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Date: Mon, 30 Nov 2009 23:34:57 -0500
From: Erez Zadok <ezk@...sunysb.edu>
To: Valerie Aurora <vaurora@...hat.com>
Cc: Jan Blunck <jblunck@...e.de>,
Alexander Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
Andy Whitcroft <apw@...onical.com>,
Scott James Remnant <scott@...onical.com>,
Sandu Popa Marius <sandupopamarius@...il.com>,
Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
"J. R. Okajima" <hooanon05@...oo.co.jp>,
Arnd Bergmann <arnd@...db.de>,
Vladimir Dronnikov <dronnikov@...il.com>,
Felix Fietkau <nbd@...nwrt.org>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 39/41] union-mount: Ignore read-only file system in permission checks
In message <1256152779-10054-40-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> In certain cases, we check a file for write access before it has been
> copied up to the top-level fs. We don't want to fail because the
> bottom layer is read-only - of course it is - so skip that check in
> those cases.
>
> Thanks to Felix Fietkau <nbd@...nwrt.org> for a bug fix.
>
> XXX - Document when to call union_permission() vs. inode_permission()
> XXX - Kinda gross. Probably a simpler solution.
>
> Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> ---
> fs/namei.c | 21 +++++++++++++++++----
> fs/open.c | 8 ++++++--
> fs/union.c | 32 ++++++++++++++++++++++++++++++--
> include/linux/fs.h | 1 +
> include/linux/union.h | 2 ++
> 5 files changed, 56 insertions(+), 8 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 61e94aa..a8d3acf 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -230,16 +230,17 @@ int generic_permission(struct inode *inode, int mask,
> }
>
> /**
> - * inode_permission - check for access rights to a given inode
> + * __inode_permission - check for access rights to a given inode
> * @inode: inode to check permission on
> * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> + * @rofs: check for read-only fs
> *
> * Used to check for read/write/execute permissions on an inode.
> * We use "fsuid" for this, letting us set arbitrary permissions
> * for filesystem access without changing the "normal" uids which
> * are used for other things.
> */
> -int inode_permission(struct inode *inode, int mask)
> +int __inode_permission(struct inode *inode, int mask, int rofs)
> {
> int retval;
rofs can be a boolean.
While I normally prefer to avoid magic flags passed to a function to change
its behavior, in this case it's a small and obvious change. I could use
your __inode_permission as is in Unionfs today, if it was upstream; in
Unionfs I had to copy inode_permission to my code, and remove the EROFS
test.
>
> @@ -249,7 +250,7 @@ int inode_permission(struct inode *inode, int mask)
> /*
> * Nobody gets write access to a read-only fs.
> */
> - if (IS_RDONLY(inode) &&
> + if ((rofs & IS_RDONLY(inode)) &&
> (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
> return -EROFS;
>
> @@ -277,6 +278,18 @@ int inode_permission(struct inode *inode, int mask)
> }
>
> /**
> + * inode_permission - check for access rights to a given inode
> + * @inode: inode to check permission on
> + * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> + *
> + * This version pays attention to the MS_RDONLY flag on the fs.
> + */
> +int inode_permission(struct inode *inode, int mask)
> +{
> + return __inode_permission(inode, mask, 1);
> +}
> +
> +/**
> * file_permission - check for additional access rights to a given file
> * @file: file to check access rights for
> * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> @@ -2129,7 +2142,7 @@ int may_open(struct path *path, int acc_mode, int flag)
> break;
> }
>
> - error = inode_permission(inode, acc_mode);
> + error = union_permission(path, acc_mode);
> if (error)
> return error;
>
> diff --git a/fs/open.c b/fs/open.c
> index dd98e80..3df5a1b 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -30,6 +30,7 @@
> #include <linux/audit.h>
> #include <linux/falloc.h>
> #include <linux/fs_struct.h>
> +#include <linux/union.h>
>
> int vfs_statfs(struct dentry *dentry, struct kstatfs *buf)
> {
> @@ -333,6 +334,7 @@ static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
> error = security_path_truncate(&file->f_path, length,
> ATTR_MTIME|ATTR_CTIME);
> if (!error)
> + /* Already copied up for union, opened with write */
> error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, file);
> out_putf:
> fput(file);
> @@ -493,7 +495,8 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
> goto out_path_release;
> }
>
> - res = inode_permission(inode, mode | MAY_ACCESS);
> + res = union_permission(&path, mode | MAY_ACCESS);
> +
> /* SuS v2 requires we report a read only fs too */
> if (res || !(mode & S_IWOTH) || special_file(inode->i_mode))
> goto out_path_release;
> @@ -507,7 +510,8 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
> * inherently racy and know that the fs may change
> * state before we even see this result.
> */
> - if (__mnt_is_readonly(path.mnt))
> + if ((!is_unionized(path.dentry, path.mnt) &&
> + (__mnt_is_readonly(path.mnt))))
> res = -EROFS;
>
> out_path_release:
> diff --git a/fs/union.c b/fs/union.c
> index d56b829..8d94b22 100644
> --- a/fs/union.c
> +++ b/fs/union.c
> @@ -390,6 +390,30 @@ static int union_relookup_topmost(struct nameidata *nd, int flags)
> return err;
> }
>
> +
> +/**
> + * union_permission - check for access rights to a given inode
> + * @inode: inode to check permission on
> + * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
> + *
> + * In a union mount, the top layer is always read-write and the bottom
> + * is always read-only. Ignore the read-only flag on the lower fs.
> + *
> + * Only need for certain activities, like checking to see if write
> + * access is ok.
> + */
> +
> +int union_permission(struct path *path, int mask)
> +{
> + struct inode *inode = path->dentry->d_inode;
> +
> + if (!is_unionized(path->dentry, path->mnt))
> + return inode_permission(inode, mask);
> +
> + /* Tell __inode_permission to ignore MS_RDONLY */
> + return __inode_permission(inode, mask, 0);
> +}
> +
> /*
> * union_create_topmost - create the topmost path component
> * @nd: pointer to nameidata of the base directory
> @@ -489,6 +513,9 @@ static int union_copy_file(struct dentry *old_dentry, struct vfsmount *old_mnt,
> if (IS_ERR(new_file))
> goto fput_old;
>
> + /* XXX be smart by using a length param, which indicates max
> + * data we'll want (e.g., we are about to truncate to 0 or 10
> + * bytes or something */
Useful comment, but not here: I'd put it right in the very first
copyup-related patch. And add it as "todo" to the design doc.
> size = i_size_read(old_file->f_path.dentry->d_inode);
> if (((size_t)size != size) || ((ssize_t)size != size)) {
> ret = -EFBIG;
> @@ -516,7 +543,8 @@ static int union_copy_file(struct dentry *old_dentry, struct vfsmount *old_mnt,
> * The topmost directory @new_nd must already be locked. Creates the topmost
> * file if it doesn't exist yet.
> */
> -int __union_copyup(struct path *old, struct nameidata *new_nd, struct path *new)
> +int __union_copyup(struct path *old, struct nameidata *new_nd,
> + struct path *new)
> {
> struct dentry *dentry;
> int error;
> @@ -581,7 +609,7 @@ out_dput:
> * @nd: nameidata pointer to the file
> * @flags: flags given to open_namei
> */
> -int union_copyup(struct nameidata *nd, int flags)
> +int union_copyup(struct nameidata *nd, int flags /* XXX not used */)
If not used, then why not remove it?
> {
> struct qstr this;
> char *name;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 57690ab..38fb113 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2106,6 +2106,7 @@ extern void emergency_remount(void);
> extern sector_t bmap(struct inode *, sector_t);
> #endif
> extern int notify_change(struct dentry *, struct iattr *);
> +extern int __inode_permission(struct inode *inode, int mask, int rofs);
> extern int inode_permission(struct inode *, int);
> extern int generic_permission(struct inode *, int,
> int (*check_acl)(struct inode *, int));
> diff --git a/include/linux/union.h b/include/linux/union.h
> index a0656b3..92654e0 100644
> --- a/include/linux/union.h
> +++ b/include/linux/union.h
> @@ -58,6 +58,7 @@ extern struct dentry *union_create_topmost(struct nameidata *, struct qstr *,
> extern int __union_copyup(struct path *, struct nameidata *, struct path *);
> extern int union_copyup(struct nameidata *, int);
> extern int union_copyup_dir(struct path *path);
> +extern int union_permission(struct path *, int);
>
> #else /* CONFIG_UNION_MOUNT */
>
> @@ -76,6 +77,7 @@ extern int union_copyup_dir(struct path *path);
> #define __union_copyup(x, y, z) ({ BUG(); (0); })
> #define union_copyup(x, y) ({ (0); })
> #define union_copyup_dir(x) ({ BUG(); (0); })
> +#define union_permission(x, y) inode_permission((x)->dentry->d_inode, y)
>
> #endif /* CONFIG_UNION_MOUNT */
> #endif /* __KERNEL__ */
> --
> 1.6.3.3
>
> --
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Erez.
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