From: Yonghong Song <yhs@fb.com>
To: Andrii Nakryiko <andrii@kernel.org>, <bpf@vger.kernel.org>,
<netdev@vger.kernel.org>, <ast@fb.com>, <daniel@iogearbox.net>
Cc: <kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 05/17] libbpf: allow gaps in BPF program sections to support overriden weak functions
Date: Wed, 21 Apr 2021 23:25:50 -0700 [thread overview]
Message-ID: <71bfd67c-c8f0-595c-e721-201ec4e8e062@fb.com> (raw)
In-Reply-To: <20210416202404.3443623-6-andrii@kernel.org>
On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> Currently libbpf is very strict about parsing BPF program isnstruction
isnstruction => instruction
> sections. No gaps are allowed between sequential BPF programs within a given
> ELF section. Libbpf enforced that by keeping track of the next section offset
> that should start a new BPF (sub)program and cross-checks that by searching for
> a corresponding STT_FUNC ELF symbol.
>
> But this is too restrictive once we allow to have weak BPF programs and link
> together two or more BPF object files. In such case, some weak BPF programs
> might be "overriden" by either non-weak BPF program with the same name and
overriden => overridden
> signature, or even by another weak BPF program that just happened to be linked
> first. That, in turn, leaves BPF instructions of the "lost" BPF (sub)program
> intact, but there is no corresponding ELF symbol, because no one is going to
> be referencing it.
>
> Libbpf already correctly handles such cases in the sense that it won't append
> such dead code to actual BPF programs loaded into kernel. So the only change
> that needs to be done is to relax the logic of parsing BPF instruction
> sections. Instead of assuming next BPF (sub)program section offset, iterate
> available STT_FUNC ELF symbols to discover all available BPF subprograms and
> programs.
>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Ack with a minor suggestion below.
Acked-by: Yonghong Song <yhs@fb.com>
> ---
> tools/lib/bpf/libbpf.c | 56 ++++++++++++++++--------------------------
> 1 file changed, 21 insertions(+), 35 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ce5558d0a61b..a0e6d6bc47f3 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -502,8 +502,6 @@ static Elf_Scn *elf_sec_by_name(const struct bpf_object *obj, const char *name);
> static int elf_sec_hdr(const struct bpf_object *obj, Elf_Scn *scn, GElf_Shdr *hdr);
> static const char *elf_sec_name(const struct bpf_object *obj, Elf_Scn *scn);
> static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn);
> -static int elf_sym_by_sec_off(const struct bpf_object *obj, size_t sec_idx,
> - size_t off, __u32 sym_type, GElf_Sym *sym);
>
> void bpf_program__unload(struct bpf_program *prog)
> {
> @@ -644,10 +642,12 @@ static int
> bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
> const char *sec_name, int sec_idx)
> {
> + Elf_Data *symbols = obj->efile.symbols;
> struct bpf_program *prog, *progs;
> void *data = sec_data->d_buf;
> size_t sec_sz = sec_data->d_size, sec_off, prog_sz;
> - int nr_progs, err;
> + size_t n = symbols->d_size / sizeof(GElf_Sym);
Maybe use "nr_syms" instead of "n" to be more descriptive?
> + int nr_progs, err, i;
> const char *name;
> GElf_Sym sym;
>
> @@ -655,14 +655,16 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
> nr_progs = obj->nr_programs;
> sec_off = 0;
>
> - while (sec_off < sec_sz) {
> - if (elf_sym_by_sec_off(obj, sec_idx, sec_off, STT_FUNC, &sym)) {
> - pr_warn("sec '%s': failed to find program symbol at offset %zu\n",
> - sec_name, sec_off);
> - return -LIBBPF_ERRNO__FORMAT;
> - }
> + for (i = 0; i < n; i++) {
> + if (!gelf_getsym(symbols, i, &sym))
> + continue;
> + if (sym.st_shndx != sec_idx)
> + continue;
> + if (GELF_ST_TYPE(sym.st_info) != STT_FUNC)
> + continue;
>
> prog_sz = sym.st_size;
> + sec_off = sym.st_value;
>
> name = elf_sym_str(obj, sym.st_name);
> if (!name) {
> @@ -711,8 +713,6 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
>
> nr_progs++;
> obj->nr_programs = nr_progs;
> -
> - sec_off += prog_sz;
> }
>
> return 0;
> @@ -2825,26 +2825,6 @@ static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn)
> return data;
> }
>
[...]
next prev parent reply other threads:[~2021-04-22 6:26 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-16 20:23 [PATCH v2 bpf-next 00/17] BPF static linker: support externs Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 01/17] bpftool: support dumping BTF VAR's "extern" linkage Andrii Nakryiko
2021-04-22 3:02 ` Yonghong Song
2021-04-16 20:23 ` [PATCH v2 bpf-next 02/17] bpftool: dump more info about DATASEC members Andrii Nakryiko
2021-04-22 3:09 ` Yonghong Song
2021-04-16 20:23 ` [PATCH v2 bpf-next 03/17] libbpf: suppress compiler warning when using SEC() macro with externs Andrii Nakryiko
2021-04-22 3:47 ` Yonghong Song
2021-04-22 3:59 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 04/17] libbpf: mark BPF subprogs with hidden visibility as static for BPF verifier Andrii Nakryiko
2021-04-22 5:43 ` Yonghong Song
2021-04-22 18:09 ` Andrii Nakryiko
2021-04-22 23:00 ` Yonghong Song
2021-04-22 23:28 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 05/17] libbpf: allow gaps in BPF program sections to support overriden weak functions Andrii Nakryiko
2021-04-22 6:25 ` Yonghong Song [this message]
2021-04-22 18:10 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 06/17] libbpf: refactor BTF map definition parsing Andrii Nakryiko
2021-04-22 15:33 ` Yonghong Song
2021-04-22 18:40 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 07/17] libbpf: factor out symtab and relos sanity checks Andrii Nakryiko
2021-04-22 16:06 ` Yonghong Song
2021-04-22 18:16 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 08/17] libbpf: make few internal helpers available outside of libbpf.c Andrii Nakryiko
2021-04-22 16:19 ` Yonghong Song
2021-04-22 18:18 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 09/17] libbpf: extend sanity checking ELF symbols with externs validation Andrii Nakryiko
2021-04-22 16:35 ` Yonghong Song
2021-04-22 18:20 ` Andrii Nakryiko
2021-04-22 23:13 ` Yonghong Song
2021-04-16 20:23 ` [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting with error checking Andrii Nakryiko
2021-04-22 16:50 ` Yonghong Song
2021-04-22 18:24 ` Andrii Nakryiko
2021-04-23 2:54 ` Alexei Starovoitov
2021-04-23 4:25 ` Andrii Nakryiko
2021-04-23 5:11 ` Alexei Starovoitov
2021-04-23 16:09 ` Andrii Nakryiko
2021-04-23 16:18 ` Alexei Starovoitov
2021-04-23 16:30 ` Andrii Nakryiko
2021-04-23 16:34 ` Alexei Starovoitov
2021-04-23 17:02 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 11/17] libbpf: add linker extern resolution support for functions and global variables Andrii Nakryiko
2021-04-22 21:27 ` Yonghong Song
2021-04-22 22:12 ` Andrii Nakryiko
2021-04-22 23:57 ` Yonghong Song
2021-04-23 2:36 ` Yonghong Song
2021-04-23 4:28 ` Andrii Nakryiko
2021-04-23 4:27 ` Andrii Nakryiko
2021-04-16 20:23 ` [PATCH v2 bpf-next 12/17] libbpf: support extern resolution for BTF-defined maps in .maps section Andrii Nakryiko
2021-04-22 22:56 ` Yonghong Song
2021-04-22 23:32 ` Andrii Nakryiko
2021-04-16 20:24 ` [PATCH v2 bpf-next 13/17] selftests/bpf: use -O0 instead of -Og in selftests builds Andrii Nakryiko
2021-04-23 0:05 ` Yonghong Song
2021-04-16 20:24 ` [PATCH v2 bpf-next 14/17] selftests/bpf: omit skeleton generation for multi-linked BPF object files Andrii Nakryiko
2021-04-23 0:13 ` Yonghong Song
2021-04-16 20:24 ` [PATCH v2 bpf-next 15/17] selftests/bpf: add function linking selftest Andrii Nakryiko
2021-04-23 0:50 ` Yonghong Song
2021-04-23 2:34 ` Alexei Starovoitov
2021-04-23 4:29 ` Andrii Nakryiko
2021-04-23 17:18 ` Andrii Nakryiko
2021-04-23 17:35 ` Yonghong Song
2021-04-23 17:55 ` Andrii Nakryiko
2021-04-23 17:58 ` Yonghong Song
2021-04-23 17:59 ` Andrii Nakryiko
2021-04-16 20:24 ` [PATCH v2 bpf-next 16/17] selftests/bpf: add global variables " Andrii Nakryiko
2021-04-23 1:01 ` Yonghong Song
2021-04-16 20:24 ` [PATCH v2 bpf-next 17/17] sleftests/bpf: add map " Andrii Nakryiko
2021-04-23 1:20 ` Yonghong Song
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