONFIG_ARCH_USER_FAULT_VOODOO) && get_user(&tmp, uaddr))
>>         return -EFAULT;
>
> get_user() may be better than find_vma(), but can we drop
> CONFIG_ARCH_USER_FAULT_VOODOO here? On those "W implies R" archs,
> get_user() always success, this can simplify the logic.

For architectures which imply R fixup_user_fault() is way more
effinicient than taking the fault on get_user() and then invoking
fixup_user_fault() to ensure that the mapping is writeable.

No, we are not making stuff less efficient just because of MIPS.

Thanks,

        tglx