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    SubjectRe: [PATCH v4 RESEND net-next] net: socket: use BIT() for MSG_*
    On Wed, Mar 17, 2021 at 10:21 AM Menglong Dong <menglong8.dong@gmail.com> wrote:
    > On Wed, Mar 17, 2021 at 9:38 AM Guenter Roeck <linux@roeck-us.net> wrote:
    > > On Wed, Mar 17, 2021 at 01:02:51AM +0200, Andy Shevchenko wrote:
    > > > On Wednesday, March 17, 2021, Guenter Roeck <linux@roeck-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

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