Re: [Libguestfs] libnbd: When are callbacks freed

On Wed, Jul 12, 2023 at 03:19:59PM +0000, Tage Johansson wrote: Can you paste the valgrind output showing the leak? It is important to view the generated lib/api.c. Note that nbd_aio_opt_info() unconditionally calls any remaining free callback prior to returning -1. If you called nbd_aio_opt_info() from the wrong state, we never even get to the nbd_unlocked_aio_opt_info() code in lib/opt.c. So the only thing we really have to worry about is if we have any exit paths where we have claimed ownership of the callback by transferring it elsewhere (that is, where we called SET_CALLBACK_TO_NULL because we have set up h->opt_cb) but still return -1 without our claimed ownership still having a reachable path for still calling the callback at the right time. True at this point, but it also has not been NULL'd. But nbd_unlocked_aio_opt_list() can ONLY return -1 if fails prior to the SET_CALLBACK_TO_NULL lines, so we also know *list still contains the free callback, and the wrapper nbd_opt_list() in lib/api.c WILL call the callback one function up in the call stack, so there is no leak here. [meta-comment: I find that when repling inline, which is common practice in patch reviews, it helps if I include a blank line before and after anything I inject, so my reply text doesn't get overlooked when scanning the wall of quoted text] But here, we've explicitly taken ownership of *list (that is, successful nbd_unlocked_aio_opt_list() transferred the callback over to h->opt_cb.fn.list), so if wait_for_option() does not free the callback, then we must do so or we indeed have a bug. But to actually figure that out, I have to inspect the state machine, because wait_for_option() itself does not currently do anything with the callbacks, on success or on failure. /me goes and digs further... I _do_ see that nbd_internal_free_option() calls the appropriate callbacks. So the question is if we can guarantee that h->opt_cb was set, do we always reach nbd_internal_free_option() prior to nbd_unlocked_poll() progressing the state machine to the point that we either succeeded or transitioned out of state_connecting. When I look over the older lib/rw.c, I remember spending quite a bit of time patching it to get the handling correct. Using nbd_unlocked_pread_structured as the comparison: if nbd_unlocked_aio_pread_structured() returns -1, then either *chunk has not yet been set to NULL (so the api.c nbd_pread_structured will call the free callback), or the code ensured that the transfer semantics happened (and now the state machine will take care of calling the free callback at the right point in time on cmd->cb). There's even code in nbd_internal_common_command() that frees the callback before returning -1 when the command could not be queued to the state machine. I even remember how much time I spent writing tests/test-errors.c (which Rich then later subdivided into finer-grained tests) covering various aspects (failure when called in the wrong state, failure with parameter validation before contacting the server, failure returned by the server, ...) and that each of them didn't leak. But I would not be surprised if my more recent additions of lib/opt.c missed some steps (unlike commands, you don't have parallel options in flight, so the code is simpler - but maybe I simplified too much), where we could have a leak. At any rate, I could argue that in the synchronous nbd_unlocked_opt functions, we should probably be able to assert that the callback was already NULL'd by the successful nbd_unlocked_aio_opt counterpart on success, rather than manually trying to re-NULL it ourselves. The synchronous calls should ALWAYS call the free callback by the time they complete, whether on success or on error. If they don't, that's a bug. Looks like I should expand tests/opt-list.c to pass in a free callback, to ensure that it is reached the correct number of times. It should always be done by libnbd. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org