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Re: [Qemu-devel] multi-bytes nop and 64bits

From: malc
Subject: Re: [Qemu-devel] multi-bytes nop and 64bits
Date: Tue, 21 Apr 2009 21:20:13 +0400 (MSD)

On Tue, 21 Apr 2009, Mark Karpeles wrote:

> Hi,
> Le mardi 21 avril 2009 ЪЪ 02:10 +0400, malc a ЪЪcrit :
> > On Mon, 20 Apr 2009, Mark Karpeles wrote:
> > > As seen by qemu (QEMU PC emulator version 0.10.1):
> > > 
> > > 0x0000000000201000:  (bad)
> > > 0x0000000000201002:  add    %bpl,%cl
> > > 0x0000000000201005:  sub    %edx,0x0(%rcx)
> > > 0x0000000000201008:  add    %al,(%rax)
> > > 0x000000000020100a:  add    %al,(%rax)
> > > (The code after this line is not meant to be executed, it's read-only
> > > data, and so the system will crash)
> > The disassembler is disconnected from the main translation engine, so
> > what you see (produced by the older version of i386 disassmbler from
> > GNU binutils) might not be what QEMU actually translates/executes. To
> > be sure use gdbstub and recent GDB to see, hopefully, more coherent view.
> Oh my... I checked more and tried running this without the qemu debug
> mode "in_asm", and it's working indeed.
> So, in_asm will decompile memory until it reaches a "stop" operation
> (ret, %cr3 changed, etc), however if it doesn't reach such an operation
> it will continue reading memory.
> I wasn't expecting this bug because the fact it was continuing to read
> memory caused a page-fault which was catched by the kernel I'm writing.
> I never though the debugger could trigger stuff like OS pagefaults (and
> of course it was happening with %rip exactly pointing to a long nop,
> that's why I wasn't understanding the origin of the page fault).
> Maybe this could be added to some documentation: "If guest OS crashes
> because of a page fault on a long nop operation, try disabling in_asm
> debugging mode".

I think fixing disassembler is a better idea, disas.c:target_disas is
clearly broken, since:

void target_disas(FILE *out, target_ulong code, target_ulong size, int 
    for (pc = code; size > 0; pc += count, size -= count) {

So here size is unsigned and as such the stop condition can only become
true when translator and disassembler have the same idea how they decode
instructions (which was clearly not the case for hint_nop).
> Or maybe this debugging mode could be fixed to handle long nops? This
> debugging mode is really helpful to understand what happened during a
> crash, and for people writing their own kernel (like me) it's a life
> savior.
> I had a look at qemu's decompiler (an old version of i386-dec.c from
> gdb), and "fixed" it to support nopl correctly (and at least avoid
> weird crashes).
> Patch:
> http://ookoo.org/svn/snip/qemu-0.10.1-nopl-fix.patch

I'm not intimately familiar with binutils' disasm but it looks correct
save for the fact that hint_nop take M and not Ev as operand, which
might be irrelevant for disassembler though...

> Anyway thanks for the tip, you just saved me more days of head
> scratching.

You are welcome..


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