linux-kernel - RE: [PATCH v4 RESEND net-next] net: socket: use BIT() for MSG

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Date:   Wed, 17 Mar 2021 15:12:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Guenter Roeck' <linux@...ck-us.net>,
        Andy Shevchenko <andy.shevchenko@...il.com>
CC:     "menglong8.dong@...il.com" <menglong8.dong@...il.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "dong.menglong@....com.cn" <dong.menglong@....com.cn>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH v4 RESEND net-next] net: socket: use BIT() for MSG_*

From: Guenter Roeck
> Sent: 17 March 2021 01:38
...
> MSG_CMSG_COMPAT (0x80000000) is set in flags, meaning its value is negative.
> This is then evaluated in
> 
>        if (flags & ~(MSG_PEEK|MSG_DONTWAIT|MSG_TRUNC|MSG_CMSG_COMPAT|MSG_ERRQUEUE))
>                 goto out;
> 
> If any of those flags is declared as BIT() and thus long, flags is
> sign-extended to long. Since it is negative, its upper 32 bits will be set,
> the if statement evaluates as true, and the function bails out.
> 
> This is relatively easy to fix here with, for example,
> 
>         if ((unsigned int)flags & ~(MSG_PEEK|MSG_DONTWAIT|MSG_TRUNC|MSG_CMSG_COMPAT|MSG_ERRQUEUE))
>                 goto out;
> 
> but that is just a hack, and it doesn't solve the real problem:
> Each function in struct proto_ops which passes flags passes it as int
> (see include/linux/net.h:struct proto_ops). Each such function, if
> called with MSG_CMSG_COMPAT set, will fail a match against
> ~(MSG_anything) if MSG_anything is declared as BIT() or long.

Isn't MSG_CMSG_COMPAT an internal value?
Could it be changed to 1u << 30 instead of 1u << 31 ?
Then it wouldn't matter if the high bit of flags got replicated.

	David

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