Re: [PATCH] Fix the EDD code misparsing the command line

["H. Peter Anvin" <hpa@zytor.com>]

Petr Vandrovec wrote:
> > +
> > +# Old-style boot protocol?
> > +old_cl:
> > +    push    %ds            # aka INITSEG
> > +    pop    %fs
> > +
> > +    cmpw    $0xa33f, (0x20)
> > +    jne    done_cl            # No command line at all?
> > +    movw    (0x22), %si        # Pointer relative to INITSEG
>
> Perhaps you should convert ds:si to flat pointer and then this flat 
> pointer to fs:si using method above, to avoid problems with dword access 
> with si > 0xfffc or word access with si > 0xfffe ?
>
> > +
> > +# fs:si has the pointer to the command line now
> > +have_cl_pointer:
> > +   
> >  # loop through kernel command line one byte at a time
> > -cl_loop:
> > -    cmpl    $EDD_CL_EQUALS, (%si)
> > +cl_atspace:
> > +    movl    %fs:(%si), %eax
>
> This looks fine for new boot protocol, but what if old boot protocol 
> puts command line so that its last byte is at INITSEG:0xffff ?  You get 
> #GP here, then, although command line is correctly zero terminated and 
> does not overflow segment.

With the old protocol, the command line is supposed to fit inside the 
64K segment, so I don't think that's an issue.  Putting "Hail Mary" 
break at 0xfffd isn't a bad idea, though (especially since even if that 
is legitimate, we can't fit "edd=" into that one.)

> If si is 0xfffb here, bad things happen.  I know, things I've pointed 
> out should not be problem in real world, and new code is definitely 
> better than old one, but if you already have code to avoid endless loop 
> if command line points to 64KB array of 0xFF let's do that right, no?

Agreed.  I'll update the patch shortly.

	-hpa
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