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

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Date:   Wed, 17 Mar 2021 11:36:02 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Jakub Kicinski <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_*

On Wed, Mar 17, 2021 at 10:21 AM Menglong Dong <menglong8.dong@...il.com> wrote:
> On Wed, Mar 17, 2021 at 9:38 AM Guenter Roeck <linux@...ck-us.net> wrote:
> > On Wed, Mar 17, 2021 at 01:02:51AM +0200, Andy Shevchenko wrote:
> > > On Wednesday, March 17, 2021, Guenter Roeck <linux@...ck-us.net> wrote:
> > >
> ...
> >
> > The problem is in net/packet/af_packet.c:packet_recvmsg(). This function,
> > as well as all other rcvmsg functions, is declared as
> >
> > static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
> >                           int flags)
> >
> > 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.
> >
> > As it turns out, I was kind of lucky to catch the problem: So far I have
> > seen it only on mips64 kernels with N32 userspace.
> >
> > Guenter
>
>  Wow, now the usages of 'msg_flag' really puzzle me. Seems that
> it is used as 'unsigned int' somewhere, but 'int' somewhere
> else.
>
> As I found, It is used as 'int' in 'netlink_recvmsg()',
> 'io_sr_msg->msg_flags', 'atalk_sendmsg()',
> 'dn_recvmsg()',  'proto_ops->recvmsg()', etc.
>
> So what should I do? Revert this patch? Or fix the usages of 'flags'?
> Or change the type of MSG_* to 'unsigned int'? I prefer the last
> one(the usages of 'flags' can be fixed too, maybe later).

The problematic code is negation of the flags when it's done in
operations like &.
It maybe fixed by swapping positions of the arguments, i.e. ~(FOO |
BAR) & flags.

All this is a beast called "integer promotions" in the C standard.

The best is to try to get flags to be unsigned. By how invasive it may be?

-- 
With Best Regards,
Andy Shevchenko