On Wed, Sep 17, 2014 at 11:44 PM, Daniel Borkmann <dborkman@xxxxxxxxxx> wrote: > On 09/18/2014 01:45 AM, Alexei Starovoitov wrote: >> >> On Wed, Sep 17, 2014 at 12:37 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> >> wrote: >>> >>> >>>> Hm, thinking out loudly ... perhaps this could be made a library >>>> problem. >>>> Such that the library which wraps the syscall needs to be aware of a >>>> marker where the initial version ends, and if the application doesn't >>>> make use of any of the new features, it would just pass in the length up >>>> to the marker as size attribute into the syscall. Similarly, if new >>>> features are always added to the end of a structure and the library >>>> truncates the overall-length after the last used member we might have >>>> a chance to load something on older kernels, haven't tried that though. >>> >>> >>> that's a 3rd option. I think it's cleaner than 2nd, since it solves it >>> completely from user space. >>> It can even be smarter than that. If this syscall wrapper library >>> sees that newer features are used and it can workaround them: >>> it can chop size and pass older fields into the older kernel >>> and when it returns, do a workaround based on newer fields. >> >> >> the more I think about 'new user space + old kernel' problem, >> the more certain I am that kernel should not try to help >> user space, since most of the time it's not going to be enough, >> but additional code in kernel would need to be maintained. >> >> syscall commands and size of bpf_attr is the least of problems. >> New map_type and prog_type will be added, new helper >> functions will be available to programs. >> One would think that md5 of uapi/linux/bpf.h would be >> enough to say that user app is compatible... In reality, >> it's not. The 'state pruning' verifier optimization I've talked >> about will not change a single bit in bpf.h, but it will be >> able to recognize more programs as safe. >> A program developed on a new kernel with more >> advanced verifier will load just fine on new kernel, but >> this valid program will not load on old kernel, only because >> verifier is not smart enough. Now we would need a version >> of verifier exposed all the way to user space?! >> imo that's too much. I think for eBPF infra kernel >> should only guarantee backwards compatibility >> (old user space must work with new kernel) and that's it. >> That's what this patch is trying to do. >> Thoughts? > > > Sure, you will never get a full compatibility on that regard > while backwards compatibility needs to be guaranteed on the > other hand. I looked at perf_copy_attr() implementation and I > think that we should mimic it in a very similar way as it > exactly solves what we need. > > For example, it will return with -EINVAL for (size > PAGE_SIZE) > and (size < PERF_ATTR_SIZE_VER0) where PAGE_SIZE has been chosen > as an arbitrary hard upper limit where it is believed that it will > never grow beyond that large limit in future. > > So this is a more loose constraint than what we currently do, > that is, -EINVAL on (size > sizeof(attr)) where attr is the > currently known size of a specific kernel. That would at least > be a start, you won't be able to cover everything though, but > it would allow to address the issue raised when running with > a basic feature set. you missed my point. We should not 'do a start', since it doesn't help user space in the long run and only makes kernel more complex. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
