netdev - [PATCH v2 bpf-next 3/6] libbpf: support static map definitions

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Date:   Fri, 23 Apr 2021 11:53:54 -0700
From:   Andrii Nakryiko <andrii@...nel.org>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>
CC:     <andrii@...nel.org>, <kernel-team@...com>
Subject: [PATCH v2 bpf-next 3/6] libbpf: support static map definitions

Change libbpf relocation logic to support references to static map
definitions. This allows to use static maps and, combined with static linking,
hide internal maps from other files, just like static variables. User-space
will still be able to look up and modify static maps.

Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
---
 tools/lib/bpf/libbpf.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a1cddd17af7d..6c755c5f037d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3633,6 +3633,8 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 
 	/* generic map reference relocation */
 	if (type == LIBBPF_MAP_UNSPEC) {
+		size_t map_offset = sym->st_value + insn->imm;
+
 		if (!bpf_object__shndx_is_maps(obj, shdr_idx)) {
 			pr_warn("prog '%s': bad map relo against '%s' in section '%s'\n",
 				prog->name, sym_name, sym_sec_name);
@@ -3642,7 +3644,7 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 			map = &obj->maps[map_idx];
 			if (map->libbpf_type != type ||
 			    map->sec_idx != sym->st_shndx ||
-			    map->sec_offset != sym->st_value)
+			    map->sec_offset != map_offset)
 				continue;
 			pr_debug("prog '%s': found map %zd (%s, sec %d, off %zu) for insn #%u\n",
 				 prog->name, map_idx, map->name, map->sec_idx,
@@ -3657,7 +3659,8 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 		reloc_desc->type = RELO_LD64;
 		reloc_desc->insn_idx = insn_idx;
 		reloc_desc->map_idx = map_idx;
-		reloc_desc->sym_off = 0; /* sym->st_value determines map_idx */
+		/* sym->st_value + insn->imm determines map_idx */
+		reloc_desc->sym_off = 0;
 		return 0;
 	}
 
-- 
2.30.2