From: Bodo Stroesser

This is the first patch of a series of four.
These patches allow the use of sysenter-systemcalls in UML
if the host support sysenter.
Some facts have to be noted:
- the sysenter instruction does not save anything, not even the
  return address. Thus the host-kernel builds a stackframe with an
  fixed return address for the backjump to the vsyscall-page. All
  kernels that support sysenter thus must have a vsyscall-page
- The hosts vsyscall-page is visible in all memory-contexts on the
  host, even in those of the processes running on UML. This cannot
  be changed.
So the best way to implement sysenter is to integrate the host's
vsyscall-page into UML, if available.

This patch creates a new source file containing an UML
initialization function. The function scans the Elf-auxiliary vector
that is prepared by the host for relevant information about:
- vsyscall elf-header
- vsyscall entry
- machine type (called "platform", e.g. "i586" or "i686")
- hardware capabilities
These informations are inserted into the Elf-auxiliary-vector that is
generated if an UML process calls "execXX()". If the information from
the auxiliray-vector is not complete, UML uses the previos default
values, with one exception: if the host has no vsyscall-page, UML now
does no longer insert AT_SYSINFO or AT_SYSINFO_EHDR elements. (I think,
that's better than writing dummies)

Since the host's vsyscall-page is always visible to UML processes, this
change is enough to let UML with an i686-compiled glibc use sysenter.

what's missing:
- is_syscall() in SKAS cannot access the code in the vsyscall-page via
  copy_from_user(), thus singlesteppers still could break out. (Note:
  that's not new, if someone jumps willingly to the sysenter-entry in
  the vsyscall-page, he can do that without the patch, too).
- a debugger cannot access the code in the vsyscall-page via
  ptrace( PEEKTEXT, ...)

Risks:
could there by any feature of the host's processor, that is indicated in
the hardware capabilities, but must not be used in UML?
Signed-off-by: Bodo Stroesser <bodo.stroesser@fujitsu-siemens.com>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>