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From: Arnd Bergmann <arnd@kernel.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
	Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	kexec@lists.infradead.org
Subject: Re: [PATCH v3 1/4] kexec: simplify compat_sys_kexec_load
Date: Tue, 18 May 2021 16:17:53 +0200	[thread overview]
Message-ID: <CAK8P3a277VggQbBnXUzpwP7TKMj-S_z6rDMYYxfjyQmzGJdpCA@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a27_z8zk6j5W4n+u3g2e90v-h+3AbaTZ6YjCQ0B7AbJaA@mail.gmail.com>

On Tue, May 18, 2021 at 4:05 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, May 18, 2021 at 3:41 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> >
> > Arnd Bergmann <arnd@kernel.org> writes:
> >
> > > From: Arnd Bergmann <arnd@arndb.de>KEXEC_ARCH_DEFAULT
> > >
> > > The compat version of sys_kexec_load() uses compat_alloc_user_space to
> > > convert the user-provided arguments into the native format.
> > >
> > > Move the conversion into the regular implementation with
> > > an in_compat_syscall() check to simplify it and avoid the
> > > compat_alloc_user_space() call.
> > >
> > > compat_sys_kexec_load() now behaves the same as sys_kexec_load().
> >
> > Nacked-by: "Eric W. Biederman" <ebiederm@xmission.com>
> >KEXEC_ARCH_DEFAULT
> > The patch is wrong.
> >
> > The logic between the compat entry point and the ordinary entry point
> > are by necessity different.   This unifies the logic and breaks the compat
> > entry point.
> >
> > The fundamentally necessity is that the code being loaded needs to know
> > which mode the kernel is running in so it can safely transition to the
> > new kernel.
> >
> > Given that the two entry points fundamentally need different logic,
> > and that difference was not preserved and the goal of this patchset
> > was to unify that which fundamentally needs to be different.  I don't
> > think this patch series makes any sense for kexec.
>
> Sorry, I'm not following that explanation. Can you clarify what different
> modes of the kernel you are referring to here, and how my patch
> changes this?

I think I figured it out now myself after comparing the two functions:

--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -269,7 +269,8 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry,
unsigned long, nr_segments,

        /* Verify we are on the appropriate architecture */
        if (((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH) &&
-               ((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH_DEFAULT))
+               (in_compat_syscall() ||
+               ((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH_DEFAULT)))
                return -EINVAL;

        /* Because we write directly to the reserved memory

Not sure if that's the best way of doing it, but it looks like folding this
in restores the current behavior.

        Arnd

  reply	other threads:[~2021-05-18 14:19 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-17 20:33 [PATCH v3 0/4] compat: remove compat_alloc_user_space callers Arnd Bergmann
2021-05-17 20:33 ` [PATCH v3 1/4] kexec: simplify compat_sys_kexec_load Arnd Bergmann
2021-05-18  3:57   ` Eric W. Biederman
2021-05-18  6:40     ` Christoph Hellwig
2021-05-18  7:44       ` Arnd Bergmann
2021-05-18  6:38   ` Christoph Hellwig
2021-05-18  7:47     ` Arnd Bergmann
2021-05-18 13:41   ` Eric W. Biederman
2021-05-18 14:05     ` Arnd Bergmann
2021-05-18 14:17       ` Arnd Bergmann [this message]
2021-05-18 16:01         ` Eric W. Biederman
2021-05-18 22:45         ` Eric W. Biederman
2021-05-19  9:55           ` Arnd Bergmann
2021-05-18 20:47   ` David Laight
2021-05-17 20:33 ` [PATCH v3 2/4] mm: simplify compat_sys_move_pages Arnd Bergmann
2021-05-18  6:42   ` Christoph Hellwig
2021-05-18 20:49   ` David Laight
2021-05-19 13:41     ` Arnd Bergmann
2021-05-17 20:33 ` [PATCH v3 3/4] mm: simplify compat numa syscalls Arnd Bergmann
2021-05-18  6:52   ` Christoph Hellwig
2021-05-17 20:33 ` [PATCH v3 4/4] compat: remove some compat entry points Arnd Bergmann
2021-05-18  6:55   ` Christoph Hellwig
2021-05-19 20:33   ` Thomas Gleixner
2021-05-19 21:00     ` Arnd Bergmann
2021-05-20  9:21       ` Arnd Bergmann

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