Re: [PATCH 4/7] ovl: add infrastructure for intercepting file ops

From: Amir Goldstein
Date: Thu Nov 24 2016 - 06:53:38 EST

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On Thu, Nov 24, 2016 at 12:55 PM, Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
> Overlayfs needs to intercept a few file operations in order to properly
> handle the following corner case:
>
> - file X is in overlayfs;
>
> - X resides on a lower (read-only) layer
> the lower file is L;
>
> - X is opened with O_RDONLY ->
> L is opened -> 'rofd';
>
> - X is opened with O_WRONLY ->
> this results in L being copied up to the upper layer file U
> U is opened -> 'rwfd';
>
> - write to 'rwfd' modifying U;
>
> - read from 'rofd' gets data from unmodified L;
>
> While this is a rare occurrence, it has been known to cause problems. To
> prevent such inconsistencies, allow intercepting read operations and fix
> them up in the unlikely case that it's needed.
>
> This patch adds an ovl_fops structure that is going to contain the pieces
> necessary for intercepting file operations:
>
> a) a new file_operations structure, based on the original but changing
> those operations that need to be intercepted;
>
> b) a pointer to the origin f_op. Intercepted operations will normally
> just call the original, unless the file was copied up;
>
> c) a hash table entry to hook this up for reuse in subsequent opens.
>
> The hash table is small (32 buckets) since there will only be a few
> different file operations used (basically the number of different
> filesystems used as lower layer of overlay). The key to the has table is
> the original fops pointer.
>
> Insertion is done under a global mutex. There will only be one insertion
> event for each unique underlying file_operations structure, so this is
> going to be rare.
>
> The hash table is accessed using rcu, so this will add minimal overhead to
> opens. The override fops are removed only at module exit time, so we don't
> even have to be worry about read-side rcu locking.
>
> There are a few assumption this scheme makes about file operation
> structures supplied by filesystems:
>
> 1) the lifetime of the structures is equal to the lifetime of the
> filesystem module; ensure this by holding a ref to the
> sb->s_type->owner (normal filesystems don't fill in f_ops->owner).
> This means that once a filesystem has been used as lower layer of
> overlayfs its module cannot be removed until the overlay module itself
> is removed. I don't believe this will be a problem.
>
> 2) Filesystems should not make use of f_op in any way except possibly
> replacing it in ->open. Overlayfs itself breaks this assumption, but
> recursion is handled by ->d_real so this is not an issue. Add a
> ->magic field to the override fops structure to verify that the
> filesystem didn't mess with file->f_op.
>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
> fs/overlayfs/inode.c | 144 ++++++++++++++++++++++++++++++++++++++++++++++-
> fs/overlayfs/overlayfs.h | 2 +
> fs/overlayfs/super.c | 1 +
> 3 files changed, 146 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 1981a5514f51..3e26615c4697 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -12,6 +12,7 @@
> #include <linux/xattr.h>
> #include <linux/posix_acl.h>
> #include <linux/module.h>
> +#include <linux/hashtable.h>
> #include "overlayfs.h"
>
> static int ovl_copy_up_truncate(struct dentry *dentry)
> @@ -334,16 +335,157 @@ static const struct inode_operations ovl_symlink_inode_operations = {
> .update_time = ovl_update_time,
> };
>
> +static DEFINE_READ_MOSTLY_HASHTABLE(ovl_fops_htable, 5);
> +static DEFINE_MUTEX(ovl_fops_mutex);
> +
> +#define OVL_FOPS_MAGIC 0x73706f462a6c764f
> +
> +struct ovl_fops {
> + struct module *owner;
> + struct file_operations fops;
> + u64 magic;
> + const struct file_operations *orig_fops;
> + struct hlist_node entry;
> +};
> +
> +void ovl_cleanup_fops_htable(void)
> +{
> + int bkt;
> + struct hlist_node *tmp;
> + struct ovl_fops *ofop;
> +
> + hash_for_each_safe(ovl_fops_htable, bkt, tmp, ofop, entry) {
> + module_put(ofop->owner);
> + fops_put(ofop->orig_fops);
> + kfree(ofop);
> + }
> +}
> +
> +#define OVL_CALL_REAL_FOP(file, call) \
> + ({ struct ovl_fops *__ofop = \
> + container_of(file->f_op, struct ovl_fops, fops); \
> + WARN_ON(__ofop->magic != OVL_FOPS_MAGIC) ? -EIO : \
> + __ofop->orig_fops->call; \
> + })
> +
> +static struct ovl_fops *ovl_fops_find(const struct file_operations *orig)
> +{
> + struct ovl_fops *ofop;
> +
> + hash_for_each_possible_rcu(ovl_fops_htable, ofop, entry, (long) orig) {
> + if (ofop->orig_fops == orig)
> + return ofop;
> + }
> + return NULL;
> +}
> +
> +static struct ovl_fops *ovl_fops_get(struct file *file)
> +{
> + const struct file_operations *orig = file->f_op;
> + struct ovl_fops *ofop = ovl_fops_find(orig);
> +
> + if (likely(ofop))
> + return ofop;
> +
> + mutex_lock(&ovl_fops_mutex);
> + ofop = ovl_fops_find(orig);
> + if (ofop)
> + goto out_unlock;
> +
> + ofop = kzalloc(sizeof(struct ovl_fops), GFP_KERNEL);

Suggestion:
if (!ofop) goto out_unlock;
and rid of all this nested chunk

> + if (ofop) {
> + /*
> + * FS don't usually fill in fops->owner, so grab ref to
> + * filesystem's module as well.
> + */
> + ofop->owner = file_inode(file)->i_sb->s_type->owner;
> + __module_get(ofop->owner);
> +
> + ofop->magic = OVL_FOPS_MAGIC;
> + ofop->orig_fops = fops_get(orig);
> +
> + /* These will need to be intercepted: */
> + ofop->fops.read_iter = orig->read_iter;
> + ofop->fops.mmap = orig->mmap;
> + ofop->fops.fsync = orig->fsync;
> +
> + /*
> + * These should be intercepted, but they are very unlikely to be
> + * a problem in practice. Leave them alone for now.

It could also be handled in vfs helpers.
Since these ops all start with establishing that src and dest are on
the same sb,
then the cost of copy up of src is the cost of clone_file_range from
lower to upper,
so it is probably worth to copy up src and leave those fops alone.

> + */
> + ofop->fops.copy_file_range = orig->copy_file_range;
> + ofop->fops.clone_file_range = orig->clone_file_range;
> + ofop->fops.dedupe_file_range = orig->dedupe_file_range;
> +
> + /* Don't intercept these: */
> + ofop->fops.llseek = orig->llseek;
> + ofop->fops.unlocked_ioctl = orig->unlocked_ioctl;
> + ofop->fops.compat_ioctl = orig->compat_ioctl;
> + ofop->fops.flush = orig->flush;
> + ofop->fops.release = orig->release;
> + ofop->fops.get_unmapped_area = orig->get_unmapped_area;
> + ofop->fops.check_flags = orig->check_flags;
> +
> + /* splice_read should be generic_file_splice_read */
> + WARN_ON(orig->splice_read != generic_file_splice_read);
> + ofop->fops.splice_read = generic_file_splice_read;
> +
> + /* These make no sense for "normal" files: */
> + WARN_ON(orig->read);
> + WARN_ON(orig->iterate);
> + WARN_ON(orig->iterate_shared);
> + WARN_ON(orig->poll);
> + WARN_ON(orig->fasync);
> + WARN_ON(orig->show_fdinfo);
> +
> + /*
> + * Don't add those which are unneeded for O_RDONLY:
> + *
> + * write
> + * write_iter
> + * splice_write
> + * sendpage
> + * fallocate
> + *
> + * Locking operations are already intercepted by vfs for ovl:
> + *
> + * lock
> + * flock
> + * setlease
> + */
> +
> + hash_add_rcu(ovl_fops_htable, &ofop->entry, (long) orig);
> + }
> +out_unlock:
> + mutex_unlock(&ovl_fops_mutex);
> +
> + return ofop;
> +}
> +
> static int ovl_open(struct inode *inode, struct file *file)
> {
> int ret = 0;
> + struct ovl_fops *ofop;
> + bool isupper = OVL_TYPE_UPPER(ovl_path_type(file->f_path.dentry));
>
> /* Want fops from real inode */
> replace_fops(file, inode->i_fop);
> if (file->f_op->open)
> ret = file->f_op->open(inode, file);
>
> - return ret;
> + /* No need to override fops for upper */
> + if (isupper || ret)
> + return ret;
> +
> + ofop = ovl_fops_get(file);
> + if (unlikely(!ofop)) {
> + if (file->f_op->release)
> + file->f_op->release(inode, file);
> + return -ENOMEM;
> + }
> + replace_fops(file, &ofop->fops);
> +
> + return 0;
> }
>
> static const struct file_operations ovl_file_operations = {
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index bdda37fa3f67..abc00f4ac247 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -209,6 +209,8 @@ static inline void ovl_copyattr(struct inode *from, struct inode *to)
> to->i_ctime = from->i_ctime;
> }
>
> +void ovl_cleanup_fops_htable(void);
> +
> /* dir.c */
> extern const struct inode_operations ovl_dir_inode_operations;
> struct dentry *ovl_lookup_temp(struct dentry *workdir, struct dentry *dentry);
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index e847cc354599..89cbb11eef58 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -960,6 +960,7 @@ static int __init ovl_init(void)
> static void __exit ovl_exit(void)
> {
> unregister_filesystem(&ovl_fs_type);
> + ovl_cleanup_fops_htable();
> }
>
> module_init(ovl_init);
> --
> 2.5.5
>
> --
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Next message: sunil . kovvuri: "[PATCH 3/4] net: thunderx: Configure RED and backpressure levels"
Previous message: Daniel Vetter: "Re: [PATCH 1/4] locking/ww_mutex: Fix a deadlock affecting ww_mutexes"
In reply to: Miklos Szeredi: "[PATCH 4/7] ovl: add infrastructure for intercepting file ops"
Next in thread: Miklos Szeredi: "Re: [PATCH 4/7] ovl: add infrastructure for intercepting file ops"
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