linux-kernel - Re: [PATCH v2 4/4] samples: Add example of using PTRACE

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Message-ID: <20191209204635.GC10721@redhat.com>
Date:   Mon, 9 Dec 2019 21:46:35 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Sargun Dhillon <sargun@...gun.me>, linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tycho@...ho.ws, jannh@...gle.com,
        cyphar@...har.com, luto@...capital.net, viro@...iv.linux.org.uk
Subject: Re: [PATCH v2 4/4] samples: Add example of using PTRACE_GETFD in
 conjunction with user trap

On 12/09, Christian Brauner wrote:
>
> >We can
> >add PTRACE_DETACH_ASYNC, but this makes me think that PTRACE_GETFD has
> >nothing
> >to do with ptrace.
> >
> >May be a new syscall which does ptrace_may_access() + get_task_file()
> >will make
> >more sense?
> >
> >Oleg.
> 
> Once more since this annoying app uses html by default...
> 
> But we can already do this right now and this is just an improvement.
> That's a bit rich for a new syscall imho...

I agree, and I won't really argue...

but the changelog in 2/4 says

	The requirement that the tracer has attached to the tracee prior to the
	capture of the file descriptor may be lifted at a later point.

so may be we should do this right now?

plus this part

	@@ -1265,7 +1295,8 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
		}
	 
		ret = ptrace_check_attach(child, request == PTRACE_KILL ||
	-				  request == PTRACE_INTERRUPT);
	+				  request == PTRACE_INTERRUPT ||
	+				  request == PTRACE_GETFD);

actually means "we do not need ptrace, but we do not know where else we
can add this fd_install(get_task_file()).

Oleg.