According to the POSIX.1-2008 manual page [1], the fchmodat() function has
a flag argument which may be passed the following value:
AT_SYMLINK_NOFOLLOW
If path names a symbolic link, then the mode of the symbolic link is
changed.
and the following error may be returned:
[EOPNOTSUPP]
The AT_SYMLINK_NOFOLLOW bit is set in the flag argument, path names a
symbolic link, and the system does not support changing the mode of a
symbolic link.
The linux kernel doesn't support changing the mode of a symbolic link, but
the current implementation doesn't even have a flag argument. It is then
up to userspace to deal with that. Unfortunately, it is impossible to
implement the POSIX behavior in a race-free manner.
This patch introduces a new fchmodat2() syscall with a flag argument to
address the issue.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html
Signed-off-by: Greg Kurz <gr...@kaod.org>
---
fs/open.c | 23 +++++++++++++++++++----
include/linux/syscalls.h | 2 ++
include/uapi/asm-generic/unistd.h | 4 +++-
scripts/checksyscalls.sh | 3 ++-
4 files changed, 26 insertions(+), 6 deletions(-)
diff --git a/fs/open.c b/fs/open.c
index 9921f70bc5ca..66a8c19f72ca 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -558,24 +558,39 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
return err;
}
-SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, umode_t,
mode)
+SYSCALL_DEFINE4(fchmodat2, int, dfd, const char __user *, filename, umode_t,
+ mode, int, flag)
{
struct path path;
- int error;
- unsigned int lookup_flags = LOOKUP_FOLLOW;
+ int error = -EINVAL;
+ unsigned int lookup_flags;
+
+ if ((flag & ~AT_SYMLINK_NOFOLLOW) != 0)
+ goto out;
+
+ lookup_flags = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
if (!error) {
- error = chmod_common(&path, mode);
+ error = -EOPNOTSUPP;
+ if (!d_is_symlink(path.dentry))
+ error = chmod_common(&path, mode);
path_put(&path);
if (retry_estale(error, lookup_flags)) {
lookup_flags |= LOOKUP_REVAL;
goto retry;
}
}
+out:
return error;
}
+SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, umode_t,
+ mode)
+{
+ return sys_fchmodat2(dfd, filename, mode, 0);
+}
+
SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
{
return sys_fchmodat(AT_FDCWD, filename, mode);
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 91a740f6b884..982089d55b31 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -775,6 +775,8 @@ asmlinkage long sys_futimesat(int dfd, const char __user
*filename,
asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode);
asmlinkage long sys_fchmodat(int dfd, const char __user * filename,
umode_t mode);
+asmlinkage long sys_fchmodat2(int dfd, const char __user *filename,
+ umode_t mode, int flag);
asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
gid_t group, int flag);
asmlinkage long sys_openat(int dfd, const char __user *filename, int flags,
diff --git a/include/uapi/asm-generic/unistd.h
b/include/uapi/asm-generic/unistd.h
index 9b1462e38b82..e8b0a00908b1 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -730,9 +730,11 @@ __SYSCALL(__NR_pkey_mprotect, sys_pkey_mprotect)
__SYSCALL(__NR_pkey_alloc, sys_pkey_alloc)
#define __NR_pkey_free 290
__SYSCALL(__NR_pkey_free, sys_pkey_free)
+#define __NR_fchmodat2 291
+__SYSCALL(__NR_fchmodat2, sys_fchmodat2)
#undef __NR_syscalls
-#define __NR_syscalls 291
+#define __NR_syscalls 292
/*
* All syscalls below here should go away really,
diff --git a/scripts/checksyscalls.sh b/scripts/checksyscalls.sh
index 2c9082ba6137..2e7471a1d308 100755
--- a/scripts/checksyscalls.sh
+++ b/scripts/checksyscalls.sh
@@ -19,7 +19,7 @@ cat << EOF
#define __IGNORE_link /* linkat */
#define __IGNORE_unlink /* unlinkat */
#define __IGNORE_mknod /* mknodat */
-#define __IGNORE_chmod /* fchmodat */
+#define __IGNORE_chmod /* fchmodat2 */
#define __IGNORE_chown /* fchownat */
#define __IGNORE_mkdir /* mkdirat */
#define __IGNORE_rmdir /* unlinkat */
@@ -39,6 +39,7 @@ cat << EOF
/* Missing flags argument */
#define __IGNORE_renameat /* renameat2 */
+#define __IGNORE_fchmodat /* fchmodat2 */
/* CLOEXEC flag */
#define __IGNORE_pipe /* pipe2 */
