properly support exec and wait with kernel pointers v2

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series first cleans up the exec code and then adds proper
kernel_execveat and kernel_wait callers instead of relying on the fact
that the early init code and kernel threads implicitly run with
the address limit set to KERNEL_DS.

Note that the cleanup removes the compat execve(at) handlers entirely, as
we can handle the compat difference very nicely in a unified codebase.
x32 needs two hacky #defines for that for now, although those can go
away if the x32 syscall rework from Brian gets merged.

Changes since v1:
 - remove a pointless ifdef from get_user_arg_ptr
 - remove the need for a compat syscall handler for x32


Diffstat:
 arch/arm64/include/asm/unistd32.h                  |    4 
 arch/mips/kernel/syscalls/syscall_n32.tbl          |    4 
 arch/mips/kernel/syscalls/syscall_o32.tbl          |    4 
 arch/parisc/kernel/syscalls/syscall.tbl            |    4 
 arch/powerpc/kernel/syscalls/syscall.tbl           |    4 
 arch/s390/kernel/syscalls/syscall.tbl              |    4 
 arch/sparc/kernel/syscalls.S                       |    4 
 arch/x86/entry/syscall_x32.c                       |    7 
 arch/x86/entry/syscalls/syscall_32.tbl             |    4 
 arch/x86/entry/syscalls/syscall_64.tbl             |    4 
 fs/exec.c                                          |  248 ++++++++-------------
 include/linux/binfmts.h                            |   10 
 include/linux/compat.h                             |    7 
 include/linux/sched/task.h                         |    1 
 include/uapi/asm-generic/unistd.h                  |    4 
 init/main.c                                        |    5 
 kernel/exit.c                                      |   16 +
 kernel/umh.c                                       |   43 ---
 tools/include/uapi/asm-generic/unistd.h            |    4 
 tools/perf/arch/powerpc/entry/syscalls/syscall.tbl |    4 
 tools/perf/arch/s390/entry/syscalls/syscall.tbl    |    4 
 tools/perf/arch/x86/entry/syscalls/syscall_64.tbl  |    4 
 22 files changed, 170 insertions(+), 223 deletions(-)