Re: [PATCH 1/2] vfs: implement fchmodat2() syscall
From: Michael Kerrisk <hidden>
Date: 2017-03-01 09:06:06
[CC += linux-api@vger.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@vger.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 [off-list ref] wrote:
quoted hunk
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 <redacted> --- 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/