From: Miklos Szeredi Date: Fri, 16 Sep 2016 10:44:21 +0000 (+0200) Subject: vfs: do get_write_access() on upper layer of overlayfs X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=4d0c5ba2ff79;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git vfs: do get_write_access() on upper layer of overlayfs The problem with writecount is: we want consistent handling of it for underlying filesystems as well as overlayfs. Making sure i_writecount is correct on all layers is difficult. Instead this patch makes sure that when write access is acquired, it's always done on the underlying writable layer (called the upper layer). We must also make sure to look at the writecount on this layer when checking for conflicting leases. Open for write already updates the upper layer's writecount. Leaving only truncate. For truncate copy up must happen before get_write_access() so that the writecount is updated on the upper layer. Problem with this is if something fails after that, then copy-up was done needlessly. E.g. if break_lease() was interrupted. Probably not a big deal in practice. Another interesting case is if there's a denywrite on a lower file that is then opened for write or truncated. With this patch these will succeed, which is somewhat counterintuitive. But I think it's still acceptable, considering that the copy-up does actually create a different file, so the old, denywrite mapping won't be touched. On non-overlayfs d_real() is an identity function and d_real_inode() is equivalent to d_inode() so this patch doesn't change behavior in that case. Signed-off-by: Miklos Szeredi Acked-by: Jeff Layton Cc: "J. Bruce Fields" --- diff --git a/fs/locks.c b/fs/locks.c index c1656cff53ee..b242d5b99589 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -1618,7 +1618,8 @@ check_conflicting_open(const struct dentry *dentry, const long arg, int flags) if (flags & FL_LAYOUT) return 0; - if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0)) + if ((arg == F_RDLCK) && + (atomic_read(&d_real_inode(dentry)->i_writecount) > 0)) return -EAGAIN; if ((arg == F_WRLCK) && ((d_count(dentry) > 1) || diff --git a/fs/open.c b/fs/open.c index 648fb9d3e97a..8aeb08bb278b 100644 --- a/fs/open.c +++ b/fs/open.c @@ -68,6 +68,7 @@ int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs, long vfs_truncate(const struct path *path, loff_t length) { struct inode *inode; + struct dentry *upperdentry; long error; inode = path->dentry->d_inode; @@ -90,7 +91,17 @@ long vfs_truncate(const struct path *path, loff_t length) if (IS_APPEND(inode)) goto mnt_drop_write_and_out; - error = get_write_access(inode); + /* + * If this is an overlayfs then do as if opening the file so we get + * write access on the upper inode, not on the overlay inode. For + * non-overlay filesystems d_real() is an identity function. + */ + upperdentry = d_real(path->dentry, NULL, O_WRONLY); + error = PTR_ERR(upperdentry); + if (IS_ERR(upperdentry)) + goto mnt_drop_write_and_out; + + error = get_write_access(upperdentry->d_inode); if (error) goto mnt_drop_write_and_out; @@ -109,7 +120,7 @@ long vfs_truncate(const struct path *path, loff_t length) error = do_truncate(path->dentry, length, 0, NULL); put_write_and_out: - put_write_access(inode); + put_write_access(upperdentry->d_inode); mnt_drop_write_and_out: mnt_drop_write(path->mnt); out: