Re: [PATCH v4 1/2] ptrace: save the type of syscall-stop in ptrace_message

["Dmitry V. Levin" <ldv-u2l5PoMzF/Vg9hUCZPvPmw@public.gmane.org>]

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On Thu, Nov 29, 2018 at 03:47:43PM +0100, Oleg Nesterov wrote:
> On 11/29, Dmitry V. Levin wrote:
> >
> > 2. Document these values
> 
> sure, they should be documented and live in include/uapi/,
> 
> > chosen to avoid collisions with ptrace_message values
> > set by other ptrace events
> 
> this is what I can't understand. But to clarify, I don't really care and
> won't argue.
> 
> If an application wants to use PTRACE_GETEVENTMSG to distinguish entry/exit
> (without PTRACE_GET_SYSCALL_INFO) it needs to do wait(status) and check status
> anyway, otherwise PTRACE_GETEVENTMSG is simply pointless (wrt syscall entry/
> exit). So we do not care if PTRACE_EVENTMSG_SYSCALL_ENTRY conflicts with, say,
> SECCOMP_RET_DATA.

Yes, once the application has verified that the kernel implements this
feature, there is no risk of collision.

> > so that PTRACE_GETEVENTMSG users can easily tell
> > whether this new semantics is supported by the kernel or not.
> 
> Yes. And how much this can help? Again, an application can trivially detect
> if this feature implemented or not, and it should do this anyway if it wants
> to (try to) use PTRACE_EVENTMSG_SYSCALL_ENTRY/EXIT ?

How an application can easily detect whether this feature is implemented?
By invoking PTRACE_GETEVENTMSG after the first syscall stop reported by
wait and checking whether the returned value is either
PTRACE_EVENTMSG_SYSCALL_ENTRY or PTRACE_EVENTMSG_SYSCALL_EXIT.

So the question is, how can this value be equal to one of these constants
when this feature is not implemented?  Can a value saved to ptrace_message
earlier by one of ptrace events be equal to one of these constants?

Imagine an application attaches to already existing process, enables
PTRACE_O_TRACESECCOMP, and a PTRACE_EVENT_SECCOMP arrives with
ptrace_message set to 1.  If this application then exits and a new invocation
of the same application attaches to the same process, it will very likely see
this 1 returned by PTRACE_GETEVENTMSG if the feature is not implemented
in the kernel.

To avoid that kind of collisions, kernel should use different ptrace_message
values for syscall stops.

> Again, I won't reallly argue. But if you insist that these values must
> be unique then you probably need to add
> 
> 	BUILD_BUG_ON(PTRACE_EVENTMSG_SYSCALL_ENTRY <= PID_MAX_LIMIT);

Yes, it's a good idea.  What is the proper place for this check?


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