Re: Bug in kernel code?

Re: Bug in kernel code?

[Stephen C. Biggs]

On 27 Aug 2002 at 17:23, David S. Miller wrote:

>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 17:24:19 -0700
> 
>    ........... Notice the --order >=0 in the do while test... since order is declared as an unsigned 
>    long, this test is a pointless comparison and never fails, causing an infinite loop if the 
>    hashtable is empty.  
>    
>    My fix to this is to change the test to have
>    while (dentry_hashtable == NULL && order-- != 0);
>    
>    Could someone check me on this?
>    
> Your analysis is right but the fix is wrong, we do want to
> try order == 0 on very small memory systems, and you should
> not decrement the order at the beginning of the loop.  We
> want to use the "order" calculated.
> 
> Just make 'order' signed long to fix this bug.
> 

NO!  That won't work either.  This is a "do while" loop so the first test is always done and if 
order is 0, the check will be done AFTER the decrement, so this works.  Changing it to a signed 
long loses you a bit.

Re: Bug in kernel code?

[David S. Miller]

   From: "Stephen C. Biggs" <s.biggs@softier.com>
   Date: Tue, 27 Aug 2002 17:32:58 -0700
   
   This is a "do while" loop so the first test is always done
   
No, a "do while" loop always executes the first iteration.
What version of the C language do you think we are using?

Re: Bug in kernel code?

[Stephen C. Biggs]

On 27 Aug 2002 at 17:29, David S. Miller wrote:

>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 17:32:58 -0700
>    
>    This is a "do while" loop so the first test is always done
>    
> No, a "do while" loop always executes the first iteration.
> What version of the C language do you think we are using?
> 

You're misunderstanding me, I meant that the  first test is done AFTER the first iteration is 
executed, so my fix is correct since, even if order is 0 because at least one iteration of the loop 
is done, and the post decrement makes sure that the test succeeds if order was 0 going into the 
loop.

Changing order to a signed long loses half of the cache entries due to the number being divided by 
2.

Re: Bug in kernel code?

[David S. Miller]

   From: "Stephen C. Biggs" <s.biggs@softier.com>
   Date: Tue, 27 Aug 2002 17:40:41 -0700
   
   You're misunderstanding me, I meant that the  first test is done
   AFTER the first iteration is   executed, so my fix is correct
   since, even if order is 0 because at least one iteration of the
   loop  is done, and the post decrement makes sure that the test
   succeeds if order was 0 going into the loop.

Order is always >= 0 when we enter the loop.

If we actually get the table allocated then the decrement of 'order'
is not executed if we allocate the table successfully.

I don't understand what the problem is with my fix.

Re: Bug in kernel code?

[Stephen C. Biggs]

On 27 Aug 2002 at 17:42, David S. Miller wrote:

>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 17:40:41 -0700
>    
>    You're misunderstanding me, I meant that the  first test is done
>    AFTER the first iteration is   executed, so my fix is correct
>    since, even if order is 0 because at least one iteration of the
>    loop  is done, and the post decrement makes sure that the test
>    succeeds if order was 0 going into the loop.
> 
> Order is always >= 0 when we enter the loop.
> 
> If we actually get the table allocated then the decrement of 'order'
> is not executed if we allocate the table successfully.
> 
> I don't understand what the problem is with my fix.
> 
[Trying again - my mail client is messing up/sending empty posts]

As I look at it, it can only be 0 <= order < 32, so your fix works, as mine does.  Yours is simpler 
and doesn't involve changing code, just declarations.

There need to be some sanity checks in this code: what if mempages is passed as some insanely huge 
number, e.g.

Re: Bug in kernel code?

[David S. Miller]

   From: "Stephen C. Biggs" <s.biggs@softier.com>
   Date: Tue, 27 Aug 2002 18:09:46 -0700
   
   There need to be some sanity checks in this code: what if mempages is passed as some insanely huge 
   number, e.g.

And then your mail ends.... let us know when you've fixed
your email client, this isn't rocket science :-)

Re: Bug in kernel code?

[Stephen C. Biggs]

On 27 Aug 2002 at 18:09, David S. Miller wrote:

>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 18:09:46 -0700
>    
>    There need to be some sanity checks in this code: what if mempages is passed as some insanely huge 
>    number, e.g.
> 
> And then your mail ends.... let us know when you've fixed
> your email client, this isn't rocket science :-)
> 

What are you talking about "And then your mail ends..."  That's all I wanted to say...

Oh, you want me to post a complete patch.... I get it.

Re: Bug in kernel code?

[David S. Miller]

   From: "Stephen C. Biggs" <s.biggs@softier.com>
   Date: Tue, 27 Aug 2002 18:24:13 -0700

   On 27 Aug 2002 at 18:09, David S. Miller wrote:
   
   > And then your mail ends.... let us know when you've fixed
   > your email client, this isn't rocket science :-)
   > 
   
   What are you talking about "And then your mail ends..."  That's all I wanted to say...
   
No, I thought you were going to give an example of a huge number.
:-)

Re: Bug in kernel code?

[Stephen C. Biggs]

On 27 Aug 2002 at 18:23, David S. Miller wrote:

>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 18:24:13 -0700
> 
>    On 27 Aug 2002 at 18:09, David S. Miller wrote:
>    
>    > And then your mail ends.... let us know when you've fixed
>    > your email client, this isn't rocket science :-)
>    > 
>    
>    What are you talking about "And then your mail ends..."  That's all I wanted to say...
>    
> No, I thought you were going to give an example of a huge number.
> :-)
> 

How about (unsigned long)(~0)?

This code never finishes:

#include <stdio.h>

#define PAGE_SHIFT 12

struct list_head {
	struct list_head *next, *prev;
};

int main(void)
{
	unsigned long order;
	unsigned long mempages = ~0;

#if PAGE_SHIFT < 13
	mempages >>= (13 - PAGE_SHIFT);
#endif
	mempages *= sizeof(struct list_head);

	for (order = 0; ((1UL << order) << PAGE_SHIFT) < mempages; order++)
		;
	printf("%d\n",order);
	return 0;
}

Re: Bug in kernel code?

[David S. Miller]

   From: "Stephen C. Biggs" <s.biggs@softier.com>
   Date: Tue, 27 Aug 2002 19:42:36 -0700

   How about (unsigned long)(~0)?
   
Realistically possible with any known configuration?

Re: Bug in kernel code?

[Luca Barbieri]

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On Wed, 2002-08-28 at 05:39, David S. Miller wrote:
>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 19:42:36 -0700
> 
>    How about (unsigned long)(~0)?
>    
> Realistically possible with any known configuration?
How about replacing the loop with ffs/__ffs that would be more elegant,
shorter and avoid any problem or doubt?


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Re: Bug in kernel code?

[David S. Miller]

   From: Luca Barbieri <ldb@ldb.ods.org>
   Date: 28 Aug 2002 05:57:50 +0200

   On Wed, 2002-08-28 at 05:39, David S. Miller wrote:
   > Realistically possible with any known configuration?

   How about replacing the loop with ffs/__ffs that would be more
   elegant, shorter and avoid any problem or doubt?
   
This is getting rediculious.  You can pursue this further
if you want, but I think we've wasted enough time with
this non-bug.

Who is getting bootup failures due to this problem?
Answer: nobody

Re: Bug in kernel code?

[Luca Barbieri]

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On Wed, 2002-08-28 at 05:58, David S. Miller wrote:
>    From: Luca Barbieri <ldb@ldb.ods.org>
>    Date: 28 Aug 2002 05:57:50 +0200
> 
>    On Wed, 2002-08-28 at 05:39, David S. Miller wrote:
>    > Realistically possible with any known configuration?
> 
>    How about replacing the loop with ffs/__ffs that would be more
>    elegant, shorter and avoid any problem or doubt?
>    
> This is getting rediculious.  You can pursue this further
> if you want, but I think we've wasted enough time with
> this non-bug.
> 
> Who is getting bootup failures due to this problem?
> Answer: nobody
I'm not saying that it's serious bug, just that using __ffs is more
appropriate than reimplementing it incorrectly and inefficiently.


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Re: Bug in kernel code?

[David S. Miller]

   From: Luca Barbieri <ldb@ldb.ods.org>
   Date: 28 Aug 2002 06:10:39 +0200

   I'm not saying that it's serious bug, just that using __ffs is more
   appropriate than reimplementing it incorrectly and inefficiently.
   
ffs won't find the smallest power of 2 >= some_arbitrary_value.
That is what this code is doing.

I've personally wasted enough typing on this thread, so enough....

Re: Bug in kernel code?

[Luca Barbieri]

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On Wed, 2002-08-28 at 06:07, David S. Miller wrote:
>    From: Luca Barbieri <ldb@ldb.ods.org>
>    Date: 28 Aug 2002 06:10:39 +0200
> 
>    I'm not saying that it's serious bug, just that using __ffs is more
>    appropriate than reimplementing it incorrectly and inefficiently.
>    
> ffs won't find the smallest power of 2 >= some_arbitrary_value.
> That is what this code is doing.
Yes you are right, fls should be used there, not ffs.


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Re: Bug in kernel code?

[Stephen Biggs]

On 27 Aug 2002 at 20:39, David S. Miller wrote:

>    How about (unsigned long)(~0)?
>    
> Realistically possible with any known configuration?
> 

You tell me.  You're saying a billion pages (((unsigned long)(~0)) >> 2) also crashes) is never 
going to be realistically possible?  Sounds like Bill Gates when he said (and I don't know the word-
for-word quote) "Who's ever going to need more than 640K??"  What if we get into 64 bit addressing? 
What if there is some sort of bug that passes all 1s on the stack for just this one instance?  
Never could "realistically" happen? Yeah, right; I've seen weirder things than that.

It's a question of mandatory paranoid sanity checking in an OS wherever possible.  Linux is trying 
to be known as robust.  Are you saying that a supposedly robust kernel should have a chance to 
crash in an infinite loop during initialization because there isn't code doing input validation 
when there isn't an optimization or speed issue?

Re: Bug in kernel code?

[David S. Miller]

   From: "Stephen Biggs" <s.biggs@softier.com>
   Date: Tue, 27 Aug 2002 21:29:06 -0700

   You tell me.  You're saying a billion pages (((unsigned long)(~0)) >> 2) also crashes) is never 
   going to be realistically possible?

On a 32-bit system?  No.  x86 cpus are architectually limited
to 64GB of memory, shift that right by PAGE_SIZE (13) and we're
still within bounds.

Larger memory will be then be found on 64-bit systems, and hey
then the limit becomes significantly higher.

So that is exactly what I am saying, it is not realistically
possible.

Re: Bug in kernel code?

[H. Peter Anvin]

Followup to:  <20020827.212649.102436426.davem@redhat.com>
By author:    "David S. Miller" <davem@redhat.com>
In newsgroup: linux.dev.kernel
>
>    From: "Stephen Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 21:29:06 -0700
> 
>    You tell me.  You're saying a billion pages (((unsigned long)(~0)) >> 2) also crashes) is never 
>    going to be realistically possible?
> 
> On a 32-bit system?  No.  x86 cpus are architectually limited
> to 64GB of memory, shift that right by PAGE_SIZE (13) and we're
> still within bounds.                   ^^^^^^^^^^^^^^
> 

PAGE_SHIFT, 12.

The maximum possible page count of any known 32-bit system (64 GB @ 4K
for x86) is 2^24 pages.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>

Re[2]: Bug in kernel code?

[Stephen C. Biggs]

On 27 Aug 2002 at 18:23, David S. Miller wrote:

>    From: "Stephen C. Biggs" <s.biggs@softier.com>
>    Date: Tue, 27 Aug 2002 18:24:13 -0700
> 
>    On 27 Aug 2002 at 18:09, David S. Miller wrote:
>    
>    > And then your mail ends.... let us know when you've fixed
>    > your email client, this isn't rocket science :-)
>    > 
>    
>    What are you talking about "And then your mail ends..."  That's all I wanted to say...
>    
> No, I thought you were going to give an example of a huge number.
> :-)
> 
> -
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> 

Argh.  I meant to post THIS code:

#include <stdio.h>

#define PAGE_SHIFT 12

struct list_head {
	struct list_head *next, *prev;
};

int main(void)
{
	unsigned long order;
	unsigned long mempages = ~0;

#if PAGE_SHIFT < 13
	mempages >>= (13 - PAGE_SHIFT);
#endif
	mempages *= sizeof(struct list_head);

	printf("mempages: %lu\n", mempages);

	for (order = 0; ((1UL << order) << PAGE_SHIFT) < mempages; order++)
		;
	printf("%lu\n",order);
	return 0;
}

Re: Bug in kernel code?

[Stephen C. Biggs]