From mboxrd@z Thu Jan 1 00:00:00 1970 From: Omar Sandoval Subject: [RFC PATCH v4 2/4] fs: add AT_LINK_REPLACE flag for linkat() which replaces the target Date: Wed, 29 Jan 2020 00:58:32 -0800 Message-ID: <1f5a197a2fdb0668f6dce8b9a4403481bc957a7c.1580251857.git.osandov__24826.2572380312$1580288346$gmane$org@fb.com> References: Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Al Viro Cc: kernel-team-b10kYP2dOMg@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, David Howells , Amir Goldstein , Xi Wang List-Id: linux-api@vger.kernel.org From: Omar Sandoval One of the most common uses of temporary files is the classic atomic replacement pattern, i.e., - write temporary file - fsync temporary file - rename temporary file over real file - fsync parent directory Now, we have O_TMPFILE, which gives us a much better way to create temporary files, but it's not possible to use it for this pattern. This patch introduces an AT_LINK_REPLACE flag which allows linkat() to replace the target file. Now, the temporary file in the pattern above can be a proper O_TMPFILE. Even without O_TMPFILE, this is a new primitive which might be useful in other contexts. The implementation on the VFS side mimics sys_renameat2(). Cc: Xi Wang Signed-off-by: Omar Sandoval --- fs/ecryptfs/inode.c | 2 +- fs/namei.c | 166 +++++++++++++++++++++++++++++-------- fs/nfsd/vfs.c | 2 +- fs/overlayfs/overlayfs.h | 2 +- include/linux/fs.h | 2 +- include/uapi/linux/fcntl.h | 1 + 6 files changed, 135 insertions(+), 40 deletions(-) diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c index eeb351b220b2..2f36b7a61a2f 100644 --- a/fs/ecryptfs/inode.c +++ b/fs/ecryptfs/inode.c @@ -440,7 +440,7 @@ static int ecryptfs_link(struct dentry *old_dentry, struct inode *dir, dget(lower_new_dentry); lower_dir_dentry = lock_parent(lower_new_dentry); rc = vfs_link(lower_old_dentry, d_inode(lower_dir_dentry), - lower_new_dentry, NULL); + lower_new_dentry, NULL, 0); if (rc || d_really_is_negative(lower_new_dentry)) goto out_lock; rc = ecryptfs_interpose(lower_new_dentry, new_dentry, dir->i_sb); diff --git a/fs/namei.c b/fs/namei.c index 9d690df17aed..78d364e99dca 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -4122,6 +4122,7 @@ SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newn * @dir: new parent * @new_dentry: where to create the new link * @delegated_inode: returns inode needing a delegation break + * @flags: link flags * * The caller must hold dir->i_mutex * @@ -4135,16 +4136,25 @@ SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newn * be appropriate for callers that expect the underlying filesystem not * to be NFS exported. */ -int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry, struct inode **delegated_inode) +int vfs_link(struct dentry *old_dentry, struct inode *dir, + struct dentry *new_dentry, struct inode **delegated_inode, + int flags) { struct inode *inode = old_dentry->d_inode; + struct inode *target = new_dentry->d_inode; unsigned max_links = dir->i_sb->s_max_links; int error; if (!inode) return -ENOENT; - error = may_create(dir, new_dentry); + if (target) { + if (inode == target) + return 0; + error = may_delete(dir, new_dentry, false); + } else { + error = may_create(dir, new_dentry); + } if (error) return error; @@ -4172,26 +4182,55 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de if (error) return error; - inode_lock(inode); + dget(new_dentry); + lock_two_nondirectories(inode, target); + + if (is_local_mountpoint(new_dentry)) { + error = -EBUSY; + goto out; + } + /* Make sure we don't allow creating hardlink to an unlinked file */ - if (inode->i_nlink == 0 && !(inode->i_state & I_LINKABLE)) + if (inode->i_nlink == 0 && !(inode->i_state & I_LINKABLE)) { error = -ENOENT; - else if (max_links && inode->i_nlink >= max_links) + goto out; + } + if (max_links && inode->i_nlink >= max_links) { error = -EMLINK; - else { - error = try_break_deleg(inode, delegated_inode); - if (!error) - error = dir->i_op->link(old_dentry, dir, new_dentry, 0); + goto out; + } + + error = try_break_deleg(inode, delegated_inode); + if (error) + goto out; + if (target) { + error = try_break_deleg(target, delegated_inode); + if (error) + goto out; + } + + error = dir->i_op->link(old_dentry, dir, new_dentry, flags); + if (error) + goto out; + + if (target) { + dont_mount(new_dentry); + detach_mounts(new_dentry); } - if (!error && (inode->i_state & I_LINKABLE)) { + if (inode->i_state & I_LINKABLE) { spin_lock(&inode->i_lock); inode->i_state &= ~I_LINKABLE; spin_unlock(&inode->i_lock); } - inode_unlock(inode); - if (!error) +out: + unlock_two_nondirectories(inode, target); + dput(new_dentry); + if (!error) { + if (target) + fsnotify_link_count(target); fsnotify_link(dir, inode, new_dentry); + } return error; } EXPORT_SYMBOL(vfs_link); @@ -4210,11 +4249,16 @@ int do_linkat(int olddfd, const char __user *oldname, int newdfd, { struct dentry *new_dentry; struct path old_path, new_path; + struct qstr new_last; + int new_type; struct inode *delegated_inode = NULL; - int how = 0; + struct filename *to; + unsigned int how = 0, target_flags; + bool should_retry = false; int error; - if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0) + if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | + AT_LINK_REPLACE)) != 0) return -EINVAL; /* * To use null names we require CAP_DAC_READ_SEARCH @@ -4229,44 +4273,94 @@ int do_linkat(int olddfd, const char __user *oldname, int newdfd, if (flags & AT_SYMLINK_FOLLOW) how |= LOOKUP_FOLLOW; + + if (flags & AT_LINK_REPLACE) + target_flags = LOOKUP_RENAME_TARGET; + else + target_flags = LOOKUP_CREATE | LOOKUP_EXCL; retry: error = user_path_at(olddfd, oldname, how, &old_path); if (error) return error; - new_dentry = user_path_create(newdfd, newname, &new_path, - (how & LOOKUP_REVAL)); - error = PTR_ERR(new_dentry); - if (IS_ERR(new_dentry)) - goto out; + to = filename_parentat(newdfd, getname(newname), how & LOOKUP_REVAL, + &new_path, &new_last, &new_type); + if (IS_ERR(to)) { + error = PTR_ERR(to); + goto exit1; + } + + if (old_path.mnt != new_path.mnt) { + error = -EXDEV; + goto exit2; + } + + if (new_type != LAST_NORM) { + if (flags & AT_LINK_REPLACE) + error = -EISDIR; + else + error = -EEXIST; + goto exit2; + } + + error = mnt_want_write(old_path.mnt); + if (error) + goto exit2; + +retry_deleg: + inode_lock_nested(new_path.dentry->d_inode, I_MUTEX_PARENT); + + new_dentry = __lookup_hash(&new_last, new_path.dentry, + (how & LOOKUP_REVAL) | target_flags); + if (IS_ERR(new_dentry)) { + error = PTR_ERR(new_dentry); + goto exit3; + } + if (!(flags & AT_LINK_REPLACE) && d_is_positive(new_dentry)) { + error = -EEXIST; + goto exit4; + } + if (new_last.name[new_last.len]) { + if (d_is_negative(new_dentry)) { + error = -ENOENT; + goto exit4; + } + if (!d_is_dir(old_path.dentry)) { + error = -ENOTDIR; + goto exit4; + } + } - error = -EXDEV; - if (old_path.mnt != new_path.mnt) - goto out_dput; error = may_linkat(&old_path); if (unlikely(error)) - goto out_dput; + goto exit4; error = security_path_link(old_path.dentry, &new_path, new_dentry); if (error) - goto out_dput; - error = vfs_link(old_path.dentry, new_path.dentry->d_inode, new_dentry, &delegated_inode); -out_dput: - done_path_create(&new_path, new_dentry); + goto exit4; + error = vfs_link(old_path.dentry, new_path.dentry->d_inode, new_dentry, + &delegated_inode, flags & AT_LINK_REPLACE); +exit4: + dput(new_dentry); +exit3: + inode_unlock(new_path.dentry->d_inode); if (delegated_inode) { error = break_deleg_wait(&delegated_inode); - if (!error) { - path_put(&old_path); - goto retry; - } + if (!error) + goto retry_deleg; } - if (retry_estale(error, how)) { - path_put(&old_path); + mnt_drop_write(old_path.mnt); +exit2: + if (retry_estale(error, how)) + should_retry = true; + path_put(&new_path); + putname(to); +exit1: + path_put(&old_path); + if (should_retry) { + should_retry = false; how |= LOOKUP_REVAL; goto retry; } -out: - path_put(&old_path); - return error; } diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c index c0dc491537a6..3f9291e76b99 100644 --- a/fs/nfsd/vfs.c +++ b/fs/nfsd/vfs.c @@ -1598,7 +1598,7 @@ nfsd_link(struct svc_rqst *rqstp, struct svc_fh *ffhp, err = nfserr_noent; if (d_really_is_negative(dold)) goto out_dput; - host_err = vfs_link(dold, dirp, dnew, NULL); + host_err = vfs_link(dold, dirp, dnew, NULL, 0); if (!host_err) { err = nfserrno(commit_metadata(ffhp)); if (!err) diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h index f283b1d69a9e..b199fc03c891 100644 --- a/fs/overlayfs/overlayfs.h +++ b/fs/overlayfs/overlayfs.h @@ -120,7 +120,7 @@ static inline int ovl_do_unlink(struct inode *dir, struct dentry *dentry) static inline int ovl_do_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry) { - int err = vfs_link(old_dentry, dir, new_dentry, NULL); + int err = vfs_link(old_dentry, dir, new_dentry, NULL, 0); pr_debug("link(%pd2, %pd2) = %i\n", old_dentry, new_dentry, err); return err; diff --git a/include/linux/fs.h b/include/linux/fs.h index 3bdb71c97e8f..93eb90eb1fdb 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1712,7 +1712,7 @@ extern int vfs_create(struct inode *, struct dentry *, umode_t, bool); extern int vfs_mkdir(struct inode *, struct dentry *, umode_t); extern int vfs_mknod(struct inode *, struct dentry *, umode_t, dev_t); extern int vfs_symlink(struct inode *, struct dentry *, const char *); -extern int vfs_link(struct dentry *, struct inode *, struct dentry *, struct inode **); +extern int vfs_link(struct dentry *, struct inode *, struct dentry *, struct inode **, int); extern int vfs_rmdir(struct inode *, struct dentry *); extern int vfs_unlink(struct inode *, struct dentry *, struct inode **); extern int vfs_rename(struct inode *, struct dentry *, struct inode *, struct dentry *, struct inode **, unsigned int); diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h index 1f97b33c840e..3704793cd5ab 100644 --- a/include/uapi/linux/fcntl.h +++ b/include/uapi/linux/fcntl.h @@ -99,6 +99,7 @@ #define AT_STATX_DONT_SYNC 0x4000 /* - Don't sync attributes with the server */ #define AT_RECURSIVE 0x8000 /* Apply to the entire subtree */ +#define AT_LINK_REPLACE 0x10000 /* Replace link() target */ #endif /* _UAPI_LINUX_FCNTL_H */ -- 2.25.0