On Fri 08-12-23 10:01:35, Amir Goldstein wrote: > With FAN_DENY response, user trying to perform the filesystem operation > gets an error with errno set to EPERM. > > It is useful for hierarchical storage management (HSM) service to be able > to deny access for reasons more diverse than EPERM, for example EAGAIN, > if HSM could retry the operation later. > > Allow userspace to response to permission events with the response value > FAN_DENY_ERRNO(errno), instead of FAN_DENY to return a custom error. > > The change in fanotify_response is backward compatible, because errno is > written in the high 8 bits of the 32bit response field and old kernels > reject respose value with high bits set. > > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx> So a couple of comments that spring to my mind when I'm looking into this now (partly maybe due to my weak memory ;): 1) Do we still need the EAGAIN return? I think we have mostly dealt with freezing deadlocks in another way, didn't we? 2) If answer to 1) is yes, then there is a second question - do we expect the errors to propagate back to the unsuspecting application doing say read(2) syscall? Because I don't think that will fly well with a big majority of applications which basically treat *any* error from read(2) as fatal. This is also related to your question about standard permission events. Consumers of these error numbers are going to be random applications and I see a potential for rather big confusion arising there (like read(1) returning EINVAL or EBADF and now you wonder why the hell until you go debug the kernel and find out the error is coming out of fanotify handler). And the usecase is not quite clear to me for ordinary fanotify permission events (while I have no doubts about creativity of implementors of fanotify handlers ;)). 3) Given the potential for confusion, maybe we should stay conservative and only allow additional EAGAIN error instead of arbitrary errno if we need it? I'm leaving the API question aside for a moment until I have a clearer picture of what we actually want to implement :). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
