[RFC PATCH bpf-next 01/12] libbpf: Prepare bpf SDT probe section for the linker

[Xu Kuohai]

From: Xu Kuohai <xukuohai@xxxxxxxxxx>

The follow up patch introduces bpf SDT probe macros that emit per-probe
metadata into a new data section named .bpf_sdt_notes. Each entry begins
with a ___sdt_jt_* symbol and contains an R_BPF_64_ABS64 relocation
referencing the probe site address in the bpf prog.

Prepare the linker for this by treating .bpf_sdt_notes like .jumptables.
Bypass the section-type check when appending ___sdt_jt_* symbols so they
can survive from linking, and relocate to real address inside each entry.

Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
---
 tools/lib/bpf/libbpf_internal.h | 1 +
 tools/lib/bpf/linker.c          | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 04cd303fb5a8..ed7a587a61ca 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -75,6 +75,7 @@
 #endif
 
 #define JUMPTABLES_SEC ".jumptables"
+#define SDT_NOTES_SEC ".bpf_sdt_notes"
 
 #define BTF_INFO_ENC(kind, kind_flag, vlen) \
 	((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 78f92c39290a..3a72276244c7 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -2028,6 +2028,8 @@ static int linker_append_elf_sym(struct bpf_linker *linker, struct src_obj *obj,
 
 		if (strcmp(src_sec->sec_name, JUMPTABLES_SEC) == 0)
 			goto add_sym;
+		if (strcmp(src_sec->sec_name, SDT_NOTES_SEC) == 0)
+			goto add_sym;
 	}
 
 	if (sym_bind == STB_LOCAL)
@@ -2274,6 +2276,13 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
 						insn->imm += sec->dst_off / sizeof(struct bpf_insn);
 					else
 						insn->imm += sec->dst_off;
+				} else if (strcmp(src_linked_sec->sec_name, SDT_NOTES_SEC) == 0) {
+					/* .bpf_sdt_notes contains R_BPF_64_ABS64 relocations
+					 * referencing code section symbols. Adjust the 64-bit
+					 * address by dst_off.
+					 */
+					__u64 *addr = dst_linked_sec->raw_data + dst_rel->r_offset;
+					*addr += sec->dst_off;
 				} else {
 					pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
 					return -EINVAL;
-- 
2.47.3