Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

From: Yu-cheng Yu
Date: Mon Jun 10 2019 - 15:50:44 EST

On Mon, 2019-06-10 at 11:02 -0700, Dave Hansen wrote:
> On 6/10/19 8:22 AM, Yu-cheng Yu wrote:
> > > How does glibc know the linear address space size? We donât want LA64 to
> > > break old binaries because the address calculation changed.
> >
> > When an application starts, its highest stack address is determined.
> > It uses that as the maximum the bitmap needs to cover.
> Huh, I didn't think we ran code from the stack. ;)
> Especially given the way that we implemented the new 5-level-paging
> address space, I don't think that expecting code to be below the stack
> is a good universal expectation.

Yes, you make a good point. However, allowing the application manage the bitmap
is the most efficient and flexible. If the loader finds a legacy lib is beyond
the bitmap can cover, it can deal with the problem by moving the lib to a lower
address; or re-allocate the bitmap. If the loader cannot allocate a big bitmap
to cover all 5-level address space (the bitmap will be large), it can put all
legacy lib's at lower address. We cannot do these easily in the kernel.