[Gluster-devel] Fixing setfsuid/gid problems in posix xlator

[Jeff Darcy]

> Jiffin found an interesting problem in posix xlator where we have never been
> using setfsuid/gid ( http://review.gluster.org/#/c/15545/ ), what I am
> seeing regressions after this is, if the files are created using non-root
> user then the file creation fails because that user doesn't have permissions
> to create the gfid-link. So it seems like the correct way forward for this
> patch is to write wrappers around sys_<syscall> to do setfsuid/gid do the
> actual operation requested and then set it back to old uid/gid and then do
> the internal operations. I am planning to write posix_sys_<syscall>() to do
> the same, may be a macro?

Kind of an aside, but I'd prefer to see a lot fewer macros in our code.
They're not type-safe, and multi-line macros often mess up line numbers for
debugging or error messages.  IMO it's better to use functions whenever
possible, and usually to let the compiler worry about how/when to inline.

> I need inputs from you guys to let me know if I am on the right path and if
> you see any issues with this approach.

I think there's a bit of an interface problem here.  The sys_xxx wrappers
don't have arguments that point to the current frame, so how would they get
the correct uid/gid?  We could add arguments to each function, but then
we'd have to modify every call.  This includes internal calls which don't
have a frame to pass, so I guess they'd have to pass NULL.  Alternatively,
we could create a parallel set of functions with frame pointers.  Contrary
to what I just said above, this might be a case where macros make sense:

   int
   sys_writev_fp (call_frame_t *frame, int fd, void *buf, size_t len)
   {
      if (frame) { setfsuid(...) ... }
      int ret = writev (fd, buf, len);
      if (frame) { setfsuid(...) ... }
      return ret;
   }
   #define sys_writev(fd,buf,len) sys_writev_fp (NULL, fd, buf, len)

That way existing callers don't have to change, but posix can use the
extended versions to get the right setfsuid behavior.