linux-kernel - Re: [PATCH -mm 1/1] ptrace: PTRACE_GETFDPIC: fix the unsafe usage of child->mm

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Date:	Mon, 24 May 2010 16:42:50 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Frysinger <vapier@...too.org>, linux-sh@...r.kernel.org,
	Paul Mundt <lethal@...ux-sh.org>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 1/1] ptrace: PTRACE_GETFDPIC: fix the unsafe usage
	of child->mm

> You mean, pass it to arch_ptrace() ?
> 
> grep, grep, grep. I guess I understand you. We have more unsafe code
> like this in arch/*/kernel/ptrace.c. Of course, it can be fixed without
> doing get_task_mm() in sys_ptrace(), but perhaps it would be more clean
> to do what you suggest.
> 
> Roland, what do you think?

The mm pointer is only used by these uncommon ptrace operations that exist
only in certain unusual arch's (and they're all ill-advised old arch ptrace
ABI additions, at that).  It doesn't seem wise to pay the overhead for
get_task_mm()/mmput() on every ptrace call, 99.44% of which don't use it
(and 100% on 90% of machines).

If you were to make any change to the signature of arch_ptrace() it should
be one big change to use a struct ptrace_params or suchlike, also passed
down to ptrace_request().  Then any future needs to pass around more
information won't require changing the code in all the arch code that
doesn't look at the new parameter.

Thanks,
Roland
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