On Thu, 8 Jul 2010 01:27:06 +1000, Nick Piggin <npiggin@xxxxxxx> wrote:
> On Tue, Jun 15, 2010 at 10:42:55PM +0530, Aneesh Kumar K.V wrote:
> > From: NeilBrown <neilb@xxxxxxx>
> >
> > This enables to use readlink to get the link target name
> > from a file descriptor point to the link. This can be used
> > with open_by_handle syscall that returns a file descriptor for a link.
> > We can then use this file descriptor to get the target name.
> >
> > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> > ---
> > fs/stat.c | 30 ++++++++++++++++++++++--------
> > 1 files changed, 22 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/stat.c b/fs/stat.c
> > index c4ecd52..49b95a7 100644
> > --- a/fs/stat.c
> > +++ b/fs/stat.c
> > @@ -284,26 +284,40 @@ SYSCALL_DEFINE2(newfstat, unsigned int, fd, struct stat __user *, statbuf)
> > SYSCALL_DEFINE4(readlinkat, int, dfd, const char __user *, pathname,
> > char __user *, buf, int, bufsiz)
> > {
> > - struct path path;
> > - int error;
> > + int error = 0;
> > + struct path path, *pp;
> > + struct file *file = NULL;
> >
> > if (bufsiz <= 0)
> > return -EINVAL;
> >
> > - error = user_path_at(dfd, pathname, 0, &path);
> > + if (pathname == NULL && dfd != AT_FDCWD) {
> > + file = fget(dfd);
> > +
> > + if (file)
> > + pp = &file->f_path;
> > + else
> > + error = -EBADF;
> > + } else {
> > + error = user_path_at(dfd, pathname, 0, &path);
> > + pp = &path;
> > + }
>
> This (and all the others) is really ugly overloading of syscall
> arguments IMO, and the changelog is seriously lacking for such
> changes.
Initially we had freadlink
http://lkml.org/lkml/2010/5/12/222
We updated the patches to use the existing readlinkat interface because
utimensat(2) already exposed a similar interface. So it should be ok to
expect that other *at call behaved in a similar way ?
>
> This also changes the the syscall API of existing calls; from reading
> the path at NULL, to switching to a completely different syscall.
> Perhaps you're assuming nobody relies on SIGSEGV / mmapped NULL address
> there, but even then you surely need to document the changed semantics
> somewhere (and document the new syscall semantics properly).
Yes this would need a documentation update. But i guess since we already
have utimensat(2) behaving similarly we are ok to extent readlinkat,
linkat and faccessat on similar lines ?
-aneesh
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