Commit 8a47a6c5 authored Jul 01, 2015 by Wang Nan Committed by Arnaldo Carvalho de Melo Aug 07, 2015

bpf tools: Relocate eBPF programs



If an eBPF program accesses a map, LLVM generates a load instruction
which loads an absolute address into a register, like this:

  ld_64   r1, <MCOperand Expr:(mymap)>
  ...
  call    2

That ld_64 instruction will be recorded in relocation section.
To enable the usage of that map, relocation must be done by replacing
the immediate value by real map file descriptor so it can be found by
eBPF map functions.

This patch to the relocation work based on information collected by
patches:

'bpf tools: Collect symbol table from SHT_SYMTAB section',
'bpf tools: Collect relocation sections from SHT_REL sections'
and
'bpf tools: Record map accessing instructions for each program'.

For each instruction which needs relocation, it inject corresponding
file descriptor to imm field. As a part of protocol, src_reg is set to
BPF_PSEUDO_MAP_FD to notify kernel this is a map loading instruction.

This is the final part of map relocation patch. The principle of map
relocation is described in commit message of 'bpf tools: Collect symbol
table from SHT_SYMTAB section'.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kaixu Xia <xiakaixu@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1435716878-189507-18-git-send-email-wangnan0@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

parent 52d3352e

tools/lib/bpf/libbpf.c

+52 −0

Original line number	Diff line number	Diff line
		@@ -630,6 +630,56 @@ bpf_object__create_maps(struct bpf_object *obj)
		return 0;
		}

		static int
		bpf_program__relocate(struct bpf_program prog, int map_fds)
		{
		int i;

		if (!prog \|\| !prog->reloc_desc)
		return 0;

		for (i = 0; i < prog->nr_reloc; i++) {
		int insn_idx, map_idx;
		struct bpf_insn *insns = prog->insns;

		insn_idx = prog->reloc_desc[i].insn_idx;
		map_idx = prog->reloc_desc[i].map_idx;

		if (insn_idx >= (int)prog->insns_cnt) {
		pr_warning("relocation out of range: '%s'\n",
		prog->section_name);
		return -ERANGE;
		}
		insns[insn_idx].src_reg = BPF_PSEUDO_MAP_FD;
		insns[insn_idx].imm = map_fds[map_idx];
		}

		zfree(&prog->reloc_desc);
		prog->nr_reloc = 0;
		return 0;
		}


		static int
		bpf_object__relocate(struct bpf_object *obj)
		{
		struct bpf_program *prog;
		size_t i;
		int err;

		for (i = 0; i < obj->nr_programs; i++) {
		prog = &obj->programs[i];

		err = bpf_program__relocate(prog, obj->map_fds);
		if (err) {
		pr_warning("failed to relocate '%s'\n",
		prog->section_name);
		return err;
		}
		}
		return 0;
		}

		static int bpf_object__collect_reloc(struct bpf_object *obj)
		{
		int i, err;
		@@ -761,6 +811,8 @@ int bpf_object__load(struct bpf_object *obj)
		obj->loaded = true;
		if (bpf_object__create_maps(obj))
		goto out;
		if (bpf_object__relocate(obj))
		goto out;

		return 0;
		out:

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