Re: [PATCH 5/8] arm64: kprobes instruction simulation support

[Marc Zyngier]

On Wed, 24 Feb 2016 01:56:52 -0500
David Long <dave.long@xxxxxxxxxx> wrote:

> On 02/19/2016 09:04 AM, Marc Zyngier wrote:
> > Hi David,
> >
> > On 18/02/16 23:48, David Long wrote:
> >> From: Sandeepa Prabhu <sandeepa.s.prabhu@xxxxxxxxx>
> >>
> >> Kprobes needs simulation of instructions that cannot be stepped
> >> from different memory location, e.g.: those instructions
> >> that uses PC-relative addressing. In simulation, the behaviour
> >> of the instruction is implemented using a copy of pt_regs.
> >>
> >> Following instruction catagories are simulated:
> >>   - All branching instructions(conditional, register, and immediate)
> >>   - Literal access instructions(load-literal, adr/adrp)
> >>
> >> Conditional execution is limited to branching instructions in
> >> ARM v8. If conditions at PSTATE do not match the condition fields
> >> of opcode, the instruction is effectively NOP. Kprobes considers
> >> this case as 'miss'.
> >>
> >> This code also replaces the use of arch/arm/opcodes.c for
> >> arm_check_condition().
> >>
> >> Thanks to Will Cohen for assorted suggested changes.
> >>
> >> Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@xxxxxxxxx>
> >> Signed-off-by: William Cohen <wcohen@xxxxxxxxxx>
> >> Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>

[...]

> >> +};
> >> +
> >> +asmlinkage unsigned int __kprobes arm_check_condition(u32 opcode, u32 psr)
> >
> > Why asmlinkage? This function is never called from assembly code on arm64.
> >
> 
> This comes from the 32-bit ARM code that tests the condition from 
> entry.S.  We include arch/arm/include/asm/opcodes.h in 
> arch/arm64/include/asm/opcodes.h so it gets declared there with 
> asmlinkage. I can remove the asmlinkage in the actual function 
> definition and it still compiles but I'm not sure that is kosher. 

asmlinkage is only meaningful if you're calling it from assembly code.
As you seem to only call it from C code, having asmlinkage is both
pointless and confusing.

> Will Deacon was advocating getting rid of the include of the 32-bit header 
> file but it looked to me like this would mean a lot of duplicated 
> defines and the work would be mostly unrelated to kprobes.

Arguably, arm_check_condition() (which only matters to 32bit code,
hence userspace) is also completely unrelated to kprobes. I still think
Will's point stands.

> 
> >> +{
> >> +	u32 cc_bits  = opcode >> 28;
> >> +
> >> +	if (cc_bits != ARM_OPCODE_CONDITION_UNCOND) {
> >> +		if ((*opcode_condition_checks[cc_bits])(psr))
> >> +			return ARM_OPCODE_CONDTEST_PASS;
> >> +		else
> >> +			return ARM_OPCODE_CONDTEST_FAIL;
> >> +	}
> >> +	return ARM_OPCODE_CONDTEST_UNCOND;
> >> +}
> >> +EXPORT_SYMBOL_GPL(arm_check_condition);
> >
> > Why do we need this to be exported at all? Also, it'd be better located
> > together with the deprecated instruction handling, possibly in a
> > separate patch (nothing uses this function in this patch).
> >
> 
> I've made the function static and moved it to armv8_deprecated.  I have 
> to leave the static functions that test the individual conditions and 
> the global array of pointers to them outside of the conditionally 
> compiled armv8_deprecated.c as they have to always be present for 
> kprobes to simulate a conditional branch.

I think that's fine.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.