linux-kernel - Re: [PATCH] net: compat: Ignore MSG_CMSG_COMPAT in compat_sys

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Message-Id: <20150212.084124.1966135201655814270.davem@davemloft.net>
Date:	Thu, 12 Feb 2015 08:41:24 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	catalin.marinas@....com
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	will.deacon@....com, luto@...capital.net
Subject: Re: [PATCH] net: compat: Ignore MSG_CMSG_COMPAT in
 compat_sys_{send,recv}msg

From: Catalin Marinas <catalin.marinas@....com>
Date: Thu, 12 Feb 2015 12:28:07 +0000

> With commit a7526eb5d06b (net: Unbreak compat_sys_{send,recv}msg), the
> MSG_CMSG_COMPAT flag is blocked at the compat syscall entry points,
> changing the kernel compat behaviour from the one before the commit it
> was trying to fix (1be374a0518a, net: Block MSG_CMSG_COMPAT in
> send(m)msg and recv(m)msg).
> 
> On 32-bit kernels (!CONFIG_COMPAT), MSG_CMSG_COMPAT is 0 and the native
> 32-bit sys_sendmsg() allows flag 0x80000000 to be set (it is ignored by
> the kernel). However, on a 64-bit kernel, the compat ABI is different
> with commit a7526eb5d06b.
> 
> This patch changes the compat_sys_{send,recv}msg behaviour to the one
> prior to commit 1be374a0518a.
> 
> The problem was found running 32-bit LTP (sendmsg01) binary on an arm64
> kernel. Arguably, LTP should not pass 0xffffffff as flags to sendmsg()
> but the general rule is not to break user ABI (even when the user
> behaviour is not entirely sane).
> 
> Fixes: a7526eb5d06b (net: Unbreak compat_sys_{send,recv}msg)
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: David S. Miller <davem@...emloft.net>
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>

I think this is a very poor LTP test.

Setting MSG_* bits that aren't supported by the protocol in any way
gives undefined semantics.  You may get an error, it may be silently
ignored, etc.

I'm not applying this, sorry.
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