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SubjectRe: writing to sysfs appears to hang

On Wed, 20 Nov 2002, Jens Axboe wrote:

> On Wed, Nov 20 2002, Paul Larson wrote:
> > On Tue, 2002-11-19 at 11:02, Jens Axboe wrote:
> > > This has been in the deadline-rbtree patches for some time (uses writes
> > > to sysfs, too).
> > >
> > > ===== fs/sysfs/inode.c 1.59 vs edited =====
> > > --- 1.59/fs/sysfs/inode.c Wed Oct 30 21:27:35 2002
> > > +++ edited/fs/sysfs/inode.c Fri Nov 8 14:33:59 2002
> > > @@ -243,7 +243,7 @@
> > > if (kobj && kobj->subsys)
> > > ops = kobj->subsys->sysfs_ops;
> > > if (!ops || !ops->store)
> > > - return 0;
> > > + return -EINVAL;
> > >
> > > page = (char *)__get_free_page(GFP_KERNEL);
> > > if (!page)
> >
> > No effect, the behaviour is still the same for me.
>
> strace the program then, what is happening? What I saw was a program
> seeing write() return 0, assuming it didn't write anything, rewrite the
> whole thing. Repeat.
>
> I bet this wouldn't happen if just sysfs didn't allow write open of a
> file that doesn't have any writeable bits. Smells like another sysfs
> bug. Pat?

Yes, it is. The attached patch (relative to current BK) should fix it. I'm
planning on sending Linus an update soon, and this is in it.


-pat


--- linux-2.5-virgin/fs/sysfs/inode.c Wed Nov 20 12:13:06 2002
+++ linux-2.5-kobject/fs/sysfs/inode.c Wed Nov 20 12:15:18 2002
@@ -23,6 +23,8 @@
* Please see Documentation/filesystems/sysfs.txt for more information.
*/

+#undef DEBUG
+
#include <linux/list.h>
#include <linux/init.h>
#include <linux/pagemap.h>
@@ -173,17 +175,11 @@
sysfs_read_file(struct file *file, char *buf, size_t count, loff_t *ppos)
{
struct attribute * attr = file->f_dentry->d_fsdata;
- struct sysfs_ops * ops = NULL;
- struct kobject * kobj;
+ struct sysfs_ops * ops = file->private_data;
+ struct kobject * kobj = file->f_dentry->d_parent->d_fsdata;
unsigned char *page;
ssize_t retval = 0;

- kobj = file->f_dentry->d_parent->d_fsdata;
- if (kobj && kobj->subsys)
- ops = kobj->subsys->sysfs_ops;
- if (!ops || !ops->show)
- return 0;
-
if (count > PAGE_SIZE)
count = PAGE_SIZE;

@@ -234,16 +230,11 @@
sysfs_write_file(struct file *file, const char *buf, size_t count, loff_t *ppos)
{
struct attribute * attr = file->f_dentry->d_fsdata;
- struct sysfs_ops * ops = NULL;
- struct kobject * kobj;
+ struct sysfs_ops * ops = file->private_data;
+ struct kobject * kobj = file->f_dentry->d_parent->d_fsdata;
ssize_t retval = 0;
char * page;

- kobj = file->f_dentry->d_parent->d_fsdata;
- if (kobj && kobj->subsys)
- ops = kobj->subsys->sysfs_ops;
- if (!ops || !ops->store)
- return -EINVAL;

page = (char *)__get_free_page(GFP_KERNEL);
if (!page)
@@ -275,21 +266,72 @@
return retval;
}

-static int sysfs_open_file(struct inode * inode, struct file * filp)
+static int check_perm(struct inode * inode, struct file * file)
{
- struct kobject * kobj;
+ struct kobject * kobj = kobject_get(file->f_dentry->d_parent->d_fsdata);
+ struct attribute * attr = file->f_dentry->d_fsdata;
+ struct sysfs_ops * ops = NULL;
int error = 0;

- kobj = filp->f_dentry->d_parent->d_fsdata;
- if ((kobj = kobject_get(kobj))) {
- struct attribute * attr = filp->f_dentry->d_fsdata;
- if (!attr)
- error = -EINVAL;
- } else
- error = -EINVAL;
+ if (!kobj || !attr)
+ goto Einval;
+
+ if (kobj->subsys)
+ ops = kobj->subsys->sysfs_ops;
+
+ /* No sysfs operations, either from having no subsystem,
+ * or the subsystem have no operations.
+ */
+ if (!ops)
+ goto Eaccess;
+
+ /* File needs write support.
+ * The inode's perms must say it's ok,
+ * and we must have a store method.
+ */
+ if (file->f_mode & FMODE_WRITE) {
+
+ if (!(inode->i_mode & S_IWUGO))
+ goto Eperm;
+ if (!ops->store)
+ goto Eaccess;
+
+ }
+
+ /* File needs read support.
+ * The inode's perms must say it's ok, and we there
+ * must be a show method for it.
+ */
+ if (file->f_mode & FMODE_READ) {
+ if (!(inode->i_mode & S_IRUGO))
+ goto Eperm;
+ if (!ops->show)
+ goto Eaccess;
+ }
+
+ /* No error? Great, store the ops in file->private_data
+ * for easy access in the read/write functions.
+ */
+ file->private_data = ops;
+ goto Done;
+
+ Einval:
+ error = -EINVAL;
+ goto Done;
+ Eaccess:
+ error = -EACCES;
+ goto Done;
+ Eperm:
+ error = -EPERM;
+ Done:
return error;
}

+static int sysfs_open_file(struct inode * inode, struct file * filp)
+{
+ return check_perm(inode,filp);
+}
+
static int sysfs_release(struct inode * inode, struct file * filp)
{
struct kobject * kobj = filp->f_dentry->d_parent->d_fsdata;
-
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