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

[Huacai Chen]

On Sat, Feb 11, 2023 at 4:44 PM Qing Zhang <zhangqing@xxxxxxxxxxx> wrote:
>
> Add regs_get_argument() which returns N th argument of the function
> call, t show This enables ftrace kprobe events to access kernel function
> arguments via $argN syntax.
Sadly, I think this message should be rewritten.

>
> eg:
> echo 'p bio_add_page arg1=$arg1' > kprobe_events
> bash: echo: write error: Invalid argument
>
> Signed-off-by: Qing Zhang <zhangqing@xxxxxxxxxxx>
> ---
>  arch/loongarch/Kconfig              |  1 +
>  arch/loongarch/include/asm/ptrace.h | 33 +++++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+)
>
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index e3eba2eb4b44..103046966893 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -97,6 +97,7 @@ config LOONGARCH
>         select HAVE_EXIT_THREAD
>         select HAVE_FAST_GUP
>         select HAVE_FTRACE_MCOUNT_RECORD
> +       select HAVE_FUNCTION_ARG_ACCESS_API
>         select HAVE_FUNCTION_GRAPH_TRACER
>         select HAVE_FUNCTION_TRACER
>         select HAVE_GENERIC_VDSO
> diff --git a/arch/loongarch/include/asm/ptrace.h b/arch/loongarch/include/asm/ptrace.h
> index 66a0e6c480a3..f6ffcc00753c 100644
> --- a/arch/loongarch/include/asm/ptrace.h
> +++ b/arch/loongarch/include/asm/ptrace.h
> @@ -120,6 +120,39 @@ static inline long regs_return_value(struct pt_regs *regs)
>         return regs->regs[4];
>  }
>
> +/**
> + * 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.
> + * This is expected to be called from kprobes or ftrace with regs
> + * where the top of stack is the return address.
> + */
> +static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
> +                                                    unsigned int n)
> +{
> +       static const unsigned int argument_offs[] = {
> +               offsetof(struct pt_regs, regs[4]),
> +               offsetof(struct pt_regs, regs[5]),
> +               offsetof(struct pt_regs, regs[6]),
> +               offsetof(struct pt_regs, regs[7]),
> +               offsetof(struct pt_regs, regs[8]),
> +               offsetof(struct pt_regs, regs[9]),
> +               offsetof(struct pt_regs, regs[10]),
> +               offsetof(struct pt_regs, regs[11]),
> +#define NR_REG_ARGUMENTS 8
> +       };
> +
> +       if (n >= NR_REG_ARGUMENTS) {
> +               n -= NR_REG_ARGUMENTS;
> +               return regs_get_kernel_stack_nth(regs, n);
> +       } else
> +               return regs_get_register(regs, argument_offs[n]);
> +}
> +
>  #define instruction_pointer(regs) ((regs)->csr_era)
>  #define profile_pc(regs) instruction_pointer(regs)
>
> --
> 2.36.0
>