So the dwarf2 annotations in low level assembly code have
become an increasing hindrance: unreadable, messy macros
mixed into some of the most security sensitive code paths
of the Linux kernel.

These debug info annotations don't even buy the upstream
kernel anything: dwarf driven stack unwinding has caused
problems in the past so it's out of tree, and the upstream
kernel only uses the much more robust framepointers based
stack unwinding method.

In addition to that there's a steady, slow bitrot going
on with these annotations, requiring frequent fixups.
There's no tooling and no functionality upstream that
keeps it correct.

So burn down the sick forest, allowing new, healthier growth:

   27 files changed, 350 insertions(+), 1101 deletions(-)

Someone who has the willingness and time to do this
properly can attempt to reintroduce dwarf debuginfo in x86
assembly code plus dwarf unwinding from first principles,
with the following conditions:

 - it should be maximally readable, and maximally low-key to
   'ordinary' code reading and maintenance.

 - find a build time method to insert dwarf annotations
   automatically in the most common cases, for pop/push
   instructions that manipulate the stack pointer. This could
   be done for example via a preprocessing step that just
   looks for common patterns - plus special annotations for
   the few cases where we want to depart from the default.
   We have hundreds of CFI annotations, so automating most of
   that makes sense.

 - it should come with build tooling checks that ensure that
   CFI annotations are sensible. We've seen such efforts from
   the framepointer side, and there's no reason it couldn't be
   done on the dwarf side.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>

Sequences:

        pushl_cfi %reg
        CFI_REL_OFFSET reg, 0

and:

        popl_cfi %reg
        CFI_RESTORE reg

happen quite often. This patch adds macros which generate them.

No assembly changes (verified with objdump -dr vmlinux.o).
Signed-off-by: NDenys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1421017655-25561-1-git-send-email-dvlasenk@redhat.com
Link: http://lkml.kernel.org/r/2202eb90f175cf45d1b2d1c64dbb5676a8ad07ad.1424989793.git.luto@amacapital.netSigned-off-by: NIngo Molnar <mingo@kernel.org>

In the "xchg" implementation, %ebx and %ecx don't need to be copied
into %eax and %edx respectively (this is only necessary when desiring
to only read the stored value).

In the "add_unless" implementation, swapping the use of %ecx and %esi
for passing arguments allows %esi to become an input only (i.e.
permitting the register to be re-used to address the same object
without reload).

In "{add,sub}_return", doing the initial read64 through the passed in
%ecx decreases a register dependency.

In "inc_not_zero", a branch can be eliminated by or-ing together the
two halves of the current (64-bit) value, and code size can be further
reduced by adjusting the arithmetic slightly.

v2: Undo the folding of "xchg" and "set".
Signed-off-by: NJan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/4F19A2BC020000780006E0DC@nat28.tlf.novell.com
Cc: Luca Barbieri <luca@luca-barbieri.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>

Cleaning up and shortening code...
Signed-off-by: NJan Beulich <jbeulich@novell.com>
Cc: Alexander van Heukelum <heukelum@fastmail.fm>
LKML-Reference: <4D6BD35002000078000341DA@vpn.id2.novell.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

atomic64_inc_not_zero must return 1 if it perfomed the add and 0 otherwise.
It was doing the opposite thing.
Signed-off-by: NLuca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267469749-11878-6-git-send-email-luca@luca-barbieri.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

atomic64_add_unless must return 1 if it perfomed the add and 0 otherwise.
The implementation did the opposite thing.
Reported-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NLuca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267469749-11878-3-git-send-email-luca@luca-barbieri.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

This patch replaces atomic64_32.c with two assembly implementations,
one for 386/486 machines using pushf/cli/popf and one for 586+ machines
using cmpxchg8b.

The cmpxchg8b implementation provides the following advantages over the
current one:

1. Implements atomic64_add_unless, atomic64_dec_if_positive and
   atomic64_inc_not_zero

2. Uses the ZF flag changed by cmpxchg8b instead of doing a comparison

3. Uses custom register calling conventions that reduce or eliminate
   register moves to suit cmpxchg8b

4. Reads the initial value instead of using cmpxchg8b to do that.
   Currently we use lock xaddl and movl, which seems the fastest.

5. Does not use the lock prefix for atomic64_set
   64-bit writes are already atomic, so we don't need that.
   We still need it for atomic64_read to avoid restoring a value
   changed in the meantime.

6. Allocates registers as well or better than gcc

The 386 implementation provides support for 386 and 486 machines.
386/486 SMP is not supported (we dropped it), but such support can be
added easily if desired.

A pure assembly implementation is required due to the custom calling
conventions, and desire to use %ebp in atomic64_add_return (we need
7 registers...), as well as the ability to use pushf/popf in the 386
code without an intermediate pop/push.

The parameter names are changed to match the convention in atomic_64.h

Changes in v3 (due to rebasing to tip/x86/asm):
- Patches atomic64_32.h instead of atomic_32.h
- Uses the CALL alternative mechanism from commit
  1b1d9258

Changes in v2:
- Merged 386 and cx8 support in the same patch
- 386 support now done in assembly, C code no longer used at all
- cmpxchg64 is used for atomic64_cmpxchg
- stop using macros, use one-line inline functions instead
- miscellanous changes and improvements
Signed-off-by: NLuca Barbieri <luca@luca-barbieri.com>
LKML-Reference: <1267005265-27958-5-git-send-email-luca@luca-barbieri.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>