Re: [RFC, PATCH] Fixup COMPAT_VDSO to work with CONFIG_PARAVIRT

Eric W. Biederman wrote:

There are three ways of finding the VDSO.
- AT_SYSINFO
- AT_SYSINFO_EHDR
- known fixed address (see x86_64)

Currently it doesn't sound like you need to deal with the known fixed
address case but COMPAT_VDSO also provides that.
  

Yes, I don't think any 32-bit userspace expects a fixed address any
more.  64-bit is another matter.

If userspace uses AT_SYSINFO the premise is that it is not expecting
not to need to perform any relocation processing.

If userspace uses AT_SYSINFO_EHDR it expects it needs to perform
relocation processing and fixes up whatever needs fixing up.
  

Correct.

With the module code we have shown the kernel is capable of performing
relocation processing at times and it works.
  

Good point.  It would be good to reuse that machinery.

So is it possible to simply relocate the normal vdso and fixup
it's program header so it shows that relocation is not necessary.
If you can do that and still export AT_SYSINFO so the problem user
space still runs you are good.  (If you can relocate the vdso
you should be able to relocate it anywhere).
  

Yes.  The plan is to map the relocated compat vdso at some fixed address
in all processes, and map a non-relocated non-compat vdso at some
randomized address (it will probably be the same bits in either case). 
We could map a relocated vdso to a randomized address (ie, only one vdso
mapping), but that would require a per-process copy of the vdso and
effort to relocate on each exec.

Otherwise it probably just make sense to simply not export a VDSO
on those systems.
  

We did that in an earlier version of the patch, and Ingo complained,
with some justification.

This would leave COMPAT_VDSO for the case where you must use one magic
fixed address, and if user space does not require that it means
COMPAT_VDSO could be completely removed.
  

FC1 and SuSE 9 both shipped with broken glibcs which require
weak-COMPAT_VDSO (not fixed address, but pre-relocated).  There are
still enough of these around that we need to cater to them.

    J