Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support

From: Daniel Thompson
Date: Wed Jul 27 2016 - 07:20:10 EST

On 26/07/16 18:54, Mark Rutland wrote:
On Tue, Jul 26, 2016 at 10:50:08AM +0100, Daniel Thompson wrote:
On 25/07/16 18:13, Catalin Marinas wrote:
You get more unexpected side effects by not saving/restoring the whole
stack. We looked into this on Friday and came to the conclusion that
there is no safe way for kprobes to know which arguments passed on the
stack should be preserved, at least not with the current API.

Basically the AArch64 PCS states that for arguments passed on the stack
(e.g. they can't fit in registers), the caller allocates memory for them
(on its own stack) and passes the pointer to the callee. Unfortunately,
the frame pointer seems to be decremented correspondingly to cover the
arguments, so we don't really have a way to tell how much to copy.
Copying just the caller's stack frame isn't safe either since a
callee/caller receiving such argument on the stack may passed it down to
a callee without copying (I couldn't find anything in the PCS stating
that this isn't allowed).

The PCS[1] seems (at least to me) to be pretty clear that "the
address of the first stacked argument is defined to be the initial
value of SP".

I think it is only the return value (when stacked via the x8
pointer) that can be passed through an intermediate function in the
way described above. Isn't it OK for a jprobe to clobber this
memory? The underlying function will overwrite whatever the jprobe
put there anyway.

Am I overlooking some additional detail in the PCS?

I suspect that the "initial value of SP" is simply meant to be relative to the
base of the region of stack reserved for callee parameters. While it also uses
the phrase "current stack-pointer value", I suspect that this is overly

I don't think so. Whilst writing my reply of yesterday I forced stacked arguments by creating a function with nine arguments (rather than large values). The ninth argument is, as expected, passed to the callee based on the value of the SP.

In practice, GCC allocates callee parameters *above* the frame record
for the caller, which is above the SP and FP. e.g. with:

> <snip>
> ----
00000000004005d0 <large_func>:
4005d0: f81f0ff3 str x19, [sp,#-16]!
4005d4: aa0003f3 mov x19, x0
4005d8: f9400260 ldr x0, [x19]
4005dc: f84107f3 ldr x19, [sp],#16
4005e0: d65f03c0 ret
> ...

Thanks for the example.

The large structure is not a stacked argument from the point of view of the PCS parameter passing algorithm (which explicitly says how large composite types will be allocated). Instead it looks like it has been implicitly passed-by-reference and the caller makes this appear as call-by-value by allocating from its own stack frame rather than from the stacked argument space. The callee joins in by implicitly dereferencing the pointer.

It is interesting to note that you force large_func() to stack its arguments (by providing 8 dummy int arguments first) then the implicit pass-by-reference behavior is still preserved even for a stacked argument; large_func() ends up as:

ldr x0, [sp]
ldr x0, [x0]

Only thing is... I *still* haven't found anything in the AArch64 PCS which describes this behavior.

I'm coming to believe that this is a mistake and this information (and the threshold at which implicit pass-by-reference kicks in) should be documented in section 7.

Or if you prefer the short version: I agree 100% with your analysis but cannot find the document that supports it.