[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 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 */ > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface", http://blog.man7.org/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
