Re: [PATCH 1/2] debugfs: prevent access to possibly dead file_operations at file open

Paul E. McKenney
2015-12-01T18:02:42Z

On Tue, Dec 01, 2015 at 12:21:31AM +0100, Nicolai Stange wrote:
> Nothing prevents a dentry found by path lookup before a return of
> __debugfs_remove() to actually get opened after that return. Now, after
> the return of __debugfs_remove(), there are no guarantees whatsoever
> regarding the memory the corresponding inode's file_operations object
> had been kept in.
>
> Since __debugfs_remove() is seldomly invoked, usually from module exit
> handlers only, the race is hard to trigger and the impact is very low.
>
> A discussion of the problem outlined above as well as a suggested
> solution can be found in the (sub-)thread rooted at
>
> http://lkml.kernel.org/g/20130401203445.GA20862@ZenIV.linux.org.uk
> ("Yet another pipe related oops.")
>
> Basically, Greg KH suggests to introduce an intermediate fops and
> Al Viro points out that a pointer to the original ones may be stored in
> ->d_fsdata.
>
> Follow this line of reasoning:
> - Add SRCU as a reverse dependency of DEBUG_FS.
> - Introduce a srcu_struct object for the debugfs subsystem.
> - In debugfs_create_file(), store a pointer to the original
> file_operations object in ->d_fsdata.
> - In __debugfs_remove(), clear out that dentry->d_fsdata member again.
> debugfs_remove() and debugfs_remove_recursive() wait for a SRCU grace
> period before returning to their caller.
> - Introduce an intermediate file_operations object named
> "debugfs_proxy_file_operations". It's ->open() functions checks,
> under the protection of a SRCU read lock, whether the "original"
> file_operations pointed to by ->d_fsdata is still valid and if so,
> tries to acquire a reference on the owning module. On success, it sets
> the file object's ->f_op to the original file_operations and forwards
> the ongoing open() call to the original ->open().
> - For clarity, rename the former debugfs_file_operations to
> debugfs_noop_file_operations -- they are in no way canonical.
>
> The choice of SRCU over "normal" RCU is justified by the fact, that the
> former may also be used to protect ->i_private data from going away
> during the execution of a file's readers and writers which may (and do)
> sleep.
>
> Signed-off-by: Nicolai Stange <nicstange@gmail.com>

One question below. Other than that, looks good from an RCU perspective.

Thanx, Paul

> ---
> Applicable to the Linus tree.
> The second of the two patches depends on the first one
>
> fs/debugfs/file.c | 29 ++++++++++++++++++++++++++++-
> fs/debugfs/inode.c | 16 +++++++++++++---
> include/linux/debugfs.h | 6 +++++-
> lib/Kconfig.debug | 1 +
> 4 files changed, 47 insertions(+), 5 deletions(-)
>
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index d2ba12e..67df2c9 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -35,13 +35,40 @@ static ssize_t default_write_file(struct file *file, const char __user *buf,
> return count;
> }
>
> -const struct file_operations debugfs_file_operations = {
> +const struct file_operations debugfs_noop_file_operations = {
> .read = default_read_file,
> .write = default_write_file,
> .open = simple_open,
> .llseek = noop_llseek,
> };
>
> +static int proxy_open(struct inode *inode, struct file *filp)
> +{
> + struct dentry * const dentry = filp->f_path.dentry;
> + const struct file_operations __rcu *rcu_real_fops;
> + const struct file_operations *real_fops;
> + int srcu_idx;
> +
> + srcu_idx = srcu_read_lock(&debugfs_srcu);
> + rcu_real_fops = (const struct file_operations __rcu *)dentry->d_fsdata;
> + real_fops = srcu_dereference(rcu_real_fops, &debugfs_srcu);
> + real_fops = fops_get(real_fops);
> + srcu_read_unlock(&debugfs_srcu, srcu_idx);
> +
> + if (!real_fops)
> + return -ENOENT;
> +
> + replace_fops(filp, real_fops);
> + if (filp->f_op->open)
> + return filp->f_op->open(inode, filp);
> +
> + return 0;
> +}
> +
> +const struct file_operations debugfs_proxy_file_operations = {
> + .open = proxy_open,
> +};
> +
> static struct dentry *debugfs_create_mode(const char *name, umode_t mode,
> struct dentry *parent, void *value,
> const struct file_operations *fops,
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index b7fcc0d..8ae2e1a 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c
> @@ -30,6 +30,8 @@
>
> #define DEBUGFS_DEFAULT_MODE 0700
>
> +DEFINE_SRCU(debugfs_srcu);
> +
> static struct vfsmount *debugfs_mount;
> static int debugfs_mount_count;
> static bool debugfs_registered;
> @@ -340,8 +342,12 @@ struct dentry *debugfs_create_file(const char *name, umode_t mode,
> return failed_creating(dentry);
>
> inode->i_mode = mode;
> - inode->i_fop = fops ? fops : &debugfs_file_operations;
> inode->i_private = data;
> +
> + inode->i_fop = fops ? &debugfs_proxy_file_operations
> + : &debugfs_noop_file_operations;
> + rcu_assign_pointer(dentry->d_fsdata, (void *)fops);
> +
> d_instantiate(dentry, inode);
> fsnotify_create(d_inode(dentry->d_parent), dentry);
> return end_creating(dentry);
> @@ -523,10 +529,12 @@ static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
>
> if (simple_positive(dentry)) {
> dget(dentry);
> - if (d_is_dir(dentry))
> + if (d_is_dir(dentry)) {
> ret = simple_rmdir(d_inode(parent), dentry);
> - else
> + } else {
> simple_unlink(d_inode(parent), dentry);
> + rcu_assign_pointer(dentry->d_fsdata, NULL);
> + }
> if (!ret)
> d_delete(dentry);
> dput(dentry);
> @@ -565,6 +573,7 @@ void debugfs_remove(struct dentry *dentry)
> mutex_unlock(&d_inode(parent)->i_mutex);
> if (!ret)
> simple_release_fs(&debugfs_mount, &debugfs_mount_count);
> + synchronize_srcu(&debugfs_srcu);

So the reason that this is OK from a performance perspective is that
people removing large quantities of debugfs entries are supposed to
use debugfs_remove_recursive()?

If there are valid use cases with lots of debugfs_remove() calls
in quick succession, and if this results in excessive latency due
to lots of synchronize_srcu() calls, then one approach would be to
have a debugfs_remove_nowait() or some such that omitted the final
synchronize_srcu(). The last call could use debugfs_remove() to clean
things up. There are of course a number of alternative approaches.

But if there is no performance issue with real workloads, then there
is of course no need to do anything.

> }
> EXPORT_SYMBOL_GPL(debugfs_remove);
>
> @@ -642,6 +651,7 @@ void debugfs_remove_recursive(struct dentry *dentry)
> if (!__debugfs_remove(child, parent))
> simple_release_fs(&debugfs_mount, &debugfs_mount_count);
> mutex_unlock(&d_inode(parent)->i_mutex);
> + synchronize_srcu(&debugfs_srcu);
> }
> EXPORT_SYMBOL_GPL(debugfs_remove_recursive);
>
> diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> index 19c066d..f8c7494 100644
> --- a/include/linux/debugfs.h
> +++ b/include/linux/debugfs.h
> @@ -19,6 +19,7 @@
> #include <linux/seq_file.h>
>
> #include <linux/types.h>
> +#include <linux/srcu.h>
>
> struct device;
> struct file_operations;
> @@ -44,7 +45,10 @@ extern struct dentry *arch_debugfs_dir;
> #if defined(CONFIG_DEBUG_FS)
>
> /* declared over in file.c */
> -extern const struct file_operations debugfs_file_operations;
> +extern const struct file_operations debugfs_noop_file_operations;
> +extern const struct file_operations debugfs_proxy_file_operations;
> +
> +extern struct srcu_struct debugfs_srcu;
>
> struct dentry *debugfs_create_file(const char *name, umode_t mode,
> struct dentry *parent, void *data,
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 8c15b29..3929878 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -257,6 +257,7 @@ config PAGE_OWNER
>
> config DEBUG_FS
> bool "Debug Filesystem"
> + select SRCU
> help
> debugfs is a virtual file system that kernel developers use to put
> debugging files into. Enable this option to be able to read and
> --
> 2.6.3
>