On Wed, Jul 31, 2019 at 1:30 AM Song Liu <songliubraving@xxxxxx> wrote: > > > > > On Jul 30, 2019, at 11:52 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > > > On Tue, Jul 30, 2019 at 10:19 PM Song Liu <songliubraving@xxxxxx> wrote: > >> > >> > >> > >>> On Jul 30, 2019, at 6:00 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > >>> > >>> On Tue, Jul 30, 2019 at 5:39 PM Song Liu <songliubraving@xxxxxx> wrote: > >>>> > >>>> > >>>> > >>>>> On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@xxxxxx> wrote: > >>>>> > >>>>> This patch implements the core logic for BPF CO-RE offsets relocations. > >>>>> Every instruction that needs to be relocated has corresponding > >>>>> bpf_offset_reloc as part of BTF.ext. Relocations are performed by trying > >>>>> to match recorded "local" relocation spec against potentially many > >>>>> compatible "target" types, creating corresponding spec. Details of the > >>>>> algorithm are noted in corresponding comments in the code. > >>>>> > >>>>> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx> > > [...] > > >>>> > >>> > >>> I just picked the most succinct and non-repetitive form. It's > >>> immediately apparent which type it's implicitly converted to, so I > >>> felt there is no need to repeat it. Also, just (void *) is much > >>> shorter. :) > >> > >> _All_ other code in btf.c converts the pointer to the target type. > > > > Most in libbpf.c doesn't, though. Also, I try to preserve pointer > > constness for uses that don't modify BTF types (pretty much all of > > them in libbpf), so it becomes really verbose, despite extremely short > > variable names: > > > > const struct btf_member *m = (const struct btf_member *)(t + 1); > > I don't think being verbose is a big problem here. Overusing Problem is too big and strong word to describe this :). It hurts readability and will often quite artificially force either wrapping the line or unnecessarily splitting declaration and assignment. Void * on the other hand is short and usually is in the same line as target type declaration, if not, you'll have to find local variable declaration to double-check type, if you are unsure. Using (void *) + implicit cast to target pointer type is not unprecedented in libbpf: $ rg ' = \((const )?struct \w+ \*\)' tools/lib/bpf/ | wc -l 52 $ rg ' = \((const )?void \*\)' tools/lib/bpf/ | wc -l 35 52 vs 35 is majority overall, but not by a landslide. > (void *) feels like a bigger problem. Why do you feel it's a problem? void * conveys that we have a piece of memory that we will need to reinterpret as some concrete pointer type. That's what we are doing, skipping btf_type and then interpreting memory after common btf_type prefix is some other type, depending on actual BTF kind. I don't think void * is misleading in any way. In any case, if you still feel strongly about this after all my arguments, please let me know and I will convert them in this patch set. It's not like I'm opposed to use duplicate type names (though it does feel sort of Java-like before it got limited type inference), it's just in practice it leads to unnecessarily verbose code which doesn't really improve anything. > > > > > Add one or two levels of nestedness and you are wrapping this line. > > > >> In some cases, it is not apparent which type it is converted to, > >> for example: > >> > >> + m = (void *)(targ_type + 1); > >> > >> I would suggest we do implicit conversion whenever possible. > > > > Implicit conversion (`m = targ_type + 1;`) is a compilation error, > > that won't work. > > I misused "implicit" here. I actually meant to say > > m = ((const struct btf_member *)(t + 1); Ah, so you meant explicit, yep. It's either `void *` or `const struct something *` then. > > >
