[PATCH] kmemcheck: support for x86_64

[Vegard Nossum]

Hi,

Here comes a particularly difficult patch. I am not submitting it for
application to any tree yet, but I have a small hope that somebody will
put their head out to look at it :-)

I am fairly sure the REX handling bits themselves are okay -- the kernel
gets to the point where it tries to mount the root partition. But before
that, there is a torrent of error reports coming from kmemcheck.

Most of them look something like this:

kmemcheck: Caught 8-bit read from freed memory (ffff81000780a904)
ifffffffifffffffifffffffifffffffifffffffifffffffifffffffifffffff
    ^

and my theory so far is that X86_64 uses some currently unhandled
instruction set extensions like MMX, SSE, etc. (Not 3DNow! because we
have a dependency for that), for a fairly common operation -- something
like memset(), and where we decode the size of the instruction to being
8 bits when in fact it is 64 and thus only mark 8 bits of the shadow
memory as being initialized.

(I guess the easiest way to catch this would be to make cases for those
instructions and WARN(), but... Did I mention I hate this opcode decoding
business? It's just too ugly.)

Do the #ifdef X86_64 parts look okay?

The patch applies to the 'current' branch of kmemcheck.git:
http://git.kernel.org/?p=linux/kernel/git/vegard/kmemcheck.git;a=shortlog;h=current

Note: kmemcheck reports from x86_64 are still not very good because of the
stacktrace issues reported earlier; in short, we can't look further than
the page fault stack entry, which makes it rather useless for debugging.
We do still have the RIP of the crash, though. End of note.

Thanks.


Vegard