Now that the kernel provides code user programs are executing directly
(I mean the vsyscall code on x86) it is necessary to add unwind
information for that code as well.  The unwind information is used not
only in C++ code.  The new thread code also uses it for the cancellation
handling.  If we have no such information available we would have to
resort to using int $0x80 for all syscalls which are also cancellation
points (read, write, ...).

Providing the information from outside the kernel is problematic.
First, we would have to recognize when a process starts which code is
actually used and install the appropriate unwind table.  Second, we
would always have to keep libc and kernel in sync.  There might be new
code sequences in future and once the kernel is changed you'd need a new
libc.  Not good at all.

Instead the best way I've found is to provide the info in the kernel.
Fortunately this is associated with almost no cost.  The unwind table is
just a block of static data which is copied at system boot time into the
vsyscall page just like the normal vsyscall code.

To advertise the existence and location of the unwind table I've added
one more AT_* constant.  It might in theory be possible to reuse
AT_SYSINFO and add a fixed offset but I'd rather not do this.  If/When
more code is added to the vsyscall page the unwind table gets larger and
you want to have the liberty to move it out of the way.

This also brings up an important point: even if more entry points into
the vsyscall page are defined, there will always have to be only one
unwind table.  It is not necessary to add more and more AT_* values to
advertise more tables.

The attached patch just adds the AT_* value, makes sure the AT_* value
is passed to applications, define the static data for the unwind blocks
(two, one for int80 and the other for sysenter), and finally code to
copy the data in place.  Very simple and unintrusive.  The patch is
verified to work nicely and unwind now works even when I use vsyscall.

I've added documentation of all the unwind info but it might still be
not easy to generate new data if the code sequences are changed or new
sequences are added.  If you want, you can add a comment somewhere which
instructs people to contact me to make the changes.


The patch also includes a bonus: so far all the tables in the
sysenter_setup() function will stay behing even if the function gets
removed after startup.  They are not marked appropriately.  I've added
__initdata marker, but actually it should be __initrodata if this would
exist.  Not that it makes much of a difference, the data is gone right
away after booting.

-- 
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Ulrich Drepper \    ,-----------------'   \ Mountain View, CA 94041 USA
Red Hat         `--' drepper at redhat.com `---------------------------