On Fri, Jan 06, 2023 at 05:04:02PM -0800, Alexei Starovoitov wrote: > On Fri, Jan 6, 2023 at 11:51 AM David Vernet <void@xxxxxxxxxxxxx> wrote: > > > > kfuncs are functions defined in the kernel, which may be invoked by BPF > > programs. They may or may not also be used as regular kernel functions, > > implying that they may be static (in which case the compiler could e.g. > > inline it away), or it could have external linkage, but potentially be > > elided in an LTO build if a function is observed to never be used, and > > is stripped from the final kernel binary. > > > > We therefore require some convenience macro that kfunc developers can > > use just add to their kfuncs, and which will prevent all of the above > > issues from happening. This is in contrast with what we have today, > > where some kfunc definitions have "noinline", some have "__used", and > > others are static and have neither. > > > > In addition to providing the obvious correctness benefits, having such a > > macro / tag also provides the following advantages: > > > > - Giving an easy and intuitive thing to query for if people are looking > > for kfuncs, as Christoph suggested at the kernel maintainers summit > > (https://lwn.net/Articles/908464/). This is currently possible by > > grepping for BTF_ID_FLAGS(func, but having something more self > > describing would be useful as well. > > > > - In the future, the tag can be expanded with other useful things such > > as the ability to suppress -Wmissing-prototype for the kfuncs rather > > than requiring developers to surround the kfunc with __diags to > > suppress the warning (this requires compiler support that as far as I > > know currently does not exist). > > Have you considered doing bpf_kfunc_start/bpf_kfunc_end ? > The former would include: > __diag_push(); __diag_ignore_all(); __used noinline Yeah that's certainly an option. The downside is that all functions within scope of the __diag_push() will be affected, and sometimes we mix kfuncs with non-kfuncs (including e.g. static helper functions that are used by the kfuncs themselves). -Wmissing-prototypes isn't a big deal, but __used and noinline are kind of unfortunate. Not a big deal though, it'll just result in a few extra __bpf_kfuncs_start() and __bpf_kfuncs_end() sprinkled throughout to avoid them being included. The upside is of course that we can get rid of the __diag_push()'es we currently have to prevent -Wmissing-prototypes. Wdyt? I do like the idea of getting rid of those ugly __diag_push()'es. And we could always go back to using a __bpf_kfunc macro if and when compilers ever support using attributes to ignore warnings for specific functions. > > Also how about using bpf_kfunc on the same line ? > Then 'git grep' will be easier. Sure, if we keep this approach I'll do this in v2.
