Re: [patch 2/4] [RFC] syscalls, x86: Add __NR_kcmp syscall v4

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Tue, Jan 24, 2012 at 01:22:22PM -0800, Andrew Morton wrote:
> 
> PIDs are not unique.  One wonders what happens in this syscall if the
> same pid appears in two namespaces.
> 
> <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 ;)
> 

Yes, caller's namespace was used intentionally, will add comments (manpage
makes me shiver).

> > At moment only x86 is supported.
> 
> Presumably you have a test app.  Please let's include that app in
> tools/testing/selftests/ for arch maintainers and others to use and
> maintain.

ok

> > +static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
> 
> This reader of this code doesn't understand why all this cookie stuff
> is in here.  Please include code comments which explain the reason for
> the existence of this code.
> 

ok

> > +static long kptr_obfuscate(long v, int type)
> > +{
> > +	return (v ^ cookies[type][0]) * cookies[type][1];
> > +}
> > +
> > +/*
> > + * 0 - equal
> > + * 1 - less than
> > + * 2 - greater than
> > + * 3 - not equal but ordering unavailable
> 
> what the heck does case 3 mean?  Why is it here?
> 

I'll add a comment. It's reserved for case where we might
need to disable gt/lt comparision result. Probably in future.

> > +
> > +#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.
> 

ok, i'll change it of course, but I personally like macros version more.

> > +/* A caller must be sure the task is presented in memory */
> "The caller must have pinned the task"
> 
> > +	if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
> > +	    !ptrace_may_access(task2, PTRACE_MODE_READ)) {
> 
> Add a comment explaining this decision.
> 

OK.

> 
> ENOENT seems inappropriate here.
>

Which one should be better?

> > +static __init int kcmp_cookie_init(void)
> > +{
> > +	int i, j;
> > +
> > +	for (i = 0; i < KCMP_TYPES; i++) {
> > +		for (j = 0; j < 2; j++) {
> > +			get_random_bytes(&cookies[i][j],
> > +					 sizeof(cookies[i][j]));
> > +		}
> > +		cookies[i][1] |= (~(~0UL >>  1) | 1);
> 
> hm, what's the point in writing a random number to cookies[i][1] and
> then immediately overwriting that with a constant?

It's '|=' , not '='.

	Cyrill