Messages in this thread | | | Date | Thu, 18 Sep 2014 08:44:05 +0200 | From | Daniel Borkmann <> | Subject | Re: [PATCH v13 net-next 07/11] bpf: verifier (add ability to receive verification log) |
| |
On 09/18/2014 01:45 AM, Alexei Starovoitov wrote: > On Wed, Sep 17, 2014 at 12:37 PM, Alexei Starovoitov <ast@plumgrid.com> 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.
| |