Re: [Qemu-devel] sha1sum segfaults on x86_64 target / i386 host

[Aurelien Jarno <aurelien@aurel32.net>]

On Tue, Sep 22, 2009 at 09:06:20PM +0200, Aurelien Jarno wrote:
> On Tue, Sep 22, 2009 at 07:48:52PM +0200, Laurent Desnogues wrote:
> > On Tue, Sep 22, 2009 at 5:49 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> > [...]
> > >
> > > Actually I am not really convinced it has been fixed, I really think the
> > > bug is still present, but not triggerable anymore this way.
> > >
> > > It looks like very long translation are not stopped correctly. This part
> > > of code looks suspicious:
> > >
> > >        /* if too long translation, stop generation too */
> > >        if (gen_opc_ptr >= gen_opc_end ||
> > >            (pc_ptr - pc_start) >= (TARGET_PAGE_SIZE - 32) ||
> > >            num_insns >= max_insns) {
> > >            gen_jmp_im(pc_ptr - dc->cs_base);
> > >            gen_eob(dc);
> > >            break;
> > >        }
> > >
> > > If I understand correctly, when the end of the buffer is reached, the
> > > translation is stopped, but some more opc are added by gen_jmp_im()
> > > and gen_eob().
> > >
> > > OTOH, on MIPS the following code leaves some space at the end of the
> > > buffer for a few more opc:
> > >
> > >    /* Leave some spare opc slots for branch handling. */
> > >    gen_opc_end = gen_opc_buf + OPC_MAX_SIZE - 16;
> > >
> > > Applying the same changes to the x86_64 target fixes the bug. However, I
> > > am not sure it is fully correct. Any comment?
> > 
> > You mean that if you sub 16 and go back just previous malc's commit
> > you don't experience the crash anymore?
> 
> Exactly, or even reverting his commit on the current tree.
> 
> > To me it looks like using gen_opc_buf + OPC_MAX_SIZE is rather
> > safe given that it gives room for 64 extra ops (cf exec-all.h).
> > 
> 
> I have played a bit with this number, it seems the buffer is too short
> by 3 opc.
> 

After some debugging with Laurent on IRC (thanks!), the problem is that
some instructions (e.g. ror $0x1b,%eax) can generate up to 77 tcg op.
This is not consistent with this line from exec-all.h:
#define MAX_OP_PER_INSTR 64

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net