[RFC] ptrace: add generic SET_SYSCALL request

[arnd@arndb.de (Arnd Bergmann)]

On Friday 07 November 2014 11:55:51 Will Deacon wrote:
> On Fri, Nov 07, 2014 at 09:30:53AM +0000, Arnd Bergmann wrote:
> > On Friday 07 November 2014 16:47:23 AKASHI Takahiro wrote:
> > > This patch adds a new generic ptrace request, PTRACE_SET_SYSCALL.
> > > It can be used to change a system call number as follows:
> > >     ret = ptrace(pid, PTRACE_SET_SYSCALL, null, new_syscall_no);
> > > 'new_syscall_no' can be -1 to skip this system call, you need to modify
> > > a register's value, in arch-specific way, as return value though.
> > > 
> > > Please note that we can't define PTRACE_SET_SYSCALL macro in
> > > uapi/linux/ptrace.h partly because its value on arm, 23, is used as another
> > > request on sparc.
> > > 
> > > This patch also contains an example of change on arch side, arm.
> > > Only syscall_set_nr() is required to be defined in asm/syscall.h.
> > > 
> > > Currently only arm has this request, while arm64 would also have it
> > > once my patch series of seccomp for arm64 is merged. It will also be
> > > usable for most of other arches.
> > > See the discussions in lak-ml:
> > > http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/300167.html
> > > 
> > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> > > 
> > 
> > Can you describe why you are moving the implementation? Is this a feature
> > that we want to have on all architectures in the future? As you say,
> > only arm32 implements is at the moment.
> 
> 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?

	Arnd