Re: [PATCH 1/2] vfs: implement fchmodat2() syscall

From: Greg Kurz
Date: Tue Apr 11 2017 - 07:47:32 EST

On Wed, 1 Mar 2017 10:01:53 +0100
Michael Kerrisk <mtk.manpages@xxxxxxxxx> wrote:

> [CC += linux-api@xxxxxxxxxxxxxxx]
>
> 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@xxxxxxxxxxxxxxx, 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
>

My bad :-\

Anyway, since I had no feedback so far, I'll send a v2 and this time I'll
rembember to Cc linux-api@xxxxxxxxxxxxxxxx

Thanks,

--
Greg

>
> On Tue, Feb 28, 2017 at 6:03 PM, Greg Kurz <groug@xxxxxxxx> 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@xxxxxxxx>
> > ---
> > 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 */
> >
>
>
>

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