v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Georg Nikodym]

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In the 2.4.18-2.4.19 timeframe:

	http://linux.bkbits.net:8080/linux-2.4/cset@1.536

brcl (Ben LaHaise, I think) pushed in a change to mm/slab.c which
(amongst other things) adds the following code:

...
	name = cachep->name; 
===>	{
===>	char tmp; 
===>	if (__get_user(tmp, name)) 
===>		name = "broken"; 
===>	}       

	seq_printf(m, "%-17s %6lu %6lu %6u %4lu %4lu %4u",
		name, active_objs, num_objs, cachep->objsize,
		active_slabs, num_slabs, (1<<cachep->gfporder));
...

to s_show() (the stuff that gets called when somebody cat's /proc/slabinfo)

Trouble is that on my ARM platform, the __get_user() call always fails
and all the slabinfo entries are labelled "broken".

For my purposes, ifdef'ing the offending block out will likely be
sufficient (and safe?) but I'd like to know:

1. Is the ARM __get_user() broken?
2. Could I be doing something else broken that is confusing __get_user()?
3. What was/is the intent of the test?  Or stated differently, why on earth
   would cachep->name be a user address?

-g

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Re: v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Georg Nikodym]

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On Wed, 27 Nov 2002 15:15:17 -0500
Georg Nikodym <georgn@somanetworks.com> wrote:

> 1. Is the ARM __get_user() broken?
> 2. Could I be doing something else broken that is confusing __get_user()?
> 3. What was/is the intent of the test?  Or stated differently, why on earth
>    would cachep->name be a user address?

In answer to my own question, reading the 2.5 source was illuminating. 
The intent of the test is obvious:

akpm     1.50         | 	/*
akpm     1.50         | 	 * Check to see if `name' resides inside a module which has been
akpm     1.50         | 	 * unloaded (someone forgot to destroy their cache)
akpm     1.50         | 	 */

Thanks to Mr. Morton for that comment.  Now I get to wrestle with questions 1 and 2.

-g

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Re: v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Manfred Spraul]

>
>
>> 1. Is the ARM __get_user() broken?
>> 2. Could I be doing something else broken that is confusing __get_user()?
>> 3. What was/is the intent of the test?  Or stated differently, why on earth
>>    would cachep->name be a user address?
>  
>

get_user is the standard test for bad pointers: If the pointer is bad, 
then the exception handler will prevent an oops.

Could you backport the get_fs()/set_fs() calls around the get_user() 
from 2.5? I assume that ARM needs it to distiguish between kernel and 
user addresses.

On i386, it's possible to skip set_fs() and use __get_user() - but 
that's i386 specific. For example the i386 oops code uses that.

--
    Manfred

Re: v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Russell King]

On Thu, Nov 28, 2002 at 11:34:20PM +0100, Manfred Spraul wrote:
> On i386, it's possible to skip set_fs() and use __get_user() - but 
> that's i386 specific. For example the i386 oops code uses that.

That isn't actually an x86 specific feature - it is a requirement across
all architectures that get_user() and friends can access kernel areas
after set_fs(get_ds())

That's how things like sys_execve() can read the binary headers, etc.
See linux/fs/exec.c:kernel_read() for one such example.

-- 
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html

Re: v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Manfred Spraul]

Russell King wrote:

>On Thu, Nov 28, 2002 at 11:34:20PM +0100, Manfred Spraul wrote:
>  
>
>>On i386, it's possible to skip set_fs() and use __get_user() - but 
>>that's i386 specific. For example the i386 oops code uses that.
>>    
>>
>
>That isn't actually an x86 specific feature - it is a requirement across
>all architectures that get_user() and friends can access kernel areas
>after set_fs(get_ds())
>  
>
It's i386 specific that
    __get_user().
is equivalent to
    set_fs(KERNEL_DS)
    get_user()
arch/i386/kernel/traps.c uses that in the fault code.

Portable code must use set_fs()/get_user(), i386 specific code can 
continue to use __get_user().

--
    Manfred

Re: v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Russell King]

On Fri, Nov 29, 2002 at 07:24:02PM +0100, Manfred Spraul wrote:
> Russell King wrote:
> >On Thu, Nov 28, 2002 at 11:34:20PM +0100, Manfred Spraul wrote:
> >  
> >
> >>On i386, it's possible to skip set_fs() and use __get_user() - but 
> >>that's i386 specific. For example the i386 oops code uses that.
> >>    
> >>
> >
> >That isn't actually an x86 specific feature - it is a requirement across
> >all architectures that get_user() and friends can access kernel areas
> >after set_fs(get_ds())
> >  
> >
> It's i386 specific that
>     __get_user().
> is equivalent to
>     set_fs(KERNEL_DS)
>     get_user()
> arch/i386/kernel/traps.c uses that in the fault code.
> 
> Portable code must use set_fs()/get_user(), i386 specific code can 
> continue to use __get_user().

That seems broken to me.  IIRC, when all this uaccess stuff went in back
in 2.1 times, the original intention was that __get_user() would fault
even when used from kernel mode, unless set_fs(KERNEL_DS) was in effect.

__get_user() at that time was always meant to be used as a faster version
of get_user(), even by generic code, and relied solely on the TLB protection
mechanisms, whereas get_user() verified the address.

Gah, I _really_ wish that we had a method to notify architecture maintainers
when this type of stuff changes.  Are we supposed to re-read the x86
implementation of everything for each kernel release to try to discover
what subtle semantics have changed?

-- 
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html

Re: v2.4.19-rmk4 slab.c: /proc/slabinfo uses broken instead of slab labels

[Manfred Spraul]

Russell King wrote:

>Gah, I _really_ wish that we had a method to notify architecture maintainers
>when this type of stuff changes.  Are we supposed to re-read the x86
>implementation of everything for each kernel release to try to discover
>what subtle semantics have changed?
>
>  
>
There was no change - i386 just permits something that doesn't work on 
all archs.