linux-kernel - Re: [PATCH] x86: clear EXTRA_REGS for all executable formats

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Message-ID: <CAMzpN2i=_jBpW5UBK=syYS7BFTxJiLCPErhbWLMTF6fkZ7yFeg@mail.gmail.com>
Date:	Mon, 6 Apr 2015 19:00:43 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: clear EXTRA_REGS for all executable formats

On Sat, Apr 4, 2015 at 2:55 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On failure, sys_execve does not clobber EXTRA_REGS, so we can just
> return to userpsace without saving/restoring them.
>
> On success, ELF_PLAT_INIT in sys_execve clears all these registers.
>
> On other executable formats:
> binfmt_flat.c has similar FLAT_PLAT_INIT, but x86 (and everyone else
> except sh) doesn't define it.
> binfmt_elf_fdpic.c has ELF_FDPIC_PLAT_INIT, but x86 (and most others)
> doesn't define it.
> There are no such hooks in binfmt_aout.c et al. We inherit
> EXTRA_REGS from the prior executable.

None of these are supported on x86-64.  The first two are for embedded
MMU-less systems.  a.out is only supported for a few old arches
(x86-32, alpha, m68k).  ia32_aout.c does explicitly clear the extra
registers.

--
Brian Gerst
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