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Date:   Fri, 12 Apr 2019 22:49:16 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: ptrace: Add function argument access API

On Thu, 11 Apr 2019 18:22:53 +0100
Will Deacon <will.deacon@....com> wrote:

> On Mon, Apr 08, 2019 at 09:33:20AM +0900, Masami Hiramatsu wrote:
> > On Wed, 3 Apr 2019 13:06:49 +0100
> > Will Deacon <will.deacon@....com> 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@...nel.org>
> > > > ---
> > > >  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?
> 
> Oh, yes, passing things by value that don't fit in the registers will
> obviously go wrong. Thanks. Do we do that in the kernel?

No, as far as I know. But I can't say it never happens in the kernel.

> 
> Anyway, please resend with the comment expanded a bit and using
> pt_regs_read_reg(), then I can queue this up for 5.2.

OK, I'll do.

Thank you!

> 
> Cheers,
> 
> Will


-- 
Masami Hiramatsu <mhiramat@...nel.org>

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