Re: [PATCHv6 4/5] x86/mm: check in_compat_syscall() instead TIF_ADDR32 for mmap(MAP_32BIT)

[Andy Lutomirski]

On Mon, Mar 13, 2017 at 6:47 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Mon, 13 Mar 2017, Dmitry Safonov wrote:
>> On 03/13/2017 12:39 PM, Thomas Gleixner wrote:
>> > On Mon, 6 Mar 2017, Dmitry Safonov wrote:
>> >
>> > > Result of mmap() calls with MAP_32BIT flag at this moment depends
>> > > on thread flag TIF_ADDR32, which is set during exec() for 32-bit apps.
>> > > It's broken as the behavior of mmap() shouldn't depend on exec-ed
>> > > application's bitness. Instead, it should check the bitness of mmap()
>> > > syscall.
>> > > How it worked before:
>> > > o for 32-bit compatible binaries it is completely ignored. Which was
>> > > fine when there were one mmap_base, computed for 32-bit syscalls.
>> > > After introducing mmap_compat_base 64-bit syscalls do use computed
>> > > for 64-bit syscalls mmap_base, which means that we can allocate 64-bit
>> > > address with 64-bit syscall in application launched from 32-bit
>> > > compatible binary. And ignoring this flag is not expected behavior.
>> >
>> > Well, the real question here is, whether we should allow 32bit applications
>> > to obtain 64bit mappings at all. We can very well force 32bit applications
>> > into the 4GB address space as it was before your mmap base splitup and be
>> > done with it.
>>
>> Hmm, yes, we could restrict 32bit applications to 32bit mappings only.
>> But the approach which I tried to follow in the patches set, it was do
>> not base the logic on the bitness of launched applications
>> (native/compat) - only base on bitness of the performing syscall.
>> The idea was suggested by Andy and I made mmap() logic here independent
>> from original application's bitness.
>>
>> It also seems to me simpler:
>> if 32-bit application wants to allocate 64-bit mapping, it should
>> long-jump with 64-bit segment descriptor and do `syscall` instruction
>> for 64-bit syscall entry path. So, in my point of view after this dance
>> the application does not differ much from native 64-bit binary and can
>> have 64-bit address mapping.

I agree.

>
> Works for me, but it lacks documentation .....
>
> Thanks,
>
>         tglx



-- 
Andy Lutomirski
AMA Capital Management, LLC