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

[Palmer Dabbelt]

On Tue, 16 Jul 2019 18:48:02 PDT (-0700), viro@xxxxxxxxxxxxxxxxxx wrote:
> On Tue, Jul 16, 2019 at 06:27:17PM -0700, Palmer Dabbelt wrote:
>
> > -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;
> > +
>
> 	Why not do that in sys_fchmodat4() itself, passing lookup_flags to
> do_fchmodat() and updating old callers to pass it 0 as extra argument?

Ya, that seems better -- passing LOOKUP_FOLLOW instead of 0, to keep the
behavior the same.  That way I could avoid the overhead of these checks for the
old syscalls, as we know they're not necessary.

I'll replace this patch with the following for a v3

   diff --git a/fs/open.c b/fs/open.c
   index b5b80469b93d..a5f99408af11 100644
   --- a/fs/open.c
   +++ b/fs/open.c
   @@ -569,11 +569,12 @@ 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 lookup_flags)
    {
           struct path path;
           int error;
   -       unsigned int lookup_flags = LOOKUP_FOLLOW;
   +
    retry:
           error = user_path_at(dfd, filename, lookup_flags, &path);
           if (!error) {
   @@ -587,15 +588,28 @@ 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)
   +{
   +       unsigned int lookup_flags;
   +
   +       if (unlikely(flags & ~AT_SYMLINK_NOFOLLOW))
   +               return -EINVAL;
   +
   +       lookup_flags = flags & AT_SYMLINK_NOFOLLOW ? 0 : LOOKUP_FOLLOW;
   +
   +       return do_fchmodat4(dfd, filename, mode, lookup_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, LOOKUP_FOLLOW);
    }
   
    SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
    {
   -       return do_fchmodat(AT_FDCWD, filename, mode);
   +       return do_fchmodat4(AT_FDCWD, filename, mode, LOOKUP_FOLLOW);
    }
   
    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..6676b1cc5485 100644
   --- a/include/linux/syscalls.h
   +++ b/include/linux/syscalls.h
   @@ -81,6 +81,7 @@ struct io_uring_params;
    #include <linux/quota.h>
    #include <linux/key.h>
    #include <linux/personality.h>
   +#include <linux/namei.h>
    #include <trace/syscall.h>
   
    #ifdef CONFIG_ARCH_HAS_SYSCALL_WRAPPER
   @@ -433,6 +434,8 @@ 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 +1323,12 @@ 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, LOOKUP_FOLLOW);
    }
   
    extern long do_faccessat(int dfd, const char __user *filename, int mode);