linux-kernel - Re: [PATCH v2 24/36] fs: add ksys_unlink() wrapper; remove in-kernel calls to sys

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Date:   Sat, 17 Mar 2018 18:09:29 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 24/36] fs: add ksys_unlink() wrapper; remove in-kernel
 calls to sys_unlink()

On Thu, Mar 15, 2018 at 09:21:39PM +0100, Arnd Bergmann wrote:
> On Thu, Mar 15, 2018 at 8:05 PM, Dominik Brodowski
> <linux@...inikbrodowski.net> wrote:
> > Using this wrapper allows us to avoid the in-kernel calls to the
> > sys_unlink() syscall.
> >
> > Cc: Al Viro <viro@...iv.linux.org.uk>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
> > ---
> >  include/linux/syscalls.h | 11 +++++++++++
> >  init/do_mounts.h         |  2 +-
> >  init/do_mounts_initrd.c  |  4 ++--
> >  init/do_mounts_rd.c      |  2 +-
> >  init/initramfs.c         |  4 ++--
> >  5 files changed, 17 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index 8f0f99702e7a..31aea3873de7 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -971,4 +971,15 @@ int ksys_chdir(const char __user *filename);
> >  int ksys_sync_file_range(int fd, loff_t offset, loff_t nbytes,
> >                          unsigned int flags);
> >
> > +/*
> > + * The following kernel syscall equivalents are just wrappers to fs-internal
> > + * functions. Therefore, provide stubs to be inlined at the callsites.
> > + */
> > +extern long do_unlinkat(int dfd, struct filename *name);
> > +
> > +static inline long ksys_unlink(const char __user *pathname)
> > +{
> > +       return do_unlinkat(AT_FDCWD, getname(pathname));
> > +}
> 
> Why does this take a __user pointer?
> 
> >  static inline int create_dev(char *name, dev_t dev)
> >  {
> > -       sys_unlink(name);
> > +       ksys_unlink(name);
> >         return sys_mknod(name, S_IFBLK|0600, new_encode_dev(dev));
> >  }
> >
> > diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
> > index c19d9070134e..784576b633fd 100644
> > --- a/init/do_mounts_initrd.c
> > +++ b/init/do_mounts_initrd.c
> > @@ -128,11 +128,11 @@ bool __init initrd_load(void)
> >                  * mounted in the normal path.
> >                  */
> >                 if (rd_load_image("/initrd.image") && ROOT_DEV != Root_RAM0) {
> > -                       sys_unlink("/initrd.image");
> > +                       ksys_unlink("/initrd.image");
> >                         handle_initrd();
> >                         return true;
> >                 }
> >         }
> > -       sys_unlink("/initrd.image");
> > +       ksys_unlink("/initrd.image");
> >         return false;
> 
> In all callers we seem to have regular kernel strings, so I think
> you should skip the getname() and change the argument to
> a regular pointer.

In line with my explanations in other messages, I have added the following
paragraph to the commit message:
	
	In the near future, all callers of ksys_unlink() should be converted to
	call do_unlinkat() directly or, at least, to operate on regular kernel
	pointers.

Thanks,
	Dominik