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Subjectsys_paccept: disable paccept() until API design is resolved
From
Andrew,

The patch below disables the new sys_paccept() for now. Please
apply for 2.6.27-rc, so that we do not release this API into
the wild before a conclusion has been reached about its design.

The reasons for disabling paccept() are as follows:

* The API is more complex than needed. There is AFAICS no demonstrated
use case that the sigset argument of this syscall serves that
couldn't equally be served by the use of pselect/ppoll/epoll_pwait +
traditional accept(). Roland seems to concur with this opinion
(http://thread.gmane.org/gmane.linux.kernel/723953/focus=732255).
I have (more than once) asked Ulrich to explain otherwise
(http://thread.gmane.org/gmane.linux.kernel/723952/focus=731018),
but he does not respond, so one is left to assume that he doesn't
know of such a case.

* The use of a sigset argument is not consistent with other I/O APIs
that can block on a single file descriptor (e.g., read(), recv(),
connect()).

* The behavior of paccept() when interrupted by a signal is IMO
strange: the kernel restarts the system call if SA_RESTART was set
for the handler. I think that it should not do this -- that it
should behave consistently with paccept()/ppoll()/epoll_pwait(),
which never restart, regardless of SA_RESTART. The reasoning here
is that the very purpose of paccept() is to wait for a connection
or a signal, and that restarting in the latter case is probably
never useful. (Note: Roland disagrees on this point, believing
that rather paccept() should be consistent with accept() in its
behavior wrt EINTR
(http://thread.gmane.org/gmane.linux.kernel/723953/focus=732255).)

I believe that instead, a simpler API, consistent with Ulrich's
other recent additions, is preferable:

accept4(int fd, struct sockaddr *sa, socklen_t *salen, ind flags);

(This simpler API was originally proposed by Ulrich:
http://thread.gmane.org/gmane.linux.network/92072)

If this simpler API is added, then if we later decide that the sigset
argument really is required, then a suitable bit in 'flags' could
be added to indicate the presence of the sigset argument.

At this point, I am hoping we either will get a counter-argument
from Ulrich about why we really do need paccept()'s sigset argument,
or that he will resubmit the original accept4() patch.

Cheers,

Michael

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>


--- linux-2.6.27-rc6/net/socket.c.orig 2008-09-16 12:38:15.000000000 +0200
+++ linux-2.6.27-rc6/net/socket.c 2008-09-16 13:07:51.000000000 +0200
@@ -1511,6 +1511,7 @@
goto out_put;
}

+#if 0
#ifdef HAVE_SET_RESTORE_SIGMASK
asmlinkage long sys_paccept(int fd, struct sockaddr __user *upeer_sockaddr,
int __user *upeer_addrlen,
@@ -1564,6 +1565,7 @@
return do_accept(fd, upeer_sockaddr, upeer_addrlen, flags);
}
#endif
+#endif

asmlinkage long sys_accept(int fd, struct sockaddr __user *upeer_sockaddr,
int __user *upeer_addrlen)

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