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SubjectRe: [PATCH v7 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL
Hi Akashi,

On Thu, Oct 02, 2014 at 10:46:11AM +0100, AKASHI Takahiro wrote:
> To allow tracer to be able to change/skip a system call by re-writing
> a syscall number, there are several approaches:
>
> (1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this case
> later on in syscall_trace_enter(), or
> (2) support ptrace(PTRACE_SET_SYSCALL) as on arm
>
> Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
> tracer as well as that secure_computing() expects a changed syscall number
> to be visible, especially case of -1, before this function returns in
> syscall_trace_enter(), we'd better take (2).
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
> arch/arm64/include/uapi/asm/ptrace.h | 1 +
> arch/arm64/kernel/ptrace.c | 14 +++++++++++++-
> 2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> index 6913643..49c6174 100644
> --- a/arch/arm64/include/uapi/asm/ptrace.h
> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> @@ -23,6 +23,7 @@
>
> #include <asm/hwcap.h>
>
> +#define PTRACE_SET_SYSCALL 23
>
> /*
> * PSR bits
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index fe63ac5..2842f9f 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1082,7 +1082,19 @@ const struct user_regset_view *task_user_regset_view(struct task_struct *task)
> long arch_ptrace(struct task_struct *child, long request,
> unsigned long addr, unsigned long data)
> {
> - return ptrace_request(child, request, addr, data);
> + int ret;
> +
> + switch (request) {
> + case PTRACE_SET_SYSCALL:
> + task_pt_regs(child)->syscallno = data;
> + ret = 0;
> + break;
> + default:
> + ret = ptrace_request(child, request, addr, data);
> + break;
> + }
> +
> + return ret;
> }

I still don't understand why this needs to be in arch-specific code. Can't
we implement this in generic code and get architectures to implement
something like syscall_set_nr if they want the generic interface?

Will


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