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

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Message-ID: <44F39A23.4000409@zytor.com>
Date:	Mon, 28 Aug 2006 18:36:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Petr Vandrovec <vandrove@...cvut.cz>
CC:	Matt Domsch <Matt_Domsch@...l.com>,
	Alon Bar-Lev <alon.barlev@...il.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	johninsd@....rr.com
Subject: Re: [PATCH] Fix the EDD code misparsing the command line

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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