[RFC] security_getprocattr() API idiocy

[Al Viro]

[apologies for resend, bogus address on the original mail]

	security_getprocattr() takes a buffer + length, copies data
to it and return the actual length.  If buffer is NULL, it just returns
the right length, a-la snprintf().  Observations:
	* at least selinux ends up actually allocating the buffer of the
right size, filling it, then copying its contents to buffer and freeing
what had been allocating.
	* all users allocate buffer, then call security_getprocattr() to
fill just allocated one.
	* one place does even worse - it calls security_getprocattr() passing
it NULL and uses obtained length to allocate buffer and call
security_getprocattr() _again_.

It's bloody bogus.  In all cases we would be just as happy if it returned
the buffer it'd allocated itself.  In the best case we end up with two
allocations; in the worst it's _three_, not to mention recalculating the
contents and size.  We end up doing
	* calculate size
	* allocate buffer of that size with GFP_ATOMIC
	* fill it
	* free it
	* allocate buffer of that size with GFP_KERNEL
	* caluclate the same size
	* allocate buffer of that size with GFP_ATOMIC
	* fill it with the same string
	* copy it to buffer we's allocated with GFP_KERNEL
	* free the buffer we'd allocated with GFP_ATOMIC
I'm sorry, but could we please not mix the kernel with Vogon poetry contest?

AFAICS, the sane solution is to make security_getprocattr() return the
allocated buffer instead.  All callers would be only happy with that.
Alternatively, we can introduce a new LSM hook (security_getprocattr_sane())
and leave the original as-is.

So, do we want to keep the original variant and add a saner one in parallel
to it or should we just switch to saner API?
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