After a lot of reading the code and thinking about how it behaves I have
managed to figure out what the current ptrace locking rules are.  The
current code is in much better that it appears at first glance.  The
troublesome code paths are actually the code paths that violate the current
rules.

ptrace uses simple exclusive access as it's locking.  You can only touch
task->ptrace if the task is stopped and you are the ptracer, or if the task
is running and are the task itself.

Very simple, very easy to maintain.  It just needs to be documented so
people know not to touch ptrace from elsewhere.

Currently we do have a few pieces of code that are in violation of this
rule.  Particularly the core dump code, and ptrace_attach.  But so far the
code looks fixable.
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

1. The tracee can go from ptrace_stop() to do_signal_stop()
   after __ptrace_unlink(p).

2. It is unsafe to __ptrace_unlink(p) while p->parent may wait
   for tasklist_lock in ptrace_detach().
Signed-off-by: NOleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

PTRACE_SYSEMU{,_SINGLESTEP} is actually arch specific, for now, and the
current allocated number clashes with a ptrace code of frv, i.e.
PTRACE_GETFDPIC.  I should have submitted this much earlier, anyway we get no
breakage for this.

CC: Daniel Jacobowitz <dan@debian.org>
Signed-off-by: NPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

The ptrace_get_task_struct() helper that I added as part of the ptrace
consolidation is useful in variety of places that currently opencode it.
Switch them to the common helpers.

Add a ptrace_traceme() helper that needs to be explicitly called, and simplify
the ptrace_get_task_struct() interface.  We don't need the request argument
now, and we return the task_struct directly, using ERR_PTR() for error
returns.  It's a bit more code in the callers, but we have two sane routines
that do one thing well now.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

The sys_ptrace boilerplate code (everything outside the big switch
statement for the arch-specific requests) is shared by most architectures.
This patch moves it to kernel/ptrace.c and leaves the arch-specific code as
arch_ptrace.

Some architectures have a too different ptrace so we have to exclude them.
They continue to keep their implementations.  For sh64 I had to add a
sh64_ptrace wrapper because it does some initialization on the first call.
For um I removed an ifdefed SUBARCH_PTRACE_SPECIAL block, but
SUBARCH_PTRACE_SPECIAL isn't defined anywhere in the tree.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NPaul Mackerras <paulus@samba.org>
Acked-by: NRalf Baechle <ralf@linux-mips.org>
Acked-By: NDavid Howells <dhowells@redhat.com>
Acked-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Acked-by: NPaul Mundt <lethal@linux-sh.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

Extract common code used by ptrace_attach() and may_ptrace_attach()
into a separate function.
Signed-off-by: NMiklos Szeredi <miklos@szeredi.hu>
Cc: <viro@parcelfarce.linux.theplanet.co.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

This patch implements the new ptrace option PTRACE_SYSEMU_SINGLESTEP, which
can be used by UML to singlestep a process: it will receive SINGLESTEP
interceptions for normal instructions and syscalls, but syscall execution will
be skipped just like with PTRACE_SYSEMU.
Signed-off-by: NBodo Stroesser <bstroesser@fujitsu-siemens.com>
Signed-off-by: NPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

      Jeff Dike <jdike@addtoit.com>,
      Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>,
      Bodo Stroesser <bstroesser@fujitsu-siemens.com>

Adds a new ptrace(2) mode, called PTRACE_SYSEMU, resembling PTRACE_SYSCALL
except that the kernel does not execute the requested syscall; this is useful
to improve performance for virtual environments, like UML, which want to run
the syscall on their own.

In fact, using PTRACE_SYSCALL means stopping child execution twice, on entry
and on exit, and each time you also have two context switches; with SYSEMU you
avoid the 2nd stop and so save two context switches per syscall.

Also, some architectures don't have support in the host for changing the
syscall number via ptrace(), which is currently needed to skip syscall
execution (UML turns any syscall into getpid() to avoid it being executed on
the host).  Fixing that is hard, while SYSEMU is easier to implement.

* This version of the patch includes some suggestions of Jeff Dike to avoid
  adding any instructions to the syscall fast path, plus some other little
  changes, by myself, to make it work even when the syscall is executed with
  SYSENTER (but I'm unsure about them). It has been widely tested for quite a
  lot of time.

* Various fixed were included to handle the various switches between
  various states, i.e. when for instance a syscall entry is traced with one of
  PT_SYSCALL / _SYSEMU / _SINGLESTEP and another one is used on exit.
  Basically, this is done by remembering which one of them was used even after
  the call to ptrace_notify().

* We're combining TIF_SYSCALL_EMU with TIF_SYSCALL_TRACE or TIF_SINGLESTEP
  to make do_syscall_trace() notice that the current syscall was started with
  SYSEMU on entry, so that no notification ought to be done in the exit path;
  this is a bit of a hack, so this problem is solved in another way in next
  patches.

* Also, the effects of the patch:
"Ptrace - i386: fix Syscall Audit interaction with singlestep"
are cancelled; they are restored back in the last patch of this series.

Detailed descriptions of the patches doing this kind of processing follow (but
I've already summed everything up).

* Fix behaviour when changing interception kind #1.

  In do_syscall_trace(), we check the status of the TIF_SYSCALL_EMU flag
  only after doing the debugger notification; but the debugger might have
  changed the status of this flag because he continued execution with
  PTRACE_SYSCALL, so this is wrong.  This patch fixes it by saving the flag
  status before calling ptrace_notify().

* Fix behaviour when changing interception kind #2:
  avoid intercepting syscall on return when using SYSCALL again.

  A guest process switching from using PTRACE_SYSEMU to PTRACE_SYSCALL
  crashes.

  The problem is in arch/i386/kernel/entry.S.  The current SYSEMU patch
  inhibits the syscall-handler to be called, but does not prevent
  do_syscall_trace() to be called after this for syscall completion
  interception.

  The appended patch fixes this.  It reuses the flag TIF_SYSCALL_EMU to
  remember "we come from PTRACE_SYSEMU and now are in PTRACE_SYSCALL", since
  the flag is unused in the depicted situation.

* Fix behaviour when changing interception kind #3:
  avoid intercepting syscall on return when using SINGLESTEP.

  When testing 2.6.9 and the skas3.v6 patch, with my latest patch and had
  problems with singlestepping on UML in SKAS with SYSEMU.  It looped
  receiving SIGTRAPs without moving forward.  EIP of the traced process was
  the same for all SIGTRAPs.

What's missing is to handle switching from PTRACE_SYSCALL_EMU to
PTRACE_SINGLESTEP in a way very similar to what is done for the change from
PTRACE_SYSCALL_EMU to PTRACE_SYSCALL_TRACE.

I.e., after calling ptrace(PTRACE_SYSEMU), on the return path, the debugger is
notified and then wake ups the process; the syscall is executed (or skipped,
when do_syscall_trace() returns 0, i.e.  when using PTRACE_SYSEMU), and
do_syscall_trace() is called again.  Since we are on the return path of a
SYSEMU'd syscall, if the wake up is performed through ptrace(PTRACE_SYSCALL),
we must still avoid notifying the parent of the syscall exit.  Now, this
behaviour is extended even to resuming with PTRACE_SINGLESTEP.
Signed-off-by: NPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!