On Thu, Jan 07, 2021 at 04:10:43PM -0800, Sargun Dhillon wrote:
> Overlayfs's volatile option allows the user to bypass all forced sync calls
> to the upperdir filesystem. This comes at the cost of safety. We can never
> ensure that the user's data is intact, but we can make a best effort to
> expose whether or not the data is likely to be in a bad state.
>
> The best way to handle this in the time being is that if an overlayfs's
> upperdir experiences an error after a volatile mount occurs, that error
> will be returned on fsync, fdatasync, sync, and syncfs. This is
> contradictory to the traditional behaviour of VFS which fails the call
> once, and only raises an error if a subsequent fsync error has occurred,
> and been raised by the filesystem.
>
> One awkward aspect of the patch is that we have to manually set the
> superblock's errseq_t after the sync_fs callback as opposed to just
> returning an error from syncfs. This is because the call chain looks
> something like this:
>
> sys_syncfs ->
> sync_filesystem ->
> __sync_filesystem ->
> /* The return value is ignored here
> sb->s_op->sync_fs(sb)
> _sync_blockdev
> /* Where the VFS fetches the error to raise to userspace */
> errseq_check_and_advance
>
> Because of this we call errseq_set every time the sync_fs callback occurs.
> Due to the nature of this seen / unseen dichotomy, if the upperdir is an
> inconsistent state at the initial mount time, overlayfs will refuse to
> mount, as overlayfs cannot get a snapshot of the upperdir's errseq that
> will increment on error until the user calls syncfs.
>
> Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx>
> Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Fixes: c86243b090bc ("ovl: provide a mount option "volatile"")
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-unionfs@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Jeff Layton <jlayton@xxxxxxxxxx>
> Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Looks good to me.
Reviewed-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
Thanks
Vivek
> ---
> Documentation/filesystems/overlayfs.rst | 8 ++++++
> fs/overlayfs/file.c | 5 ++--
> fs/overlayfs/overlayfs.h | 1 +
> fs/overlayfs/ovl_entry.h | 2 ++
> fs/overlayfs/readdir.c | 5 ++--
> fs/overlayfs/super.c | 34 ++++++++++++++++++++-----
> fs/overlayfs/util.c | 27 ++++++++++++++++++++
> 7 files changed, 71 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index 580ab9a0fe31..137afeb3f581 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -575,6 +575,14 @@ without significant effort.
> The advantage of mounting with the "volatile" option is that all forms of
> sync calls to the upper filesystem are omitted.
>
> +In order to avoid a giving a false sense of safety, the syncfs (and fsync)
> +semantics of volatile mounts are slightly different than that of the rest of
> +VFS. If any writeback error occurs on the upperdir's filesystem after a
> +volatile mount takes place, all sync functions will return an error. Once this
> +condition is reached, the filesystem will not recover, and every subsequent sync
> +call will return an error, even if the upperdir has not experience a new error
> +since the last sync call.
> +
> When overlay is mounted with "volatile" option, the directory
> "$workdir/work/incompat/volatile" is created. During next mount, overlay
> checks for this directory and refuses to mount if present. This is a strong
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index a1f72ac053e5..5c5c3972ebd0 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> const struct cred *old_cred;
> int ret;
>
> - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb)))
> - return 0;
> + ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb));
> + if (ret <= 0)
> + return ret;
>
> ret = ovl_real_fdget_meta(file, &real, !datasync);
> if (ret)
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index f8880aa2ba0e..9f7af98ae200 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry);
> bool ovl_is_metacopy_dentry(struct dentry *dentry);
> char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry,
> int padding);
> +int ovl_sync_status(struct ovl_fs *ofs);
>
> static inline bool ovl_is_impuredir(struct super_block *sb,
> struct dentry *dentry)
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index 1b5a2094df8e..b208eba5d0b6 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -79,6 +79,8 @@ struct ovl_fs {
> atomic_long_t last_ino;
> /* Whiteout dentry cache */
> struct dentry *whiteout;
> + /* r/o snapshot of upperdir sb's only taken on volatile mounts */
> + errseq_t errseq;
> };
>
> static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs)
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 01620ebae1bd..a273ef901e57 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
> struct file *realfile;
> int err;
>
> - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb)))
> - return 0;
> + err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb));
> + if (err <= 0)
> + return err;
>
> realfile = ovl_dir_real_file(file, true);
> err = PTR_ERR_OR_ZERO(realfile);
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 290983bcfbb3..d23177a53c95 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
> struct super_block *upper_sb;
> int ret;
>
> - if (!ovl_upper_mnt(ofs))
> - return 0;
> + ret = ovl_sync_status(ofs);
> + /*
> + * We have to always set the err, because the return value isn't
> + * checked in syncfs, and instead indirectly return an error via
> + * the sb's writeback errseq, which VFS inspects after this call.
> + */
> + if (ret < 0) {
> + errseq_set(&sb->s_wb_err, -EIO);
> + return -EIO;
> + }
> +
> + if (!ret)
> + return ret;
>
> - if (!ovl_should_sync(ofs))
> - return 0;
> /*
> * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).
> * All the super blocks will be iterated, including upper_sb.
> @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> sb->s_op = &ovl_super_operations;
>
> if (ofs->config.upperdir) {
> + struct super_block *upper_sb;
> +
> if (!ofs->config.workdir) {
> pr_err("missing 'workdir'\n");
> goto out_err;
> @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> if (err)
> goto out_err;
>
> + upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
> + if (!ovl_should_sync(ofs)) {
> + ofs->errseq = errseq_sample(&upper_sb->s_wb_err);
> + if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) {
> + err = -EIO;
> + pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n");
> + goto out_err;
> + }
> + }
> +
> err = ovl_get_workdir(sb, ofs, &upperpath);
> if (err)
> goto out_err;
> @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> if (!ofs->workdir)
> sb->s_flags |= SB_RDONLY;
>
> - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth;
> - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran;
> -
> + sb->s_stack_depth = upper_sb->s_stack_depth;
> + sb->s_time_gran = upper_sb->s_time_gran;
> }
> oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers);
> err = PTR_ERR(oe);
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 23f475627d07..6e7b8c882045 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry,
> kfree(buf);
> return ERR_PTR(res);
> }
> +
> +/*
> + * ovl_sync_status() - Check fs sync status for volatile mounts
> + *
> + * Returns 1 if this is not a volatile mount and a real sync is required.
> + *
> + * Returns 0 if syncing can be skipped because mount is volatile, and no errors
> + * have occurred on the upperdir since the mount.
> + *
> + * Returns -errno if it is a volatile mount, and the error that occurred since
> + * the last mount. If the error code changes, it'll return the latest error
> + * code.
> + */
> +
> +int ovl_sync_status(struct ovl_fs *ofs)
> +{
> + struct vfsmount *mnt;
> +
> + if (ovl_should_sync(ofs))
> + return 1;
> +
> + mnt = ovl_upper_mnt(ofs);
> + if (!mnt)
> + return 0;
> +
> + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq);
> +}
> --
> 2.25.1
>
