On Mon, Jan 09, 2023 at 04:47:54AM IST, Alexei Starovoitov wrote: > On Fri, Jan 6, 2023 at 6:09 PM David Vernet <void@xxxxxxxxxxxxx> wrote: > > > > 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. > > I meant to use bpf_kfunc_start/bpf_kfunc_end around every kfunc. > Ideally bpf_kfunc_start would be on the same line as func proto > for nice grepping. > Maybe it's an overkill. > Maybe 3 macroses then? > bpf_kfunc_start to hide __diag > bpf_kfunc on the proto line > bpf_kfunc_end to finish __diag_pop > There's also the option of doing this: #define BPF_KFUNC(proto) proto; __used noinline proto BPF_KFUNC(void kfunc(arg1, arg2)) { ... } No need to disable the warning with diag push/pop, just put a declaration before the definition to silence the compiler. The only awkward part is entire function prototype becoming a macro argument (unlike the common case void MACRO(...)) but it becomes less noisy and easy to grep as well.
