linux-kernel - signal frame ss in a 64-bit process?

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Date:	Wed, 9 Jul 2014 15:14:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: signal frame ss in a 64-bit process?

How is restoring ss and ds in a 64-bit process supposed to work?
There's this comment:

        /* Kernel saves and restores only the CS segment register on signals,
         * which is the bare minimum needed to allow mixed 32/64-bit code.
         * App's signal handler can save/restore other segments if needed. */

AFAICT this is bogus: syscall (the 64-bit version) sets regs->ss to
__USER_DS.  I think that the actual ss register is untouched until the
syscall returns, so in theory sigreturn could copy ss to regs, but
that seems kind of nasty.

Can we steal __pad0 and put ss in there?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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