[PATCH v2 2/4] Add fchmodat4(), a new syscall

Palmer Dabbelt
2019-07-17T01:27:17Z

man 3p says that fchmodat() takes a flags argument, but the Linux
syscall does not. There doesn't appear to be a good userspace
workaround for this issue but the implementation in the kernel is pretty
straight-forward. The specific use case where the missing flags came up
was WRT a fuse filesystem implemenation, but the functionality is pretty
generic so I'm assuming there would be other use cases.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
---
fs/open.c | 20 ++++++++++++++++----
include/linux/syscalls.h | 7 +++++--
2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index b5b80469b93d..2f72b4d6a2c1 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -569,11 +569,17 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
return ksys_fchmod(fd, mode);
}

-int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
+int do_fchmodat4(int dfd, const char __user *filename, umode_t mode, int flags)
{
struct path path;
int error;
- unsigned int lookup_flags = LOOKUP_FOLLOW;
+ unsigned int lookup_flags;
+
+ if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
+ return -EINVAL;
+
+ lookup_flags = flags & AT_SYMLINK_NOFOLLOW ? 0 : LOOKUP_FOLLOW;
+
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
if (!error) {
@@ -587,15 +593,21 @@ int do_fchmodat(int dfd, const char __user *filename, umode_t mode)
return error;
}

+SYSCALL_DEFINE4(fchmodat4, int, dfd, const char __user *, filename,
+ umode_t, mode, int, flags)
+{
+ return do_fchmodat4(dfd, filename, mode, flags);
+}
+
SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename,
umode_t, mode)
{
- return do_fchmodat(dfd, filename, mode);
+ return do_fchmodat4(dfd, filename, mode, 0);
}

SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
{
- return do_fchmodat(AT_FDCWD, filename, mode);
+ return do_fchmodat4(AT_FDCWD, filename, mode, 0);
}

static int chown_common(const struct path *path, uid_t user, gid_t group)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index e1c20f1d0525..a4bde25ad264 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -433,6 +433,8 @@ asmlinkage long sys_chroot(const char __user *filename);
asmlinkage long sys_fchmod(unsigned int fd, umode_t mode);
asmlinkage long sys_fchmodat(int dfd, const char __user *filename,
umode_t mode);
+asmlinkage long sys_fchmodat4(int dfd, const char __user *filename,
+ umode_t mode, int flags);
asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
gid_t group, int flag);
asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group);
@@ -1320,11 +1322,12 @@ static inline long ksys_link(const char __user *oldname,
return do_linkat(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
}

-extern int do_fchmodat(int dfd, const char __user *filename, umode_t mode);
+extern int do_fchmodat4(int dfd, const char __user *filename, umode_t mode,
+ int flags);

static inline int ksys_chmod(const char __user *filename, umode_t mode)
{
- return do_fchmodat(AT_FDCWD, filename, mode);
+ return do_fchmodat4(AT_FDCWD, filename, mode, 0);
}

extern long do_faccessat(int dfd, const char __user *filename, int mode);
--
2.21.0