Rework how shared page related data is stored

[kd]

Summary

Store the shared page address in struct vmspace.
Instead of storing base addresses of various shared page segments, save their offsets with respect to the shared page base address.
This will be more useful when the shared page address is randomized.

[kd]

Copy offsets unconditionally in exec_sysvec_init_secondary (as suggested by kib in D35349).

[kib]

sys/arm/arm/exec_machdep.c

349

I wonder if it is better to check for sv_shared_obj != NULL there and everywhere this kind of test is done. It is done so in kern_exec.c.

Might be, even add some macro like PROC_HAS_SHP() ?

sys/sys/exec.h

94–95

I would remove mention of a.out from the comment, not trying to enumerate all cases where shared page is present or not.

sys/vm/vm_map.h

298

'sp' sounds to much as a 'stack pointer' to me. I used 'shp' elsewere.

[kd]

• Rename vm_sp_base to vm_shp_base
• Introduce the PROC_HAS_SHP macro
• Don't mention a.out in a comment.

[kib]

sys/kern/kern_sharedpage.c

323–324

Perhaps change this instance of 'sigtramp' to 'vdso', since you reused the same string below.