linux-kernel - Re: [patch 2/4] [RFC] syscalls, x86: Add __NR

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Date:	Tue, 24 Jan 2012 14:54:18 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>, Valdis.Kletnieks@...edu
Subject: Re: [patch 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v4

Andrew Morton <akpm@...ux-foundation.org> writes:


> <reads the code>
>
> Seems that it performs lookups only in the caller's PID namespace. 
> Maybe this is appropriate but it should be described and justified in
> the changelog and in code comments, please.  And in the forthcoming
> manpage ;)

Well pids should always and only be looked up in the callers pid
namespace.  Any other behavior is broken.  It is probably worth
a mention in a manpage but you should not need to justify using
abstractions as they were designed to be used.


>> +static int kcmp_ptr(long v1, long v2, int type)
>> +{
>> +	long ret;
>> +
>> +	ret = kptr_obfuscate(v1, type) - kptr_obfuscate(v2, type);
>> +
>> +	return (ret < 0) | ((ret > 0) << 1);
>> +}
>> +
>> +#define KCMP_TASK_PTR(task1, task2, member, type)	\
>> +	kcmp_ptr((long)(task1)->member,			\
>> +		 (long)(task2)->member,			\
>> +		 type)
>> +
>> +#define KCMP_PTR(ptr1, ptr2, type)			\
>> +	kcmp_ptr((long)ptr1, (long)ptr2, type)
>
> ugh.  This:
>
> static long kptr_obfuscate(void *p, enum you_forgot_to_name_the_enum type)
> {
> 	return ((long)p ^ cookies[type][0]) * cookies[type][1];
> }
>
> static int kcmp_task_pointers(void *task1, void *task2, size_t field_offset,
> 				enum you_forgot_to_name_the_enum type)
> {
> 	void **field1 = t1 + field_offset;	/* points to a pointer in the task_struct */
> 	void **field2 = t1 + field_offset;
> 	long diff;
>
> 	diff = kptr_obfuscate(*field1, type) - kptr_obfuscate(*field2, type);
> 	return (diff < 0) | ((diff > 0) << 1);
> }
>
> 	...
> 	ret = kcmp_task_pointers(task1, task2, offsetof(task_struct, mm),
> 				KCMP_VM);
> 	...
>
> see?  No nasty macros, it's type-correct and it uses only a single
> explicit typecast.

Seriously?  Simply open coding the comparison would be better. 

        ret = kcmp_ptr(task1->files, task2->files, type);

All pointers are not encoded the same as void * pointers.  Admittedly
the only case I can think of are function pointers on Itanium, but
what is a little wrong today can easily become a lot wrong tomorrow.

Making the kcmp_ptr arguments void * seems the way to go though.

Now there is one interesting case we are not handling properly.
If any of our pointers can be NULL which I think happens in the
file case we should return -EBADF instead of reporting two NULL
pointers point to the same object.

Eric
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