On Fri, Feb 09, 2007 at 01:00:37AM -0800, Andrew Morton wrote:
> On Thu, 8 Feb 2007 16:20:12 +0300 Alexey Dobriyan <[EMAIL PROTECTED]> wrote:
> > +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.
> > + */
> > + spin_lock(&de->pde_unload_lock);
> > + de->proc_fops = NULL;
> > + /* Wait until all readers/writers are done. */
> > + if (de->pde_users > 0) {
> > + spin_unlock(&de->pde_unload_lock);
> > + spin_unlock(&proc_subdir_lock);
> > + schedule();
> > + goto again;
> > + }
> > + spin_unlock(&de->pde_unload_lock);
>
> aergh. This will devolve into busy-wait-until-we-expire-our-timeslice.
>
> Would be nicer to do this with a wait_for_completion().
>
> I guess it doesn't happen very often - if another process happens to
> be in the middle or a read or write syscall to that /proc file.
Yes, that's rare. OK, I read LDD3 text on completions, hope I got it
right.
Does it pass everyone's bullshit detectors?
[PATCH v4] Fix rmmod/read/write races in /proc entries
Differences from version 3:
Use completion instead of unlock/schedule/lock
Move refcount waiting business after removing PDE from lists,
so that *cough* possible concurrent remove_proc_entry() will
work.
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 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 except that users dynamically creating ->proc_fops need to
be careful to not get leak. Patch fixes races of type 1.
Patch survives infinite modprobe/rmmod loop in parallel with infinite
read loops with many debugging options turned on including lockdep
(albeit on UP-PREEMPT box).
Signed-off-by: Alexey Dobriyan <[EMAIL PROTECTED]>
---
fs/proc/generic.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++--
include/linux/proc_fs.h | 19 ++++++++++
2 files changed, 100 insertions(+), 2 deletions(-)
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -20,6 +20,7 @@ #include <linux/idr.h>
#include <linux/namei.h>
#include <linux/bitops.h>
#include <linux/spinlock.h>
+#include <linux/completion.h>
#include <asm/uaccess.h>
#include "internal.h"
@@ -76,6 +77,21 @@ proc_file_read(struct file *file, char _
if (!(page = (char*) __get_free_page(GFP_KERNEL)))
return -ENOMEM;
+ spin_lock(&dp->pde_unload_lock);
+ if (!dp->proc_fops)
+ /*
+ * remove_proc_entry() marked PDE as "going away".
+ * No new readers allowed.
+ */
+ goto out_unlock;
+ /*
+ * 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.
+ */
+ dp->pde_users++;
+ spin_unlock(&dp->pde_unload_lock);
+
while ((nbytes > 0) && !eof) {
count = min_t(size_t, PROC_BLOCK_SIZE, nbytes);
@@ -195,6 +211,13 @@ proc_file_read(struct file *file, char _
buf += n;
retval += n;
}
+
+ spin_lock(&dp->pde_unload_lock);
+ dp->pde_users--;
+ if (dp->pde_unload_completion && dp->pde_users == 0)
+ complete(dp->pde_unload_completion);
+out_unlock:
+ spin_unlock(&dp->pde_unload_lock);
free_page((unsigned long) page);
return retval;
}
@@ -205,14 +228,41 @@ proc_file_write(struct file *file, const
{
struct inode *inode = file->f_path.dentry->d_inode;
struct proc_dir_entry * dp;
+ ssize_t rv = -EIO;
dp = PDE(inode);
if (!dp->write_proc)
- return -EIO;
+ goto out;
+
+ spin_lock(&dp->pde_unload_lock);
+ if (!dp->proc_fops)
+ /*
+ * remove_proc_entry() marked PDE as "going away".
+ * No new writers allowed.
+ */
+ goto out_unlock;
+ /*
+ * 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.
+ */
+ dp->pde_users++;
+ spin_unlock(&dp->pde_unload_lock);
+ /* PDE is ready, refcount bumped, call into module. */
/* FIXME: does this routine need ppos? probably... */
- return dp->write_proc(file, buffer, count, dp->data);
+ rv = dp->write_proc(file, buffer, count, dp->data);
+
+ spin_lock(&dp->pde_unload_lock);
+ dp->pde_users--;
+ if (dp->pde_unload_completion && dp->pde_users == 0)
+ complete(dp->pde_unload_completion);
+out_unlock:
+ spin_unlock(&dp->pde_unload_lock);
+out:
+ return rv;
}
@@ -604,6 +654,9 @@ static struct proc_dir_entry *proc_creat
ent->namelen = len;
ent->mode = mode;
ent->nlink = nlink;
+ ent->pde_users = 0;
+ spin_lock_init(&ent->pde_unload_lock);
+ ent->pde_unload_completion = NULL;
out:
return ent;
}
@@ -725,6 +778,32 @@ void remove_proc_entry(const char *name,
de = *p;
*p = de->next;
de->next = NULL;
+
+ spin_lock(&de->pde_unload_lock);
+ /*
+ * Stop accepting new readers/writers. If you're dynamically
+ * allocating ->proc_fops, save a pointer somewhere.
+ */
+ de->proc_fops = NULL;
+ /* Wait until all existing readers/writers are done. */
+ if (de->pde_users > 0) {
+ struct completion c;
+
+ init_completion(&c);
+ if (!de->pde_unload_completion)
+ de->pde_unload_completion = &c;
+
+ spin_unlock(&de->pde_unload_lock);
+ spin_unlock(&proc_subdir_lock);
+
+ wait_for_completion(de->pde_unload_completion);
+
+ spin_lock(&proc_subdir_lock);
+ goto continue_removing;
+ }
+ spin_unlock(&de->pde_unload_lock);
+
+continue_removing:
if (S_ISDIR(de->mode))
parent->nlink--;
proc_kill_inodes(de);
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -7,6 +7,8 @@ #include <linux/spinlock.h>
#include <linux/magic.h>
#include <asm/atomic.h>
+struct completion;
+
/*
* The proc filesystem constants/structures
*/
@@ -56,6 +58,19 @@ 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
+ * can 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 +81,10 @@ struct proc_dir_entry {
atomic_t count; /* use count */
int deleted; /* delete flag */
void *set;
+ int pde_users; /* number of readers + number of writers via
+ * ->read_proc, ->write_proc, ->get_info */
+ spinlock_t pde_unload_lock;
+ struct completion *pde_unload_completion;
};
struct kcore_list {
-
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