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From: Hao Luo <haoluo@google.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Song Liu <songliubraving@fb.com>,
	Quentin Monnet <quentin@isovalent.com>
Subject: Re: [RFC PATCH bpf-next 2/8] libbpf: add support for extracting kernel symbol addresses
Date: Tue, 16 Jun 2020 01:05:56 -0700	[thread overview]
Message-ID: <CA+khW7iU4oT3N2fYK6ym7XtWAnyD4fmiMpkuNybrJSROJeuk8A@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzYVY-sA_SRqxr-dxrkR5DPW6tv3tnNonK=4WPx6eEiZFQ@mail.gmail.com>

On Mon, Jun 15, 2020 at 12:08 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
> On Mon, Jun 15, 2020 at 9:44 AM Hao Luo <haoluo@google.com> wrote:
> >
> > Thanks, Andrii,
> >
> > This change looks nice! A couple of comments:
> >
> > 1. A 'void' type variable looks slightly odd from a user's perspective. How about using 'u64' or 'void *'? Or at least, a named type, which aliases to 'void'?
> That choice is very deliberate one. `extern const void` is the right
> way in C language to access linker-generated symbols, for instance,
> which is quite similar to what the intent is her. Having void type is
> very explicit that you don't know/care about that value pointed to by
> extern address, the only operation you can perform is to get it's
> address.
> Once we add kernel variables support, that's when types will start to
> be specified and libbpf will do extra checks (type matching) and extra
> work (generating ldimm64 with BTF ID, for instance), to allow C code
> to access data pointed to by extern address.
> Switching type to u64 would be misleading in allowing C code to
> implicitly dereference value of extern. E.g., there is a big
> difference between:
> extern u64 bla;
> printf("%lld\n", bla); /* de-reference happens here, we get contents
> of memory pointed to by "bla" symbol */
> printf("%p\n", &bla); /* here we get value of linker symbol/address of
> extern variable */
> Currently I explicitly support only the latter and want to prevent the
> former, until we have kernel variables in BTF. Using `extern void`
> makes compiler enforce that only the &bla form is allowed. Everything
> else is compilation error.

Ah, I see. I've been taking the extern variable as an actual variable
that contains the symbol's address, which is the first case. Your
approach makes sense. Thanks for explaining.

> > 2. About the type size of ksym, IIUC, it looks strange that the values read from kallsyms have 8 bytes but their corresponding vs->size is 4 bytes and vs->type points to 4-byte int. Can we make them of the same size?
> That's a bit of a hack on my part. Variable needs to point to some
> type, which size will match the size of datasec's varinfo entry. This
> is checked and enforced by kernel. I'm looking for 4-byte int, because
> it's almost guaranteed that it will be present in program's BTF and I
> won't have to explicitly add it (it's because all BPF programs return
> int, so it must be in program's BTF already). While 8-byte long is
> less likely to be there.
> In the future, if we have a nicer way to extend BTF (and we will
> soon), we can do this a bit better, but either way that .ksyms DATASEC
> type isn't used for anything (there is no map with that DATASEC as a
> value type), so it doesn't matter.

Thanks for explaining, Andrii.

These explanations as comments in the code would be quite helpful, IMHO.

  reply	other threads:[~2020-06-16  8:06 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-12 22:31 [RFC PATCH bpf-next 0/8] libbpf ksym support and bpftool show PIDs Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 1/8] libbpf: generalize libbpf externs support Andrii Nakryiko
     [not found]   ` <CA+khW7hAYVdoQX5-j0z1iGEVZeww4BBu4NXzy5eS5OwDRYqe2w@mail.gmail.com>
2020-06-15 18:55     ` Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 2/8] libbpf: add support for extracting kernel symbol addresses Andrii Nakryiko
     [not found]   ` <CA+khW7hFZzp_K_xydSFw0O3LYB22_fC=Z4wG7i9Si+phGHn4cQ@mail.gmail.com>
2020-06-15 19:08     ` Andrii Nakryiko
2020-06-16  8:05       ` Hao Luo [this message]
2020-06-17  1:24         ` Hao Luo
2020-06-17  1:36           ` Andrii Nakryiko
2020-06-18  7:53             ` Hao Luo
2020-06-12 22:31 ` [RFC PATCH bpf-next 3/8] selftests/bpf: add __ksym extern selftest Andrii Nakryiko
     [not found]   ` <CA+khW7jxdS1KRpk2syVGjDqbyn3wAd3Eh_LEMAEhkPUehuXMwg@mail.gmail.com>
2020-06-15 19:11     ` Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 4/8] tools/bpftool: move map/prog parsing logic into common Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 5/8] tools/bpftool: minimize bootstrap bpftool Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 6/8] tools/bpftool: generalize BPF skeleton support and generate vmlinux.h Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 7/8] libbpf: wrap source argument of BPF_CORE_READ macro in parentheses Andrii Nakryiko
2020-06-12 22:31 ` [RFC PATCH bpf-next 8/8] tools/bpftool: show PIDs with FDs open against BPF map/prog/link/btf Andrii Nakryiko
2020-06-13  3:45   ` Alexei Starovoitov
2020-06-13  5:57     ` Andrii Nakryiko
2020-06-13 22:14       ` Arnaldo Carvalho de Melo
2020-06-15  9:04         ` Toke Høiland-Jørgensen
2020-06-15  9:30           ` Quentin Monnet

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