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Message-ID: <CAEf4BzaR5WyZ8h_6-0D91wvu2OOHAFAsWTMKEHkUbVqducLy_Q@mail.gmail.com>
Date: Wed, 14 Apr 2021 17:21:35 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 00/17] BPF static linker: support externs
On Wed, Apr 14, 2021 at 1:02 PM Andrii Nakryiko <andrii@...nel.org> wrote:
>
> Add BPF static linker support for extern resolution of global variables,
> functions, and BTF-defined maps.
>
> This patch set consists of 4 parts:
> - few patches are extending bpftool to simplify working with BTF dump;
> - libbpf object loading logic is extended to support __hidden functions and
> overriden (unused) __weak functions; also BTF-defined map parsing logic is
> refactored to be re-used by linker;
> - the crux of the patch set is BPF static linker logic extension to perform
> extern resolution for three categories: global variables, BPF
> sub-programs, BTF-defined maps;
> - a set of selftests that validate that all the combinations of
> extern/weak/__hidden are working as expected.
>
> See respective patches for more details.
>
> One aspect hasn't been addressed yet and is going to be resolved in the next
> patch set, but is worth mentioning. With BPF static linking of multiple .o
> files, dealing with static everything becomes more problematic for BPF
> skeleton and in general for any by name look up APIs. This is due to static
> entities are allowed to have non-unique name. Historically this was never
> a problem due to BPF programs were always confined to a single C file. That
> changes now and needs to be addressed. The thinking so far is for BPF static
> linker to prepend filename to each static variable and static map (which is
> currently not supported by libbpf, btw), so that they can be unambiguously
> resolved by (mostly) unique name. Mostly, because even filenames can be
> duplicated, but that should be rare and easy to address by wiser choice of
> filenames by users. Fortunately, static BPF subprograms don't suffer from this
> issues, as they are not independent entities and are neither exposed in BPF
> skeleton, nor is lookup-able by any of libbpf APIs (and there is little reason
> to do that anyways).
>
> This and few other things will be the topic of the next set of patches.
>
I forgot to mention that selftests are relying on Clang fix [0], which
is not yet in nightly builds, so kernel-patches CI is currently
failing a test. So, when/if testing locally, please make sure to build
Clang from latest main.
[0] https://reviews.llvm.org/D100362
> Andrii Nakryiko (17):
> bpftool: support dumping BTF VAR's "extern" linkage
> bpftool: dump more info about DATASEC members
> libbpf: suppress compiler warning when using SEC() macro with externs
> libbpf: mark BPF subprogs with hidden visibility as static for BPF
> verifier
> libbpf: allow gaps in BPF program sections to support overriden weak
> functions
> libbpf: refactor BTF map definition parsing
> libbpf: factor out symtab and relos sanity checks
> libbpf: make few internal helpers available outside of libbpf.c
> libbpf: extend sanity checking ELF symbols with externs validation
> libbpf: tighten BTF type ID rewriting with error checking
> libbpf: add linker extern resolution support for functions and global
> variables
> libbpf: support extern resolution for BTF-defined maps in .maps
> section
> selftests/bpf: use -O0 instead of -Og in selftests builds
> selftests/bpf: omit skeleton generation for multi-linked BPF object
> files
> selftests/bpf: add function linking selftest
> selftests/bpf: add global variables linking selftest
> sleftests/bpf: add map linking selftest
>
> tools/bpf/bpftool/btf.c | 30 +-
> tools/lib/bpf/bpf_helpers.h | 19 +-
> tools/lib/bpf/btf.c | 5 -
> tools/lib/bpf/libbpf.c | 370 +++--
> tools/lib/bpf/libbpf_internal.h | 45 +
> tools/lib/bpf/linker.c | 1292 ++++++++++++++---
> tools/testing/selftests/bpf/Makefile | 18 +-
> .../selftests/bpf/prog_tests/linked_funcs.c | 42 +
> .../selftests/bpf/prog_tests/linked_maps.c | 33 +
> .../selftests/bpf/prog_tests/linked_vars.c | 43 +
> .../selftests/bpf/progs/linked_funcs1.c | 73 +
> .../selftests/bpf/progs/linked_funcs2.c | 73 +
> .../selftests/bpf/progs/linked_maps1.c | 102 ++
> .../selftests/bpf/progs/linked_maps2.c | 112 ++
> .../selftests/bpf/progs/linked_vars1.c | 60 +
> .../selftests/bpf/progs/linked_vars2.c | 61 +
> 16 files changed, 2028 insertions(+), 350 deletions(-)
> create mode 100644 tools/testing/selftests/bpf/prog_tests/linked_funcs.c
> create mode 100644 tools/testing/selftests/bpf/prog_tests/linked_maps.c
> create mode 100644 tools/testing/selftests/bpf/prog_tests/linked_vars.c
> create mode 100644 tools/testing/selftests/bpf/progs/linked_funcs1.c
> create mode 100644 tools/testing/selftests/bpf/progs/linked_funcs2.c
> create mode 100644 tools/testing/selftests/bpf/progs/linked_maps1.c
> create mode 100644 tools/testing/selftests/bpf/progs/linked_maps2.c
> create mode 100644 tools/testing/selftests/bpf/progs/linked_vars1.c
> create mode 100644 tools/testing/selftests/bpf/progs/linked_vars2.c
>
> --
> 2.30.2
>
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