[RFC] ptrace: add generic SET_SYSCALL request

[Arnd Bergmann]

On Friday 07 November 2014 12:11:19 Russell King - ARM Linux wrote:
> On Fri, Nov 07, 2014 at 01:03:00PM +0100, Arnd Bergmann wrote:
> > On Friday 07 November 2014 11:55:51 Will Deacon wrote:
> > > We need this for arm64 and, since all architectures seem to have a mechanism
> > > for setting a system call via ptrace, moving it to generic code should make
> > > sense for new architectures too, no?
> > 
> > It makes a little more sense now, but I still don't understand why you
> > need to set the system call number via ptrace. What is this used for,
> > and why doesn't any other architecture have this?
> 
> All other architectures have a way.  x86, for example, you set orig_eax
> (or orig_rax) to change the syscall number.  On ARM, that doesn't work
> because we don't always pass the syscall number in a register.
> 

Sorry for being slow today, but why can't we use the same interface that
s390 has on arm64:

static int s390_system_call_get(struct task_struct *target,
                                const struct user_regset *regset,
                                unsigned int pos, unsigned int count,
                                void *kbuf, void __user *ubuf)
{
        unsigned int *data = &task_thread_info(target)->system_call;
        return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
                                   data, 0, sizeof(unsigned int));
}

static int s390_system_call_set(struct task_struct *target,
                                const struct user_regset *regset,
                                unsigned int pos, unsigned int count,
                                const void *kbuf, const void __user *ubuf)
{
        unsigned int *data = &task_thread_info(target)->system_call;
        return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
                                  data, 0, sizeof(unsigned int));
}

static const struct user_regset s390_regsets[] = {
	...
        {
                .core_note_type = NT_S390_SYSTEM_CALL,
                .n = 1,
                .size = sizeof(unsigned int),
                .align = sizeof(unsigned int),
                .get = s390_system_call_get,
                .set = s390_system_call_set,
        },
	...
};

Is it just preference for being consistent with ARM32, or is there a
reason this won't work?

It's not that I care strongly about the interface, my main point is
that the changelog doesn't describe why one interface was used instead
the other.

	Arnd