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

[Will Deacon <will.deacon@arm.com>]

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@kernel.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?

> +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.

Will