Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx> writes: [...] >> IIUC, the bpf_timer callback is just a function (subprog) from the >> verifier PoV, so it is verified as whatever program type is creating the >> timer. So in other words, as long as you setup the timer from inside a >> tracing prog type, you should have access to all the same kfuncs, I >> think? > > Yep, you are correct. But as mentioned above, I am now in trouble with > the sleepable state: > - I need to call timer_start() from a non sleepable tracing function > (I'm in hard IRQ when dealing with a physical device) > - but then, ideally, the callback function needs to be tagged as a > sleepable one, so I can export my kfuncs which are doing kzalloc and > device IO as such. > > However, I can not really teach the BPF verifier to do so: > - it seems to check for the callback first when it is loaded, and > there is no SEC() equivalent for static functions > - libbpf doesn't have access to the callback as a prog as it has to be > a static function, and thus isn't exported as a full-blown prog. > - the verifier only checks for the callback when dealing with > BPF_FUNC_timer_set_callback, which doesn't have a "flag" argument > (though the validation of the callback has already been done while > checking it first, so we are already too late to change the sleppable > state of the callback) > > Right now, the only OK-ish version I have is declaring the kfunc as > non-sleepable, but checking that we are in a different context than > the IRQ of the initial event. This way, I am not crashing if this > function is called from the initial IRQ, but will still crash if used > outside of the hid context. > > This is not satisfactory, but I feel like it's going to be hard to > teach the verifier that the callback function is sleepable in that > case (maybe we could suffix the callback name, like we do for > arguments, but this is not very clean either). The callback is only set once when the timer is first setup; I *think* it works to do the setup (bpf_timer_init() and bpf_timer_set_callback()) in the context you need (from a sleepable prog), but do the arming (bpf_timer_start()) from a different program that is not itself sleepable? -Toke
