Re: [PATCH 6/8] ptrace: arch_ptrace -ENOSYS return

Roland McGrath
2008-03-20T08:16:58Z

> Hmm.. I see the whole series, and I see this patch, but I think it adds
> new code and new complexity, and I don't really see *why*.

The motivation is to get the arch function out of the code path for the
machine-independent request handling. I want to be able to change the
implementation later without touching the arch code again.

The arguments passed down to arch_ptrace are sufficient for what the arch
code itself needs and for the current implementation in ptrace_request.
In future, I'd like the option of changing the code for the standard
requests to use a local data structure set up at the start of ptrace, and
such like (so more pointers and the like would need to be passed down to
ptrace_request). These patches let me remove ptrace_request or change
its calling convention without touching the arch code again.

> Wouldn't it be nicer to just let "arch_ptrace()" return a flag saying
> whether it handled things or not?

It would certainly be nicer. I would prefer:

extern int arch_ptrace(struct task_struct *child, long request,
long addr, long data, long *retval);

where it returns an error code or it returns 0 and *retval is the value
or it returns 1 and it didn't do anything.

The reason I took the approach I did instead is incrementalism.
I can't change that signature without breaking about 22 arch builds.
I'm only really prepared to thoroughly verify a change on 2 of those
myself. It should be a simple enough change to make blind and get
right. But I've gotten a lot of things wrong before. On principle,
I wouldn't really expect anyone to sign off on stuff I won't even
claim to have tried. I did the forced_successful_syscall_return()
macro for arch's I don't try to build, and was just awful sure golly
that I hadn't got them wrong, because the generic change would break
those few arch's (not 20) without it.

So this ugliness seemed like a better bet than waiting for 20 more
arch sign-offs before any of it could go in. You are certainly in a
position to just change the generic signature and make every arch do
the update (or fix your typos if you just tweak them all blind), and
let them grumble. I did not presume to do so.

If you'd like a patch that changes this signature, updates all arch
implementations, and is actually verified to compile and work only
on x86 and powerpc, I'll be happy to provide that.

Thanks,
Roland