linux-kernel - Re: Compat 32-bit syscall entry from 64-bit task!?

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Date:	Fri, 20 Jan 2012 09:56:58 -0800
From:	Roland McGrath <mcgrathr@...gle.com>
To:	Will Drewry <wad@...omium.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jamie Lokier <jamie@...reable.org>,
	Andrew Lutomirski <luto@....edu>,
	Indan Zupancic <indan@....nu>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	ak@...ux.intel.com, eric.dumazet@...il.com, gregkh@...e.de,
	dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

In arch_ptrace, task_user_regset_view is called on current.  On an x86-64
kernel, that path is only reached for a 64-bit syscall.  compat_arch_ptrace
doesn't use it at all, always using the 32-bit view.  So your change would
have no effect on PTRACE_GETREGS.

It would only affect PTRACE_GETREGSET, which calls task_user_regset_view on
the target task.  Is that what you meant?  I think that would be confusing
at best.  A caller of PTRACE_GETREGSET is expecting a particular layout
based on what type of task he thinks he's dealing with.  The caller can
look at the iov_len in the result to discern which layout it actually got
filled in, but I don't think that's what callers expect.

The other use of task_user_regset_view is in core dump
(binfmt_elf.c:fill_note_info).  Off hand I don't think there's a way a core
dump can be started while still "inside" a syscall so that TS_COMPAT could
ever be set.  But that should be double-checked.

As to whether it was considered before, I doubt that it was.  I don't
really recall the sequence of events, but I think that I did all the
user_regset code before I was really cognizant of the TS_COMPAT subtleties.

Thanks,
Roland
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