Re: [PATCH v9 1/6] arm64: ptrace: add NT_ARM_SYSTEM_CALL regset

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

On Thu, Nov 27, 2014 at 05:53:36AM +0000, AKASHI Takahiro wrote:
> On 11/26/2014 09:41 PM, Will Deacon wrote:
> > On Wed, Nov 26, 2014 at 04:49:46AM +0000, AKASHI Takahiro wrote:
> >> This regeset is intended to be used to get and set a system call number
> >> while tracing.
> >> There was some discussion about possible approaches to do so:
> >>
> >> (1) modify x8 register with ptrace(PTRACE_SETREGSET) indirectly,
> >>      and update regs->syscallno later on in syscall_trace_enter(), or
> >> (2) define a dedicated regset for this purpose as on s390, or
> >> (3) support ptrace(PTRACE_SET_SYSCALL) as on arch/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,
> >> especially case of -1, to be visible before this function returns in
> >> syscall_trace_enter(), (1) doesn't work well.
> >> We will take (2) since it looks much cleaner.
> >>
> >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> >> ---
> >>   arch/arm64/kernel/ptrace.c |   35 +++++++++++++++++++++++++++++++++++
> >>   include/uapi/linux/elf.h   |    1 +
> >>   2 files changed, 36 insertions(+)
> >>
> >> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> >> index 8a4ae8e..8b98781 100644
> >> --- a/arch/arm64/kernel/ptrace.c
> >> +++ b/arch/arm64/kernel/ptrace.c
> >> @@ -551,6 +551,32 @@ static int tls_set(struct task_struct *target, const struct user_regset *regset,
> >>   	return ret;
> >>   }
> >>
> >> +static int system_call_get(struct task_struct *target,
> >> +			   const struct user_regset *regset,
> >> +			   unsigned int pos, unsigned int count,
> >> +			   void *kbuf, void __user *ubuf)
> >> +{
> >> +	struct pt_regs *regs = task_pt_regs(target);
> >> +
> >> +	return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
> >> +				   &regs->syscallno, 0, -1);
> >
> > Does this work for big-endian machines? regs->syscallno is a u64, but the
> > regset defines it as an int. I think you need to copy to a temporary
> > register first.
> 
> Right. I will fix it.
> Do you prefer to use s32, instead of int, like other regsets?

I don't have a preference either way.

It would be great to have a new revision of these patches ASAP if you're
targetting 3.19.

Will