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

[Joe Stringer]

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 <j...@wand.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