netdev - [PATCH bpf-next 3/4] libbpf: Support relocations for bss.

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Date:   Mon, 11 Feb 2019 16:47:28 -0800
From:   Joe Stringer <joe@...d.net.nz>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, daniel@...earbox.net, ast@...nel.org
Subject: [PATCH bpf-next 3/4] libbpf: Support relocations for bss.

The BSS section in an ELF generated by LLVM represents constants for
uninitialized variables or variables that are configured with a zero
value. Support initializing zeroed static data by parsing the
relocations with references to the .bss section and zeroing them.

Signed-off-by: Joe Stringer <joe@...d.net.nz>
---
 tools/lib/bpf/libbpf.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index da35d5559b22..ff66d7e970c9 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -141,6 +141,7 @@ struct bpf_program {
 			RELO_LD64,
 			RELO_CALL,
 			RELO_DATA,
+			RELO_ZERO,
 		} type;
 		int insn_idx;
 		union {
@@ -222,6 +223,7 @@ struct bpf_object {
 		int maps_shndx;
 		int text_shndx;
 		int data_shndx;
+		int bss_shndx;
 	} efile;
 	/*
 	 * All loaded bpf_object is linked in a list, which is
@@ -901,6 +903,8 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 				obj->efile.reloc[n].shdr = sh;
 				obj->efile.reloc[n].data = data;
 			}
+		} else if (sh.sh_type == SHT_NOBITS && strcmp(name, ".bss") == 0) {
+			obj->efile.bss_shndx = idx;
 		} else {
 			pr_debug("skip section(%d) %s\n", idx, name);
 		}
@@ -971,6 +975,7 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 	int text_shndx = obj->efile.text_shndx;
 	int maps_shndx = obj->efile.maps_shndx;
 	int data_shndx = obj->efile.data_shndx;
+	int bss_shndx = obj->efile.bss_shndx;
 	struct bpf_map *maps = obj->maps;
 	size_t nr_maps = obj->nr_maps;
 	int i, nrels;
@@ -1010,7 +1015,7 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 			 (long long) sym.st_value, sym.st_name);
 
 		if (sym.st_shndx != maps_shndx && sym.st_shndx != text_shndx &&
-		    sym.st_shndx != data_shndx) {
+		    sym.st_shndx != data_shndx && sym.st_shndx != bss_shndx) {
 			pr_warning("Program '%s' contains unrecognized relo data pointing to section %u\n",
 				   prog->section_name, sym.st_shndx);
 			return -LIBBPF_ERRNO__RELOC;
@@ -1070,6 +1075,9 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 			prog->reloc_desc[i].type = RELO_DATA;
 			prog->reloc_desc[i].insn_idx = insn_idx;
 			prog->reloc_desc[i].data = *static_data;
+		} else if (sym.st_shndx == bss_shndx) {
+			prog->reloc_desc[i].type = RELO_ZERO;
+			prog->reloc_desc[i].insn_idx = insn_idx;
 		}
 	}
 	return 0;
@@ -1429,6 +1437,10 @@ bpf_program__relocate(struct bpf_program *prog, struct bpf_object *obj)
 
 			insn_idx = prog->reloc_desc[i].insn_idx;
 			insns[insn_idx].imm = prog->reloc_desc[i].data;
+		} else if (prog->reloc_desc[i].type == RELO_ZERO) {
+			int insn_idx = prog->reloc_desc[i].insn_idx;
+
+			prog->insns[insn_idx].imm = 0;
 		}
 	}
 
-- 
2.19.1