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Date: Wed, 28 Apr 2010 22:18:43 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: paulmck@...ux.vnet.ibm.com
Cc: Miles Lane <miles.lane@...il.com>, Vivek Goyal <vgoyal@...hat.com>,
Eric Paris <eparis@...hat.com>,
Lai Jiangshan <laijs@...fujitsu.com>,
Ingo Molnar <mingo@...e.hu>,
Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>, nauman@...gle.com,
netdev@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
Gui Jianfeng <guijianfeng@...fujitsu.com>,
Li Zefan <lizf@...fujitsu.com>,
Johannes Berg <johannes@...solutions.net>,
shemminger@...tta.com
Subject: Re: 2.6.34-rc5-git7 (plus all patches) -- another suspicious
rcu_dereference_check() usage.
Le mercredi 28 avril 2010 à 13:09 -0700, Paul E. McKenney a écrit :
> On Wed, Apr 28, 2010 at 09:38:11PM +0200, Eric Dumazet wrote:
> > Le mercredi 28 avril 2010 à 10:54 -0700, Paul E. McKenney a écrit :
> > > On Mon, Apr 26, 2010 at 08:51:06PM -0400, Miles Lane wrote:
> > > > This one occurred during the wakeup from suspend to RAM.
> > > >
> > > > [ 984.724697] [ INFO: suspicious rcu_dereference_check() usage. ]
> > > > [ 984.724700] ---------------------------------------------------
> > > > [ 984.724703] include/linux/fdtable.h:88 invoked
> > > > rcu_dereference_check() without protection!
> > > > [ 984.724706]
> > > > [ 984.724707] other info that might help us debug this:
> > > > [ 984.724708]
> > > > [ 984.724711]
> > > > [ 984.724711] rcu_scheduler_active = 1, debug_locks = 1
> > > > [ 984.724714] no locks held by dbus-daemon/4680.
> > > > [ 984.724717]
> > > > [ 984.724717] stack backtrace:
> > > > [ 984.724721] Pid: 4680, comm: dbus-daemon Not tainted 2.6.34-rc5-git7 #33
> > > > [ 984.724724] Call Trace:
> > > > [ 984.724734] [<ffffffff81074556>] lockdep_rcu_dereference+0x9d/0xa6
> > > > [ 984.724740] [<ffffffff810fc785>] fcheck_files+0xb1/0xc9
> > > > [ 984.724745] [<ffffffff810fc7f5>] fget_light+0x35/0xab
> > > > [ 984.724751] [<ffffffff81433e1b>] ? sock_poll_wait+0x13/0x18
> > > > [ 984.724755] [<ffffffff81433e39>] ? unix_poll+0x19/0x95
> > > > [ 984.724762] [<ffffffff8110aa95>] do_sys_poll+0x1ff/0x3e5
> > > > [ 984.724766] [<ffffffff8110a19e>] ? __pollwait+0x0/0xc7
> > > > [ 984.724771] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724776] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724780] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724784] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724788] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724793] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724797] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724802] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724806] [<ffffffff8110a265>] ? pollwake+0x0/0x4f
> > > > [ 984.724812] [<ffffffff8110ae0f>] sys_poll+0x50/0xbb
> > > > [ 984.724818] [<ffffffff81009d82>] system_call_fastpath+0x16/0x1b
> > >
> > > Hmmm... I am not convinced that this is a false positive. Couldn't
> > > there be a multi-threaded process where one thread is invoking poll()
> > > on a UNIX socket just as another thread is calling close() on it?
> > >
> > > The current fcheck_files() logic requires that the caller either (1) be in
> > > an RCU read-side critical section, (2) hold ->files_lock, or (3) passing
> > > in a files_struct with ->count equal to 1 (initialization or cleanup).
> > >
> > > So I don't feel comfortable just slapping an RCU read-side critical
> > > section around this one, at least not unless someone who understands
> > > the locking says that doing so is OK.
> > >
> > >
> >
> > Its a single threaded program.
> >
> > So fget_light() calls fcheck_files(files, fd); without rcu lock,
> > but some /proc/pid/fd/... user temporarly raised files->count just
> > before we perform the condition check.
>
> So I should add a single-threaded check. My first thought was to use
> current_is_single_threaded(), but the bit about scanning the full list
> of processes does give me pause. However, thread_group_empty() looks
> like a much lighter-weight alternative.
>
> I believe that it is possible for a pair of single-threaded processes
> to share a file descriptor, but that should not be a problem, as both
> of them would need to close it for it to go away.
>
> But what happens if someone does a clone() with CLONE_FILES, as some
> of the AIO stuff seems to do? Won't that allow one of the resulting
> processes to close the file for both of them, even though both are
> otherwise single-threaded? And the ->count seems to be the only
> distinction between these two cases.
>
> And AIO does CLONE_VM as well as CLONE_FILES, but that seems to mean that
> the check must scan the processes with current_is_single_threaded().
> Besides which, a user could invoke clone() with only CLONE_FILES
> specified, right?
>
> Or am I just confused here?
>
> Thanx, Paul
If a program is mono threaded, and doing a fget_light() syscall, it
cannot possibly do a clone() in // ;)
If we want to be picky, we could add a user provided condition, aka "we
are sure we are allowed to do this because we are the owner of the files
struct".
diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 6da962c..027f5e1 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -2694,7 +2694,7 @@ void __do_SAK(struct tty_struct *tty)
spin_lock(&p->files->file_lock);
fdt = files_fdtable(p->files);
for (i = 0; i < fdt->max_fds; i++) {
- filp = fcheck_files(p->files, i);
+ filp = fcheck_files(p->files, i, false);
if (!filp)
continue;
if (filp->f_op->read == tty_read &&
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 452d02f..dabf4d8 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -119,7 +119,7 @@ SYSCALL_DEFINE2(dup2, unsigned int, oldfd, unsigned int, newfd)
int retval = oldfd;
rcu_read_lock();
- if (!fcheck_files(files, oldfd))
+ if (!fcheck_files(files, oldfd, false))
retval = -EBADF;
rcu_read_unlock();
return retval;
diff --git a/fs/file_table.c b/fs/file_table.c
index 32d12b7..2865f72 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -274,7 +274,7 @@ struct file *fget(unsigned int fd)
struct files_struct *files = current->files;
rcu_read_lock();
- file = fcheck_files(files, fd);
+ file = fcheck_files(files, fd, false);
if (file) {
if (!atomic_long_inc_not_zero(&file->f_count)) {
/* File object ref couldn't be taken */
@@ -303,10 +303,10 @@ struct file *fget_light(unsigned int fd, int *fput_needed)
*fput_needed = 0;
if (likely((atomic_read(&files->count) == 1))) {
- file = fcheck_files(files, fd);
+ file = fcheck_files(files, fd, true);
} else {
rcu_read_lock();
- file = fcheck_files(files, fd);
+ file = fcheck_files(files, fd, false);
if (file) {
if (atomic_long_inc_not_zero(&file->f_count))
*fput_needed = 1;
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 8418fcc..0e89448 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1716,7 +1716,7 @@ static int proc_fd_info(struct inode *inode, struct path *path, char *info)
* hold ->file_lock.
*/
spin_lock(&files->file_lock);
- file = fcheck_files(files, fd);
+ file = fcheck_files(files, fd, false);
if (file) {
if (path) {
*path = file->f_path;
@@ -1755,7 +1755,7 @@ static int tid_fd_revalidate(struct dentry *dentry, struct nameidata *nd)
files = get_files_struct(task);
if (files) {
rcu_read_lock();
- if (fcheck_files(files, fd)) {
+ if (fcheck_files(files, fd, false)) {
rcu_read_unlock();
put_files_struct(files);
if (task_dumpable(task)) {
@@ -1813,7 +1813,7 @@ static struct dentry *proc_fd_instantiate(struct inode *dir,
* hold ->file_lock.
*/
spin_lock(&files->file_lock);
- file = fcheck_files(files, fd);
+ file = fcheck_files(files, fd, false);
if (!file)
goto out_unlock;
if (file->f_mode & FMODE_READ)
@@ -1899,7 +1899,7 @@ static int proc_readfd_common(struct file * filp, void * dirent,
char name[PROC_NUMBUF];
int len;
- if (!fcheck_files(files, fd))
+ if (!fcheck_files(files, fd, false))
continue;
rcu_read_unlock();
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 013dc52..76423ad 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -57,11 +57,12 @@ struct files_struct {
struct file * fd_array[NR_OPEN_DEFAULT];
};
-#define rcu_dereference_check_fdtable(files, fdtfd) \
+#define rcu_dereference_check_fdtable(files, fdtfd, cond) \
(rcu_dereference_check((fdtfd), \
rcu_read_lock_held() || \
lockdep_is_held(&(files)->file_lock) || \
- atomic_read(&(files)->count) == 1))
+ atomic_read(&(files)->count) == 1 || \
+ cond))
#define files_fdtable(files) \
(rcu_dereference_check_fdtable((files), (files)->fdt))
@@ -79,13 +80,13 @@ static inline void free_fdtable(struct fdtable *fdt)
call_rcu(&fdt->rcu, free_fdtable_rcu);
}
-static inline struct file * fcheck_files(struct files_struct *files, unsigned int fd)
+static inline struct file * fcheck_files(struct files_struct *files, unsigned int fd, bool cond)
{
struct file * file = NULL;
struct fdtable *fdt = files_fdtable(files);
if (fd < fdt->max_fds)
- file = rcu_dereference_check_fdtable(files, fdt->fd[fd]);
+ file = rcu_dereference_check_fdtable(files, fdt->fd[fd], cond);
return file;
}
--
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