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Subject[PATCH 28/38] union-mount: Implement union-aware access()/faccessat()
Date
For union mounts, a file located on the lower layer will incorrectly
return EROFS on an access check. To fix this, use the new
path_permission() call, which ignores a read-only lower layer file
system if the target will be copied up to the topmost file system.
---
fs/open.c | 21 +++++++++++++++++----
1 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/fs/open.c b/fs/open.c
index 74e5cd9..7f7958e 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -32,6 +32,7 @@
#include <linux/ima.h>

#include "internal.h"
+#include "union.h"

int vfs_statfs(struct dentry *dentry, struct kstatfs *buf)
{
@@ -454,7 +455,10 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
const struct cred *old_cred;
struct cred *override_cred;
struct path path;
+ struct nameidata nd;
+ struct vfsmount *mnt;
struct inode *inode;
+ char *tmp;
int res;

if (mode & ~S_IRWXO) /* where's F_OK, X_OK, W_OK, R_OK? */
@@ -478,10 +482,17 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)

old_cred = override_creds(override_cred);

- res = user_path_at(dfd, filename, LOOKUP_FOLLOW, &path);
+ res = user_path_nd(dfd, filename, LOOKUP_FOLLOW,
+ &nd, &path, &tmp);
if (res)
goto out;

+ /* For union mounts, use the topmost mnt's permissions */
+ if (IS_DIR_UNIONED(nd.path.dentry))
+ mnt = nd.path.mnt;
+ else
+ mnt = path.mnt;
+
inode = path.dentry->d_inode;

if ((mode & MAY_EXEC) && S_ISREG(inode->i_mode)) {
@@ -490,11 +501,11 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
* with the "noexec" flag.
*/
res = -EACCES;
- if (path.mnt->mnt_flags & MNT_NOEXEC)
+ if (mnt->mnt_flags & MNT_NOEXEC)
goto out_path_release;
}

- res = inode_permission(inode, mode | MAY_ACCESS);
+ res = path_permission(&path, &nd.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;
@@ -508,11 +519,13 @@ 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 (__mnt_is_readonly(mnt))
res = -EROFS;

out_path_release:
path_put(&path);
+ path_put(&nd.path);
+ putname(tmp);
out:
revert_creds(old_cred);
put_cred(override_cred);
--
1.6.3.3


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