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

[David Long]

On 02/24/2016 04:05 AM, Marc Zyngier wrote:
> 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.

Yes, I would not argue about that cross-architecture include needing to 
be fixed.  Can I assume you agree that need not be a part of this 
kprobes patch though, nor a prerequisite patch for it?

> > > > +{
> > > > +	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.