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From: ebiederm@xmission.com (Eric W. Biederman)
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
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Subject: Re: [patch 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v4
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Date: Tue, 24 Jan 2012 14:54:18 -0800
In-Reply-To: <20120124132222.d78bc0d4.akpm@linux-foundation.org> (Andrew
	Morton's message of "Tue, 24 Jan 2012 13:22:22 -0800")
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Andrew Morton <akpm@linux-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