Re: [PATCH] arm64: ptrace: Add function argument access API

[Masami Hiramatsu]

Hi Will,

On Wed, 3 Apr 2019 13:06:49 +0100
Will Deacon <will.deacon@xxxxxxx> wrote:

> On Mon, Mar 18, 2019 at 04:59:02PM +0900, Masami Hiramatsu wrote:
> > Add regs_get_argument() which returns N th argument of the function
> > call. On arm64, it supports up to 8th argument.
> > Note that this chooses most probably assignment, in some case
> > it can be incorrect (e.g. passing data structure or floating
> > point etc.)
> > 
> > This enables ftrace kprobe events to access kernel function
> > arguments via $argN syntax.
> > 
> > Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > ---
> >  arch/arm64/Kconfig              |    1 +
> >  arch/arm64/include/asm/ptrace.h |   18 ++++++++++++++++++
> >  2 files changed, 19 insertions(+)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 117b2541ef3d..6ba0da4be73c 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -148,6 +148,7 @@ config ARM64
> >  	select HAVE_PERF_REGS
> >  	select HAVE_PERF_USER_STACK_DUMP
> >  	select HAVE_REGS_AND_STACK_ACCESS_API
> > +	select HAVE_FUNCTION_ARG_ACCESS_API
> >  	select HAVE_RCU_TABLE_FREE
> >  	select HAVE_RCU_TABLE_INVALIDATE
> >  	select HAVE_RSEQ
> > diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> > index ec60174c8c18..cfa1bc9b8b70 100644
> > --- a/arch/arm64/include/asm/ptrace.h
> > +++ b/arch/arm64/include/asm/ptrace.h
> > @@ -305,6 +305,24 @@ static inline unsigned long regs_return_value(struct pt_regs *regs)
> >  	return regs->regs[0];
> >  }
> >  
> > +/**
> > + * regs_get_kernel_argument() - get Nth function argument in kernel
> > + * @regs:	pt_regs of that context
> > + * @n:		function argument number (start from 0)
> > + *
> > + * regs_get_argument() returns @n th argument of the function call.
> > + * Note that this chooses most probably assignment, in some case
> > + * it can be incorrect.
> 
> In which cases would it be incorrect? I can imagine varargs causing
> problems, but are there others?

As far as I can read "Procedure Call Standard for the ARM 64-bit 
Architecture(AArch64) 5.4.2 Parameter Passing Rules", it may not return
correct data if the target function has a parameter which is 16bytes
or bigger size. Of course that is just a limitation of this interface.
But anyway, it can return wrong data for the parameter after such big
parameters. I think, for func(data-struct-128bits p1, u64 p2), p1
is stored into r0 and r1, and p2 is stored r2, is that correct?

(Of course any SIMD/float parameter passed to the function, it also can
not correctly get, but in the kernel we can ignore it.)

> > +static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
> > +						     unsigned int n)
> > +{
> > +#define NR_REG_ARGUMENTS 8
> > +	if (n < NR_REG_ARGUMENTS)
> > +		return regs_get_register(regs, n << 3);
> 
> You can use pt_regs_read_reg() to avoid the shift here.

Ah, right!

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>