Re: [PATCH v2 5/6] tools/virtiofsd: xattr name mappings: Map server xattr names

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Christophe de Dinechin (dinechin@redhat.com) wrote:
> 
> On 2020-08-27 at 17:36 CEST, Dr. David Alan Gilbert (git) wrote...
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> > Map xattr names coming from the server, i.e. the host filesystem;
> > currently this is only from listxattr.
> >
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  tools/virtiofsd/passthrough_ll.c | 88 ++++++++++++++++++++++++++++++++
> >  1 file changed, 88 insertions(+)
> >
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index 9b9c8f3ab1..7cd99186f7 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -2188,6 +2188,42 @@ static int xattr_map_client(const char *client_name, char **out_name)
> >      abort();
> >  }
> >
> > +/*
> > + * For use with listxattr where the server fs gives us a name and we may need
> > + * to sanitize this for the client.
> > + * Returns a pointer to the result in *out_name
> > + *   This is always the original string or the current string with some prefix
> > + *   removed; no reallocation is done.
> > + * Returns 0 on success
> > + * Can return -ENODATA to indicate the name should be dropped from the list.
> > + */
> > +static int xattr_map_server(const char *server_name, const char **out_name)
> 
> > +{
> > +    const XattrMapEntry *cur_entry;
> > +    for (cur_entry = xattr_map_list; ; cur_entry++) {
> > +        if ((cur_entry->flags & XATTR_MAP_FLAG_SERVER) &&
> > +            (!strncmp(cur_entry->prepend,
> > +                      server_name,
> > +                      strlen(cur_entry->prepend)))) {
> 
> Might be slightly clearer (and possibly faster) as
> 
>     strstart(cur_entry->prepend, server_name, &end)


I think this is actually,
   strstart(server_name, cur_entry->prepend, &end)

because I think strstart writes the offset into the 1st string
into &end expecting the 2nd string to be the thing to test for.
Agreed?


> > +            if (cur_entry->flags & XATTR_MAP_FLAG_END_BAD) {
> > +                return -ENODATA;
> > +            }
> > +            if (cur_entry->flags & XATTR_MAP_FLAG_END_OK) {
> > +                *out_name = server_name;
> > +                return 0;
> > +            }
> > +            if (cur_entry->flags & XATTR_MAP_FLAG_PREFIX) {
> > +                /* Remove prefix */
> > +                *out_name = server_name + strlen(cur_entry->prepend);
> 
> With the above, that would be
> 
>     *out_name = end;

OK.

> 
> > +                return 0;
> > +            }
> > +        }
> > +    }
> > +
> > +    /* Shouldn't get here - rules should have an END_* */
> 
> You probably want to point the finger back to parse_xattrmap() in the comment?

Done.

> > +    abort();
> 
> > +}
> > +
> >  static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
> >                          size_t size)
> >  {
> > @@ -2342,8 +2378,60 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
> >          if (ret == 0) {
> >              goto out;
> >          }
> > +
> > +        if (lo->xattrmap) {
> 
> If you put the "XattrMapEntry" pointer in lo, then you would probably test that.

Done

> 
> > +            /*
> > +             * Map the names back, some attributes might be dropped,
> > +             * some shortened, but not increased, so we shouldn't
> > +             * run out of room.
> > +             */
> > +            size_t out_index, in_index;
> > +            out_index = 0;
> > +            in_index = 0;
> > +            while (in_index < ret) {
> > +                const char *map_out;
> > +                char *in_ptr = value + in_index;
> > +                /* Length of current attribute name */
> > +                size_t in_len = strlen(value + in_index) + 1;
> > +
> > +                int mapret = xattr_map_server(in_ptr, &map_out);
> > +                if (mapret != -ENODATA && mapret != 0) {
> > +                    /* Shouldn't happen */
> > +                    saverr = -mapret;
> > +                    goto out;
> > +                }
> > +                if (mapret == 0) {
> > +                    /* Either unchanged, or truncated */
> > +                    size_t out_len;
> > +                    if (map_out != in_ptr) {
> > +                        /* +1 copies the NIL */
> > +                        out_len = strlen(map_out) + 1;
> > +                    } else {
> > +                        /* No change */
> > +                        out_len = in_len;
> > +                    }
> > +                    /*
> > +                     * Move result along, may still be needed for an unchanged
> > +                     * entry if a previous entry was changed.
> > +                     */
> > +                    memmove(value + out_index, map_out, out_len);
> > +
> > +                    out_index += out_len;
> > +                }
> > +                in_index += in_len;
> > +            }
> > +            ret = out_index;
> > +            if (ret == 0) {
> > +                goto out;
> > +            }
> > +        }
> >          fuse_reply_buf(req, value, ret);
> >      } else {
> > +        /*
> > +         * xattrmap only ever shortens the result,
> > +         * so we don't need to do anything clever with the
> > +         * allocation length here.
> > +         */
> 
> I don't understand the comment above. We are in the !lo->xattrmap) case, no?

No, we're in the else for if (size) {  for the case where the caller is
trying not to actually get a list of the attributes, but to get a guess
of the size of storage needed.

Dave


> >          fuse_reply_xattr(req, ret);
> >      }
> >  out_free:
> 
> 
> --
> Cheers,
> Christophe de Dinechin (IRC c3d)
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK