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Message-ID: <20150121085900.GA25434@ad.nay.redhat.com>
Date: Wed, 21 Jan 2015 16:59:00 +0800
From: Fam Zheng <famz@...hat.com>
To: Omar Sandoval <osandov@...ndov.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>,
Andy Lutomirski <luto@...capital.net>,
David Herrmann <dh.herrmann@...il.com>,
Alexei Starovoitov <ast@...mgrid.com>,
Miklos Szeredi <mszeredi@...e.cz>,
David Drysdale <drysdale@...gle.com>,
Oleg Nesterov <oleg@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Vivek Goyal <vgoyal@...hat.com>,
Mike Frysinger <vapier@...too.org>,
"Theodore Ts'o" <tytso@....edu>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Rashika Kheria <rashika.kheria@...il.com>,
Hugh Dickins <hughd@...gle.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Peter Zijlstra <peterz@...radead.org>,
linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH RFC 5/6] epoll: Add implementation for epoll_mod_wait
On Tue, 01/20 23:56, Omar Sandoval wrote:
> On Tue, Jan 20, 2015 at 05:57:57PM +0800, Fam Zheng wrote:
> > This syscall is a sequence of
> >
> > 1) a number of epoll_ctl calls
> > 2) a epoll_pwait, with timeout enhancement.
> >
> > The epoll_ctl operations are embeded so that application doesn't have to use
> > separate syscalls to insert/delete/update the fds before poll. It is more
> > efficient if the set of fds varies from one poll to another, which is the
> > common pattern for certain applications. For example, depending on the input
> > buffer status, a data reading program may decide to temporarily not polling an
> > fd.
> >
> > Because the enablement of batching in this interface, even that regular
> > epoll_ctl call sequence, which manipulates several fds, can be optimized to one
> > single epoll_ctl_wait (while specifying spec=NULL to skip the poll part).
> >
> > The only complexity is returning the result of each operation. For each
> > epoll_mod_cmd in cmds, the field "error" is an output field that will be stored
> > the return code *iff* the command is executed (0 for success and -errno of the
> > equivalent epoll_ctl call), and will be left unchanged if the command is not
> > executed because some earlier error, for example due to failure of
> > copy_from_user to copy the array.
> >
> > Applications can utilize this fact to do error handling: they could initialize
> > all the epoll_mod_wait.error to a positive value, which is by definition not a
> > possible output value from epoll_mod_wait. Then when the syscall returned, they
> > know whether or not the command is executed by comparing each error with the
> > init value, if they're different, they have the result of the command.
> > More roughly, they can put any non-zero and not distinguish "not run" from
> > failure.
> >
> > Also, timeout parameter is enhanced: timespec is used, compared to the old ms
> > scalar. This provides higher precision. The parameter field in struct
> > epoll_wait_spec, "clockid", also makes it possible for users to use a different
> > clock than the default when it makes more sense.
> >
> > Signed-off-by: Fam Zheng <famz@...hat.com>
> > ---
> > fs/eventpoll.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++
> > include/linux/syscalls.h | 5 ++++
> > 2 files changed, 65 insertions(+)
> >
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index e7a116d..2cc22c9 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -2067,6 +2067,66 @@ SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
> > sigmask ? &ksigmask : NULL);
> > }
> >
> > +SYSCALL_DEFINE5(epoll_mod_wait, int, epfd, int, flags,
> > + int, ncmds, struct epoll_mod_cmd __user *, cmds,
> > + struct epoll_wait_spec __user *, spec)
> > +{
> > + struct epoll_mod_cmd *kcmds = NULL;
> > + int i, ret = 0;
> > + int cmd_size = sizeof(struct epoll_mod_cmd) * ncmds;
> > +
> > + if (flags)
> > + return -EINVAL;
> > + if (ncmds) {
> > + if (!cmds)
> > + return -EINVAL;
> > + kcmds = kmalloc(cmd_size, GFP_KERNEL);
> > + if (!kcmds)
> > + return -ENOMEM;
> > + if (copy_from_user(kcmds, cmds, cmd_size)) {
> > + ret = -EFAULT;
> > + goto out;
> > + }
> > + }
> > + for (i = 0; i < ncmds; i++) {
> > + struct epoll_event ev = (struct epoll_event) {
> > + .events = kcmds[i].events,
> > + .data = kcmds[i].data,
> > + };
> > + if (kcmds[i].flags) {
> > + kcmds[i].error = ret = -EINVAL;
> > + goto out;
> > + }
> > + kcmds[i].error = ret = ep_ctl_do(epfd, kcmds[i].op, kcmds[i].fd, ev);
> > + if (ret)
> > + goto out;
> > + }
> > + if (spec) {
> > + sigset_t ksigmask;
> > + struct epoll_wait_spec kspec;
> > + ktime_t timeout;
> > +
> > + if(copy_from_user(&kspec, spec, sizeof(struct epoll_wait_spec)))
> > + return -EFAULT;
> This should probably be goto out, or you'll leak kcmds.
>
> > + if (kspec.sigmask) {
> > + if (kspec.sigsetsize != sizeof(sigset_t))
> > + return -EINVAL;
> Same here...
>
> > + if (copy_from_user(&ksigmask, kspec.sigmask, sizeof(ksigmask)))
> > + return -EFAULT;
> and here.
>
> > + }
> > + timeout = timespec_to_ktime(kspec.timeout);
> > + ret = epoll_pwait_do(epfd, kspec.events, kspec.maxevents,
> > + kspec.clockid, timeout,
> > + kspec.sigmask ? &ksigmask : NULL);
> > + }
> > +
> > +out:
> > + if (ncmds && copy_to_user(cmds, kcmds, cmd_size))
> > + return -EFAULT;
> This will also leak kcmds, it should be ret = -EFAULT. This case, however, seems
> to lead to a weird corner case: if cmds is read-only, we'll end up executing
> every command but fail to copy out the return values, so when userspace gets the
> EFAULT, it won't know whether anything was executed. But, getting an EFAULT here
> means you're probably doing something wrong anyways, so maybe not the biggest
> concern.
Yes, thanks! Will fix this.
Fam
>
> > + kfree(kcmds);
> > + return ret;
> > +}
> > +
> > #ifdef CONFIG_COMPAT
> > COMPAT_SYSCALL_DEFINE6(epoll_pwait, int, epfd,
> > struct epoll_event __user *, events,
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index 85893d7..7156c80 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -12,6 +12,8 @@
> > #define _LINUX_SYSCALLS_H
> >
> > struct epoll_event;
> > +struct epoll_mod_cmd;
> > +struct epoll_wait_spec;
> > struct iattr;
> > struct inode;
> > struct iocb;
> > @@ -630,6 +632,9 @@ asmlinkage long sys_epoll_pwait(int epfd, struct epoll_event __user *events,
> > int maxevents, int timeout,
> > const sigset_t __user *sigmask,
> > size_t sigsetsize);
> > +asmlinkage long sys_epoll_mod_wait(int epfd, int flags,
> > + int ncmds, struct epoll_mod_cmd __user * cmds,
> > + struct epoll_wait_spec __user * spec);
> > asmlinkage long sys_gethostname(char __user *name, int len);
> > asmlinkage long sys_sethostname(char __user *name, int len);
> > asmlinkage long sys_setdomainname(char __user *name, int len);
> > --
> > 1.9.3
> >
> > --
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>
> --
> Omar
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