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

[Dmitry Safonov]

On 03/13/2017 04:47 PM, Thomas Gleixner 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.
>
> Works for me, but it lacks documentation .....

Sure, could you recommend a better place for it?
Should it be in-code comment in x86 mmap() code or Documentation/*
change or a patch to man-pages?

CC'ing Michael.


--
             Dmitry