linux-kernel - Re: [PATCH 1/6] POWERPC: use KSYM_NAME

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Date:	Wed, 23 Jan 2008 21:43:43 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Paulo Marques <pmarques@...popie.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/6] POWERPC: use KSYM_NAME_LEN

[Paulo Marques - Wed, Jan 23, 2008 at 06:26:28PM +0000]
> Cyrill Gorcunov wrote:
>> Use KSYM_NAME_LEN instead of numeric value.
>
> The patch series looks like a nice cleanup, except for a few things in this 
> patch.
>
>> Actually because of too small 'tmp' there is
>> a potential buffer overflow.
>
> I don't think there is. "tmp" is not being passed to kallsyms to be filled 
> with a symbol name, but it's being used to hold a name written by the user 
> to lookup an address.
>
> If the powerpc/xmon people feel that 63 characters is enough to hold a 
> symbol name, it's their problem, but there is no buffer overflow.
>
>> Signed-off-by: Cyrill Gorcunov <gorcunov@...il.com>
>> ---
>> Index: linux-2.6.git/arch/powerpc/xmon/xmon.c
>> ===================================================================
>> --- linux-2.6.git.orig/arch/powerpc/xmon/xmon.c	2008-01-23 
>> 19:04:42.000000000 +0300
>> +++ linux-2.6.git/arch/powerpc/xmon/xmon.c	2008-01-23 19:12:45.000000000 
>> +0300
>> @@ -69,7 +69,7 @@ static unsigned long ndump = 64;
>>  static unsigned long nidump = 16;
>>  static unsigned long ncsum = 4096;
>>  static int termch;
>> -static char tmpstr[128];
>> +static char tmpstr[KSYM_NAME_LEN];
>
> This one seems ok, since "tmpstr" is used everywhere to hold symbol names.
>
>>  #define JMP_BUF_LEN	23
>>  static long bus_error_jmp[JMP_BUF_LEN];
>> @@ -2354,7 +2354,7 @@ scanhex(unsigned long *vp)
>>  		}
>>  	} else if (c == '$') {
>>  		int i;
>> -		for (i=0; i<63; i++) {
>> +		for (i = 0; i < sizeof(tmpstr) / 2; i++) {
>
> This one is completely out of the blue. Why "sizeof(tmpstr) / 2"?
>

the original code was 63 but 63 is 128/2-1 so to not change the
original idea of 'use a half size in this case' I made it like
that.

> It would make more sense to use "sizeof(tmpstr) - 1", but either way it is 
> a change in behavior.
>
>>  			c = inchar();
>>  			if (isspace(c)) {
>>  				termch = c;
>> @@ -2467,7 +2467,7 @@ symbol_lookup(void)
>>  {
>>  	int type = inchar();
>>  	unsigned long addr;
>> -	static char tmp[64];
>> +	static char tmp[KSYM_NAME_LEN];
>>   	switch (type) {
>>  	case 'a':
>> @@ -2476,7 +2476,7 @@ symbol_lookup(void)
>>  		termch = 0;
>>  		break;
>>  	case 's':
>> -		getstring(tmp, 64);
>> +		getstring(tmp, sizeof(tmp));
>>  		if (setjmp(bus_error_jmp) == 0) {
>>  			catch_memory_errors = 1;
>>  			sync();

just after that poin in the original code a call to kallsyms_lookup_name
is done - so i think it could be an overflow (of course it depends
on what *exactly* the name is being searched, and Paulo - I didn't
managed to get *the whole picture* of what is going on in this
code - so the thoughs were like: kallsyms_lookup_name could find
a quite long name restricted by KSYM_NAME_LEN (dunno how it could
happens - due to buggy code or due to memory corruption outside,
it does not matter - the only matter - it *could* find that long
name).

Anyway - it's just an attempt ;) we always could drop it far-far away ;)

>
> This also introduces a change in behavior. It is still a nice cleanup, 
> though. So, if the powerpc people feel they can spare an extra 64 bytes of 
> stack here, I guess it's ok.

Thanks a lot for review Paulo!

>
> -- 
> Paulo Marques - www.grupopie.com
>
> "As far as we know, our computer has never had an undetected error."
> Weisert
>
		- Cyrill -
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