Re: [PATCH v5 2/3] fanotify: define struct members to hold response decision context

On Thu 22-12-22 15:47:21, Richard Guy Briggs wrote: > On 2022-12-16 17:43, Jan Kara wrote: > > On Mon 12-12-22 09:06:10, Richard Guy Briggs wrote: > > > This patch adds a flag, FAN_INFO and an extensible buffer to provide > > > additional information about response decisions. The buffer contains > > > one or more headers defining the information type and the length of the > > > following information. The patch defines one additional information > > > type, FAN_RESPONSE_INFO_AUDIT_RULE, to audit a rule number. This will > > > allow for the creation of other information types in the future if other > > > users of the API identify different needs. > > > > > > Suggested-by: Steve Grubb <sgrubb(a)redhat.com&gt; > > > Link: https://lore.kernel.org/r/2745105.e9J7NaK4W3@x2 > > > Suggested-by: Jan Kara <jack(a)suse.cz&gt; > > > Link: https://lore.kernel.org/r/20201001101219.GE17860@quack2.suse.cz > > > Signed-off-by: Richard Guy Briggs <rgb(a)redhat.com&gt;

> > > +{ > > > + if (fd == FAN_NOFD) > > > + return -ENOENT; > > > > I would not test 'fd' in this function at all. After all it is not part of > > the response info structure and you do check it in > > process_access_response() anyway. > > I wrestled with that. I was even tempted to swallow the following fd > check too, but the flow would not have made as much sense for the > non-INFO case. > > My understanding from Amir was that FAN_NOFD was only to be sent in in > conjuction with FAN_INFO to test if a newer kernel was present. Yes, that is correct. But we not only want to check that FAN_INFO flag is understood (as you do in your patch) but also whether a particular response type is understood (which you don't verify for FAN_NOFD). Currently, there is only one response type (FAN_RESPONSE_INFO_AUDIT_RULE) but if there are more in the future we need old kernels to refuse new response types even for FAN_NOFD case.

> I presumed that if FAN_NOFD was present without FAN_INFO that was an > invalid input to an old kernel. Yes, that is correct and I agree the conditions I've suggested below are wrong in that regard and need a bit of tweaking. Thanks for catching it. > > > + > > > + if (info_len != sizeof(*friar)) > > > + return -EINVAL; > > > + > > > + if (copy_from_user(friar, info, sizeof(*friar))) > > > + return -EFAULT; > > > + > > > + if (friar->hdr.type != FAN_RESPONSE_INFO_AUDIT_RULE) > > > + return -EINVAL; > > > + if (friar->hdr.pad != 0) > > > + return -EINVAL; > > > + if (friar->hdr.len != sizeof(*friar)) > > > + return -EINVAL; > > > + > > > + return info_len; > > > +} > > > + > > > > ... > > > > > @@ -327,10 +359,18 @@ static int process_access_response(struct fsnotify_group *group, > > > return -EINVAL; > > > } > > > > > > - if (fd < 0) > > > + if ((response & FAN_AUDIT) && !FAN_GROUP_FLAG(group, FAN_ENABLE_AUDIT)) > > > return -EINVAL; > > > > > > - if ((response & FAN_AUDIT) && !FAN_GROUP_FLAG(group, FAN_ENABLE_AUDIT)) > > > + if (response & FAN_INFO) { > > > + ret = process_access_response_info(fd, info, info_len, &friar); > > > + if (ret < 0) > > > + return ret; > > > + } else { > > > + ret = 0; > > > + } > > > + > > > + if (fd < 0) > > > return -EINVAL; > > > > And here I'd do: > > > > if (fd == FAN_NOFD) > > return 0; > > if (fd < 0) > > return -EINVAL; > > > > As we talked in previous revisions we'd specialcase FAN_NOFD to just verify > > extra info is understood by the kernel so that application writing fanotify > > responses has a way to check which information it can provide to the > > kernel. > > The reason for including it in process_access_response_info() is to make > sure that it is included in the FAN_INFO case to detect this extension. > If it were included here I see what you're getting at now. So the condition if (fd == FAN_NOFD) return 0; needs to be moved into if (response & FAN_INFO) branch after process_access_response_info(). I still prefer to keep it outside of the process_access_response_info() function itself as it looks more logical to me. Does it address your concerns?

Jan Kara <jack(a)suse.com&gt;