Re: [PATCH] net: compat: Ignore MSG_CMSG_COMPAT in compat_sys_{send,recv}msg

[David Miller <davem@davemloft.net>]

From: Catalin Marinas <catalin.marinas@arm.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@amacapital.net>
> Cc: David S. Miller <davem@davemloft.net>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.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.