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msgop.2: Add restriction on ENOSYS error 

When calling msgrcv() with the MSG_COPY flag, it will report
EINVAL error even we if have disabled CONFIG_CHECKPOINT_RESTORE.
ENOSYS will be reported only if we also specify the IPC_NOWAIT
flag.

[mtk: edited commit message]

Notes from mtk:

The relevant kernel code is this:

[[
#ifdef CONFIG_CHECKPOINT_RESTORE
...
#else
static inline struct msg_msg *prepare_copy(void __user *buf, size_t bufsz)
{
        return ERR_PTR(-ENOSYS);
}

...
static long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long
msgtyp, int msgflg,
               long (*msg_handler)(void __user *, struct msg_msg *, size_t))
{
...
        if (msgflg & MSG_COPY) {
                if ((msgflg & MSG_EXCEPT) || !(msgflg & IPC_NOWAIT))
                        return -EINVAL;
                copy = prepare_copy(buf, min_t(size_t, bufsz, ns->msg_ctlmax));
...
}
]]

We'll only hit the ENOSYS error if:
(1) MSG_COPY was specified;
(2) IPC_NOWAIT was not specified; and
(3) CONFIG_CHECKPOINT_RESTORE was not enabled.

Signed-off-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Yang Xu 2020-09-09 11:57:42 +08:00 committed by Michael Kerrisk
parent c3e8ceb883
commit ab365f43e7
1 changed files with 3 additions and 1 deletions
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4
man2/msgop.2
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@ -467,7 +467,9 @@ and the queue contains less than
messages.
.TP
.BR ENOSYS " (since Linux 3.8)"
.I MSG_COPY
.B IPC_NOWAIT
and
.B MSG_COPY
was specified in
.IR msgflg ,
and this kernel was configured without