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Message-ID: <CAHO5Pa2dPUQRXEs7W198M0wbxWAkjKzsVPo4yrc1VPfbNM5R9w@mail.gmail.com>
Date: Wed, 1 Mar 2017 10:01:53 +0100
From: Michael Kerrisk <mtk.manpages@...il.com>
To: Greg Kurz <groug@...d.org>
Cc: Al Viro <viro@...iv.linux.org.uk>,
Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
Eric Blake <eblake@...hat.com>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 1/2] vfs: implement fchmodat2() syscall
[CC += linux-api@...r.kernel.org]
Hello Greg,
Since this is a kernel-user-space API change, please CC linux-api@.
The kernel source file Documentation/SubmitChecklist notes that all
Linux kernel patches that change userspace interfaces should be CCed
to linux-api@...r.kernel.org, so that the various parties who are
interested in API changes are informed. For further information, see
https://www.kernel.org/doc/man-pages/linux-api-ml.html
Thanks,
Michael
On Tue, Feb 28, 2017 at 6:03 PM, Greg Kurz <groug@...d.org> wrote:
> 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 <groug@...d.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 */
>
--
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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