[RFC PATCH] uprobes: copy to user-space xol page with proper cache flushing

[Russell King - ARM Linux]

On Tue, Apr 15, 2014 at 02:50:06PM -0400, David Miller wrote:
> From: David Miller <davem at davemloft.net>
> Date: Tue, 15 Apr 2014 14:30:54 -0400 (EDT)
> 
> > Russell, if for example userland stores into the instruction stream
> > to patch an instruction, what needs to be done on ARM?
> 
> Looking around I suspect something like:
> 
> 	mcrne	p15, 0, INSN_ADDR, c7, c5, 1
> 
> after the instruction stores will do it.

It does still need to be pushed out of the D-cache first though.  So,
for ARMv7 for example:

	str	NEW_INSN, [INSN_ADDR]		@ store new instruction
	mcr	p15, 0, INSN_ADDR, c7, c11, 1	@ clean d line
	mcr	p15, 0, INSN_ADDR, c7, c5, 1	@ flush i line

would do it.  We of course need the user access marking on that (so that
any fault doesn't oops the kernel) - not only for the store, but also the
following two instructions which could fault (and oops unless they're
marked with a fixup) if someone were to munmap() this page in another
thread.  All those fixups can just do the "lets return -EFAULT" from
the operation.

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