Date:	Wed, 29 Sep 2010 18:22:37 -0400
From:	Oren Laadan <orenl@...columbia.edu>
To:	Matt Helsley <matthltc@...ibm.com>
CC:	containers@...ts.linux-foundation.org,
	Eric Sandeen <sandeen@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Jamie Lokier <jamie@...reable.org>,
	Amir Goldstein <amir73il@...rs.sf.net>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH 5/6] [RFC] Checkpoint/restart unlinked files


On 09/23/2010 05:53 PM, Matt Helsley wrote:
> Implement checkpoint of unlinked files by relinking them into their
> filesystem at lost+found/checkpoint/unlink-at-restart-<ktime>-<N>.
> We can change the function to generate better paths -- I just need to
> know what that path should be. For example, perhaps sys_checkpoint should
> take a template string somewhat like mkstemp().

/lost+found/checkpoint/.... relative to what ?
security concerns ?

> Relinking allows userspace to leverage the snapshotting capabilities
> of various linux block devices and filesystems. sys_checkpoint relinks
> the files and returns. Userspace then checkpoints the filesystem contents
> using any backup-like method prior to thawing. That backup would then be
> available for use during an optional migration followed by restore and
> restart. In the case of network and cluster/distributed filesystems copying
> the filesystem contents explicitly for migration may not be necessary at
> all -- it would be part of normal file writes. For non-migration uses
> of checkpoint/restart filesystems like btrfs a snapshot could simply be
> taken during checkpoint and mounted during restart -- again without
> requiring IO proportional to the aggregate size of filesystem contents
> being checkpointed.
>
> In addition to the original path of the file we save the newly-linked
> path. This newly-linked path is opened during restart instead of the
> original path (which is only useful files that were linked at the time
> of checkpoint). The newly-linked location is also useful in order to
> identify which relinked files in lost+found/checkpoint were created by
> this particular invokation of sys_checkpoint. This enables userspace

s/invokation/invocation/

> to cleanup after checkpoints which failed yet successfully relinked.
>
> Note that we'd still be restricted by the limitations of hardlinks.
> Furthermore, as Aneesh Kumar mentioned in the LKML threads leading up to
> the v19 file handle patches, this kind of linking seems to require
> CAP_DAC_READ_SEARCH because the files to be linked lack a path to
> search in the first place. Aneesh added the check in response to Al Viro's
> point that being able to relink open files (passed via SCM_RIGHTS for
> example) has non-trivial security ramifications.
>
> To understand why relinking is extremely useful for checkpoint/restart
> consider this simple pseudocode program and a specific example checkpoint
> of it:
>
> 	a_fd = open("a"); /* example: size of the file at "a" is 1GB */
> 	link("a", "b");
> 	unlink("a");
> 	creat("a");
> 	<---- example: checkpoint happens here
> 	write(a_fd, "bar");
>
> The file "a" is unlinked and a different file has been placed at that
> path. a_fd still refers to the inode shared with "b".
>
> Without relinking we would need to walk the entire filesystem to find out
> that "b" is a path to the same inode (another variation on this case: "b"
> would also have been unlinked). We'd need to do this for every
> unlinked file that remains open in every task to checkpoint. Even then
> there is no guarantee such a "b" exists for every unlinked file -- the
> inodes could be "orphans" -- and we'd need to preserve their contents
> some other way.
>
> I considered a couple alternatives to preserving unlinked file contents:
> copying and file handles. Each has significant drawbacks.
>
> First I attempted to copy the file contents into the image and then
> recreate and unlink the file during restart. Using a simple version of
> that method the write above would not reach "b". One fix would be to search
> the filesystem for a file with the same inode number (inode of "b") and
> either open it or hardlink it to "a". Another would be to record the inode
> number. This either shifts the search from checkpoint time to restart time
> or has all the drawbacks of the second method I considered: file handles.

Maybe worth to explicitly mention the obvious, just in case:

Only re-create and unlink the file if at checkpoint time the
link count was 0 (no other references existed); and do it under
a temporary name, to not overwrite existing (newer) files.

If the link count was greater than 0, then the only correct
solution is to somehow find the other pathname for the same
inode, make a new temporary link, open the new link, and finally
unlink the new link.

>
> Instead of copying contents or recording inodes I also considered using
> file handles. We'd need to ensure that the filehandles persist in storage,
> can be snapshotted/backed up, and can be migrated. Can handlefs or any
> generic file handle system do this? My _guess_ is "no" but folks are
> welcome to tell me I'm wrong.
>
> In contrast, linking the file from a_fd back into its filesystem can avoid
> these complexities. Relinking avoids the search for matching inodes and
> copying large quantities of data from storage only to write it back (in
> fact the data would be read-and-written twice -- once for checkpoint and
> once for restart). Like file handles it does require changes to the
> filesystem code. Unlike file handles, enabling relinking does not require
> every filesystem to support a new kind of filesystem "object" -- only
> an operation that is quite similar to one that already exists: link.
>
> Signed-off-by: Matt Helsley<matthltc@...ibm.com>
> Cc: Eric Sandeen<sandeen@...hat.com>
> Cc: Theodore Ts'o<tytso@....edu>
> Cc: Andreas Dilger<adilger.kernel@...ger.ca>
> Cc: linux-ext4@...r.kernel.org
> Cc: Jan Kara<jack@...e.cz>
> Cc: containers@...ts.linux-foundation.org
> Cc: Oren Laadan<orenl@...columbia.edu>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: Al Viro<viro@...iv.linux.org.uk>
> Cc: Christoph Hellwig<hch@...radead.org>
> Cc: Jamie Lokier<jamie@...reable.org>
> Cc: Amir Goldstein<amir73il@...rs.sf.net>
> Cc: Aneesh Kumar<aneesh.kumar@...ux.vnet.ibm.com>
> Cc: Miklos Szeredi<miklos@...redi.hu>
> ---
>   fs/checkpoint.c                  |   51 ++++++++++++++-----
>   fs/namei.c                       |  102 ++++++++++++++++++++++++++++++++++++++
>   fs/pipe.c                        |    2 +-
>   include/linux/checkpoint.h       |    3 +-
>   include/linux/checkpoint_hdr.h   |    3 +
>   include/linux/checkpoint_types.h |    3 +
>   6 files changed, 149 insertions(+), 15 deletions(-)
>
> diff --git a/fs/checkpoint.c b/fs/checkpoint.c
> index 87d7c6e..9c7caec 100644
> --- a/fs/checkpoint.c
> +++ b/fs/checkpoint.c
> @@ -16,6 +16,7 @@
>   #include<linux/sched.h>
>   #include<linux/file.h>
>   #include<linux/namei.h>
> +#include<linux/mount.h>
>   #include<linux/fs_struct.h>
>   #include<linux/fs.h>
>   #include<linux/fdtable.h>
> @@ -26,6 +27,7 @@
>   #include<linux/checkpoint.h>
>   #include<linux/eventpoll.h>
>   #include<linux/eventfd.h>
> +#include<linux/sys-wrapper.h>
>   #include<net/sock.h>
>
>   /**************************************************************************
> @@ -174,6 +176,9 @@ int checkpoint_file_common(struct ckpt_ctx *ctx, struct file *file,
>   	h->f_pos = file->f_pos;
>   	h->f_version = file->f_version;
>
> +	if (d_unlinked(file->f_dentry))
> +		/* Perform post-checkpoint and post-restart unlink() */
> +		h->f_restart_flags |= RESTART_FILE_F_UNLINK;

Follow the convention for these ?  s/RESTART_.../CKPT_.../

>   	h->f_credref = checkpoint_obj(ctx, f_cred, CKPT_OBJ_CRED);
>   	if (h->f_credref<  0)
>   		return h->f_credref;
> @@ -197,16 +202,6 @@ int generic_file_checkpoint(struct ckpt_ctx *ctx, struct file *file)
>   	struct ckpt_hdr_file_generic *h;
>   	int ret;
>
> -	/*
> -	 * FIXME: when we'll add support for unlinked files/dirs, we'll
> -	 * need to distinguish between unlinked filed and unlinked dirs.
> -	 */
> -	if (d_unlinked(file->f_dentry)) {
> -		ckpt_err(ctx, -EBADF, "%(T)%(P)Unlinked files unsupported\n",
> -			 file);
> -		return -EBADF;
> -	}

We still need to handle unlinked directories separately (see
below). And they are actually easier to handle. So you probably
also need a flag to indicate that it is a directory.

Perhaps rename flag from above, e.g.:
   CKPT_FILE_UNLINKED_FILE
   CKPT_FILE_UNLINKED_DIR

> -
>   	h = ckpt_hdr_get_type(ctx, sizeof(*h), CKPT_HDR_FILE);
>   	if (!h)
>   		return -ENOMEM;
> @@ -220,6 +215,9 @@ int generic_file_checkpoint(struct ckpt_ctx *ctx, struct file *file)
>   	if (ret<  0)
>   		goto out;
>   	ret = checkpoint_fname(ctx,&file->f_path,&ctx->root_fs_path);
> +	if (ret<  0)
> +		goto out;
> +	ret = checkpoint_file_links(ctx, file);
>    out:
>   	ckpt_hdr_put(ctx, h);
>   	return ret;
> @@ -570,9 +568,11 @@ static int ckpt_read_fname(struct ckpt_ctx *ctx, char **fname)
>   /**
>    * restore_open_fname - read a file name and open a file
>    * @ctx: checkpoint context
> + * @restore_unlinked: unlink the opened file

nit: s/restore_unlinked/unlinked/   ?

>    * @flags: file flags
>    */
> -struct file *restore_open_fname(struct ckpt_ctx *ctx, int flags)
> +struct file *restore_open_fname(struct ckpt_ctx *ctx,
> +				int restore_unlinked, int flags)
>   {
>   	struct file *file;
>   	char *fname;
> @@ -586,8 +586,33 @@ struct file *restore_open_fname(struct ckpt_ctx *ctx, int flags)
>   	if (len<  0)
>   		return ERR_PTR(len);
>   	ckpt_debug("fname '%s' flags %#x\n", fname, flags);
> -
> +	if (restore_unlinked) {
> +		kfree(fname);

This is too early to discard the original name (why would we
have saved it in the first place anyway ?) because ...

> +		fname = NULL;
> +		len = ckpt_read_payload(ctx, (void **)&fname, PATH_MAX,
> +					CKPT_HDR_BUFFER);
> +		if (len<  0)
> +			return ERR_PTR(len);
> +		fname[len] = '\0';
> +	}

Uhm... for the following to work you need to explicitly handle
the case of unlinked directory:

	if (restore_unlinked & CKPT_FILE_UNLINKED_DIR)
		mkdir(...);

>   	file = filp_open(fname, flags, 0);
> +	if (IS_ERR(file)) {
> +		ckpt_err(ctx, PTR_ERR(file), "Could not open file \"%s\"\n", fname);
> +
> +		goto out;
> +	}
> +	if (!restore_unlinked)
> +		goto out;
> +	if (S_ISDIR(file->f_mapping->host->i_mode))
> +		len = kernel_sys_rmdir(fname);
> +	else
> +		len = kernel_sys_unlink(fname);

... because now you need to manually modify the dentry to have
the original name, so that doing 'ls -l /proc/PID/fd' will give
   "NN -> original_name (deleted)"
rather than
   "NN -> relinked_obfuscated_name (deleted)"

> +	if (len<  0) {
> +		ckpt_err(ctx, len, "Could not unlink \"%s\"\n", fname);
> +		fput(file);
> +		file = ERR_PTR(len);
> +	}
> +out:
>   	kfree(fname);
>
>   	return file;
> @@ -692,7 +717,7 @@ static struct file *generic_file_restore(struct ckpt_ctx *ctx,
>   	    ptr->h.len != sizeof(*ptr) || ptr->f_type != CKPT_FILE_GENERIC)
>   		return ERR_PTR(-EINVAL);
>
> -	file = restore_open_fname(ctx, ptr->f_flags);
> +	file = restore_open_fname(ctx, !!(ptr->f_restart_flags&  RESTART_FILE_F_UNLINK), ptr->f_flags);

Now f_restart_flags can have more than one flag, so remove the "!!"
(and I would rename it to f_ckpt_flags)

>   	if (IS_ERR(file))
>   		return file;
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 8c9663d..69c4f4e 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -32,6 +32,9 @@
>   #include<linux/fcntl.h>
>   #include<linux/device_cgroup.h>
>   #include<linux/fs_struct.h>
> +#ifdef CONFIG_CHECKPOINT
> +#include<linux/checkpoint.h>
> +#endif
>   #include<asm/uaccess.h>
>
>   #include "internal.h"
> @@ -2543,6 +2546,105 @@ SYSCALL_DEFINE2(link, const char __user *, oldname, const char __user *, newname
>   	return sys_linkat(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
>   }
>
> +#ifdef CONFIG_CHECKPOINT
> +
> +/* Path relative to the mounted filesystem's root -- not a "global" root or even a namespace root. The unique_name_count is unique for the entire checkpoint. */
> +#define CKPT_RELINKAT_FMT "lost+found/checkpoint/unlink-at-restart-%08llx-%u"

A few existential questions:

* Do we want to hard-code to /lost+found/checkpoint/... or allow
  userspace to set it per checkpoint ?

* If we put it all in some place, could it cause some security
  or privacy issues, if we hold data from multiple users on the
  same filesystem ?

* This doesn't guarantee unique-ness among checkpoints. One option
  is to add the ctx->crid to make it unique - but I'm unsure that
  we'll keep it in the future.

* For clean-up purposes, maybe get a subdir for each checkpoint ?

* If checkpoint fails, who is responsible for the cleanup ?  I'd
  prefer that sys_checkpoint() won't leave garbage behind (so using
  the subdir approach above, we could just delete that dir ?)

* If restart fails, e.g. because of lack of memory, then another
  restart would fail because some (or all) relinked files will
  have already been re-unlinked and gone.  Maybe defer all unlinks
  to the end of the restart ?
  (This is less of a problem when we had a filesystem snapshot,
  just go back to that snapshot. But when we, e.g. suspend a vnc
  session to disk using checkpoint, and then try to resume - it
  will be a problem).

* By using the unique counter, you assume that the sequence of
  file-open's (or mkdir) will never change at restart. But if a
  future user tool will alter the checkpoint image (assume it is
  in a valid and desirable way) and drop one file, then we are
  doomed ...  Instead, you should save the unique unlink id
  explcitily in the ckpt_hdr_file.

> +
> +static int checkpoint_fill_relink_fname(struct ckpt_ctx *ctx,
> +					struct file *for_file,
> +					char relink_dir_pathname[PATH_MAX],
> +					int *lenp)
> +{
> +	struct path relink_dir_path;
> +	char *tmp;
> +	int len;
> +
> +	/* Find path to mount */
> +	relink_dir_path.mnt = for_file->f_path.mnt;
> +	relink_dir_path.dentry = relink_dir_path.mnt->mnt_root;
> +	tmp = d_path(&relink_dir_path, relink_dir_pathname, PATH_MAX);

This path is relative to whom ?

Don't we need similar precautions as in ckpt_fill_name(), in
particular verify that that path is "reachable" from the
checkpointer ?    Can it be made to reuse the code from there ?

> +	if (IS_ERR(tmp))
> +		return PTR_ERR(tmp);
> +
> +	/* Append path to relinked file. */
> +	len = strlen(tmp);
> +	if (len<= 0)
> +		return -ENOENT;
> +	memmove(relink_dir_pathname, tmp, len);
> +	tmp = relink_dir_pathname + len - 1;
> +	/* Ensure we've got a single dir separator */
> +	if (*tmp == '/')
> +		tmp++;
> +	else {
> +		tmp++;
> +		*tmp = '/';
> +		tmp++;
> +		len++;
> +	}
> +	len += snprintf(tmp, PATH_MAX - len, CKPT_RELINKAT_FMT,
> +			ctx->ktime_begin.tv64,
> +			 ++ctx->unique_name_count);

Should test for truncation, and bail...

> +	relink_dir_pathname[len] = '\0';
> +	*lenp = len;
> +	return 0;
> +}
> +
> +static int checkpoint_file_relink(struct ckpt_ctx *ctx,
> +				  struct file *file,
> +				  char new_path[PATH_MAX])
> +{
> +	int ret, len;
> +
> +	/*
> +	 * Relinking arbitrary files without searching a path
> +	 * (which non-existent if the file is unlinked) requires
> +	 * special privileges.
> +	 */
> +	if (!capable(CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH)) {
> +		ckpt_err(ctx, -EPERM, "%(T)Relinking unlinked files requires CAP_DAC_{OVERRIDE,READ_SEARCH}\n");
> +		return -EPERM;
> +	}
> +	ret = checkpoint_fill_relink_fname(ctx, file, new_path,&len);
> +	if (ret)
> +		return ret;
> +	ret = do_kern_linkat(&file->f_path, file->f_dentry,
> +			     AT_FDCWD, new_path, 0);
> +	if (ret)
> +		ckpt_err(ctx, ret, "%(T)%(P)%(V)Failed to relink unlinked file.\n", file, file->f_op);
> +	return ret;
> +}
> +
> +int checkpoint_file_links(struct ckpt_ctx *ctx, struct file *file)
> +{
> +	char *new_link_path;
> +	int ret, len;
> +
> +	if (!d_unlinked(file->f_dentry))
> +		return 0;
> +
> +	/*
> +	 * Unlinked files need at least one hardlink for the post-sys_checkpoint
> +	 * filesystem backup/snapshot.
> +	 */
> +	new_link_path = kmalloc(PATH_MAX, GFP_KERNEL);
> +	if (!new_link_path)
> +		return -ENOMEM;
> +	ret = checkpoint_file_relink(ctx, file, new_link_path);
> +	if (ret<  0)
> +		goto out_free;
> +	len = strlen(new_link_path);
> +	ret = ckpt_write_obj_type(ctx, NULL, len + 1, CKPT_HDR_BUFFER);

Do we need to save the entire pathname ?  Only the pathname to
the relevant mount point suffices, no ?  (the timestamp and the
unique id are available at restart).

Also worth to think (and at least comment with a big TODO) that
when we work on c/r support for mount points, we could just
refer to a (shared object) mount point instead of carrying the
path.

> +	if (ret<  0)
> +		goto out_free;
> +	ret = ckpt_kwrite(ctx, new_link_path, len + 1);
> +out_free:
> +	kfree(new_link_path);
> +
> +	return ret;
> +}
> +#endif /* CONFIG_CHECKPOINT */
> +
>   /*
>    * The worst of all namespace operations - renaming directory. "Perverted"
>    * doesn't even start to describe it. Somebody in UCB had a heck of a trip...
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 7f00e58..1325e84 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -1021,7 +1021,7 @@ struct file *fifo_file_restore(struct ckpt_ctx *ctx, struct ckpt_hdr_file *ptr)
>   	 * To avoid blocking, always open the fifo with O_RDWR;
>   	 * then fix flags below.
>   	 */
> -	file = restore_open_fname(ctx, (ptr->f_flags&  ~O_ACCMODE) | O_RDWR);
> +	file = restore_open_fname(ctx, 0, (ptr->f_flags&  ~O_ACCMODE) | O_RDWR);
>   	if (IS_ERR(file))
>   		return file;
>
> diff --git a/include/linux/checkpoint.h b/include/linux/checkpoint.h
> index 4e25042..6ca7b24 100644
> --- a/include/linux/checkpoint.h
> +++ b/include/linux/checkpoint.h
> @@ -258,7 +258,8 @@ extern int restore_obj_file_table(struct ckpt_ctx *ctx, int files_objref);
>   /* files */
>   extern int checkpoint_fname(struct ckpt_ctx *ctx,
>   			    struct path *path, struct path *root);
> -extern struct file *restore_open_fname(struct ckpt_ctx *ctx, int flags);
> +extern int checkpoint_file_links(struct ckpt_ctx *ctx, struct file *file);
> +extern struct file *restore_open_fname(struct ckpt_ctx *ctx, int restore_unlinked, int flags);
>
>   extern int ckpt_collect_file(struct ckpt_ctx *ctx, struct file *file);
>
> diff --git a/include/linux/checkpoint_hdr.h b/include/linux/checkpoint_hdr.h
> index f4f9577..ea50e7d 100644
> --- a/include/linux/checkpoint_hdr.h
> +++ b/include/linux/checkpoint_hdr.h
> @@ -575,6 +575,9 @@ struct ckpt_hdr_file {
>   	__u64 f_pos;
>   	__u64 f_version;
>   	__s32 f_secref;
> +
> +	__u32 f_restart_flags;
> +#define RESTART_FILE_F_UNLINK (1<<0)
>   } __attribute__((aligned(8)));
>
>   struct ckpt_hdr_file_generic {
> diff --git a/include/linux/checkpoint_types.h b/include/linux/checkpoint_types.h
> index 3ffe9bd..ceaa671 100644
> --- a/include/linux/checkpoint_types.h
> +++ b/include/linux/checkpoint_types.h
> @@ -57,6 +57,9 @@ struct ckpt_ctx {
>
>   	struct path root_fs_path;     /* container root (FIXME) */
>
> +	/* relink unlinked files to<mnt_root>/<unique_name>  */
> +	unsigned int unique_name_count;

[nit] why not straightforward "unique_relink_count" ?

> +
>   	struct task_struct *tsk;/* checkpoint: current target task */
>   	char err_string[256];	/* checkpoint: error string */
>

Phew ... end of round one :)

Oren.
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