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Date: Wed, 24 Jan 2007 18:22:51 +0300
From: Alexey Dobriyan <adobriyan@...nvz.org>
To: Andrew Morton <akpm@...l.org>
Cc: Al Viro <viro@....linux.org.uk>, linux-kernel@...r.kernel.org,
devel@...nvz.org
Subject: Re: [PATCH] Fix rmmod/read/write races in /proc entries
On Tue, Jan 23, 2007 at 12:58:01PM -0800, Andrew Morton wrote:
> <head spins>
>
> Looks a bit hacky. Can this race not be fixed by addition of suitable
> locking, or possibly refcounting-under-locking?
I'll think about it.
> > @@ -76,6 +77,12 @@ proc_file_read(struct file *file, char _
> > if (!(page = (char*) __get_free_page(GFP_KERNEL)))
> > return -ENOMEM;
> >
> > + if (!dp->proc_fops)
> > + goto out_free;
> > + atomic_inc(&dp->pde_users);
> > + if (!dp->proc_fops)
> > + goto out_dec;
> > +
>
> You'll be shocked to know that I'd prefer more comments in there. Enough
> for a later maintainer to be able to understand what's going on.
Here is replacement patch with rewritten changelog and comments in
place. HTH.
[PATCH] Fix rmmod/read/write races in /proc entries
Current /proc creation interfaces suffer from at least two types of races:
--------------------------------------------------------
1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
meanwhile.
pde = create_proc_entry()
if (!pde)
return -ENOMEM;
pde->write_proc = ...
open
write
copy_from_user
pde = create_proc_entry();
if (!pde) {
remove_proc_entry();
return -ENOMEM;
/* module unloaded */
}
*boom*
--------------------------------------------------------
2. Read/write happens when PDE only partially initialized. ->data is NULL
when create_proc_entry() returns. Almost all ->read_proc and
->write_proc handlers assume that ->data is valid.
pde = create_proc_entry();
if (pde) {
/* which dereferences ->data */
pde->write_proc = ...
open
write
pde->data = ...
}
--------------------------------------------------------
The following plan is going to be executed (as per Al Viro's explanations):
PDE gets atomic counter counting reads and writes in progress done
via ->read_proc, ->write_proc, ->get_info . Generic proc code will bump
PDE's counter before calling into module-specific method and decrement
it after it returns.
remove_proc_entry() will wait until all readers and writers are done.
To do this reliably it will set ->proc_fops to NULL and generic proc
code won't call into module it it sees NULL ->proc_fops.
This patch implements part above. So far, no changes in proc users
required. Patch fixes races of type 1.
Unfortunately, fixing races of type #2 will require changing in some modules.
We need an indicator of PDE readinness of accepting reads and writes.
->proc_fops nicely fits. It is going to get new semantics:
* if ->proc_fops is valid, PDE will accept reads and writes via ->read_proc,
->write_proc, ->get_info.
* if ->proc_fops is NULL, PDE won't call into module's code.
remove_proc_entry() and only remove_proc_entry() will clear ->proc_fops.
create_proc_entry() will not set ->proc_fops. Helpers will be required.
Helpers will set ->proc_fops last (after ->data, particularly).
set_proc_entry_data_fops(pde, data, fops);
set_proc_entry_data_read_write(pde, data, read_proc, write_proc);
When all necessary helpers will be plugged, create_proc_entry will stop
setting default proc_fops and helpers will start setting it. Races of
type #2 will be fixed.
If module sets ->proc_fops only, or uses create_proc_read_entry(), or uses
create_proc_info_entry(), there won't be any changes for module.
---
fs/proc/generic.c | 61 +++++++++++++++++++++++++++++++++++++++++++++---
include/linux/proc_fs.h | 15 +++++++++++
2 files changed, 73 insertions(+), 3 deletions(-)
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -19,6 +19,7 @@ #include <linux/init.h>
#include <linux/idr.h>
#include <linux/namei.h>
#include <linux/bitops.h>
+#include <linux/delay.h>
#include <linux/spinlock.h>
#include <asm/uaccess.h>
@@ -76,6 +77,25 @@ proc_file_read(struct file *file, char _
if (!(page = (char*) __get_free_page(GFP_KERNEL)))
return -ENOMEM;
+ if (!dp->proc_fops)
+ /*
+ * remove_proc_entry() marked PDE as "going away".
+ * No new readers allowed.
+ */
+ goto out_free;
+ /*
+ * We are going to call into module's code via ->get_info or
+ * ->read_proc. Bump refcount so that remove_proc_entry() will
+ * wait for read to complete.
+ */
+ atomic_inc(&dp->pde_users);
+ if (!dp->proc_fops)
+ /*
+ * While we're busy bumping refcount, remove_proc_entry()
+ * marked PDE as "going away". Obey.
+ */
+ goto out_dec;
+
while ((nbytes > 0) && !eof) {
count = min_t(size_t, PROC_BLOCK_SIZE, nbytes);
@@ -195,6 +215,9 @@ proc_file_read(struct file *file, char _
buf += n;
retval += n;
}
+out_dec:
+ atomic_dec(&dp->pde_users);
+out_free:
free_page((unsigned long) page);
return retval;
}
@@ -205,14 +228,33 @@ proc_file_write(struct file *file, const
{
struct inode *inode = file->f_path.dentry->d_inode;
struct proc_dir_entry * dp;
+ ssize_t rv;
dp = PDE(inode);
if (!dp->write_proc)
return -EIO;
+ /*
+ * remove_proc_entry() marked PDE as "going away".
+ * No new writers allowed.
+ */
+ if (!dp->proc_fops)
+ return -EIO;
- /* FIXME: does this routine need ppos? probably... */
- return dp->write_proc(file, buffer, count, dp->data);
+ rv = -EIO;
+ /*
+ * We are going to call into module's code via ->write_proc .
+ * Bump refcount so that module won't dissapear while ->write_proc
+ * sleeps in copy_from_user(). remove_proc_entry() will wait for
+ * write to complete.
+ */
+ atomic_inc(&dp->pde_users);
+ if (dp->proc_fops)
+ /* PDE is ready, refcount bumped, call into module. */
+ /* FIXME: does this routine need ppos? probably... */
+ rv = dp->write_proc(file, buffer, count, dp->data);
+ atomic_dec(&dp->pde_users);
+ return rv;
}
@@ -717,12 +759,25 @@ void remove_proc_entry(const char *name,
if (!parent && xlate_proc_name(name, &parent, &fn) != 0)
goto out;
len = strlen(fn);
-
+again:
spin_lock(&proc_subdir_lock);
for (p = &parent->subdir; *p; p=&(*p)->next ) {
if (!proc_match(len, fn, *p))
continue;
de = *p;
+
+ /*
+ * Stop accepting new readers/writers. If you're dynamically
+ * allocating ->proc_fops, save a pointer somewhere.
+ */
+ de->proc_fops = NULL;
+ /* Wait until all readers/writers are done. */
+ if (atomic_read(&de->pde_users) > 0) {
+ spin_unlock(&proc_subdir_lock);
+ msleep(1);
+ goto again;
+ }
+
*p = de->next;
de->next = NULL;
if (S_ISDIR(de->mode))
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -56,6 +56,19 @@ struct proc_dir_entry {
gid_t gid;
loff_t size;
struct inode_operations * proc_iops;
+ /*
+ * NULL ->proc_fops means "PDE is going away RSN" or
+ * "PDE is just created". In either case ->get_info, ->read_proc,
+ * ->write_proc won't be called because it's too late or too early,
+ * respectively.
+ *
+ * Valid ->proc_fops means "use this file_operations" or
+ * "->data is setup, it's safe to call ->read_proc, ->write_proc which
+ * dereference it".
+ *
+ * If you're allocating ->proc_fops dynamically, save a pointer
+ * somewhere.
+ */
const struct file_operations * proc_fops;
get_info_t *get_info;
struct module *owner;
@@ -66,6 +79,8 @@ struct proc_dir_entry {
atomic_t count; /* use count */
int deleted; /* delete flag */
void *set;
+ atomic_t pde_users; /* number of readers + number of writers via
+ * ->read_proc, ->write_proc, ->get_info */
};
struct kcore_list {
-
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