[PATCH bpf-next 3/4] libbpf: Ignore relocations for .jumptables sections

[Puranjay Mohan <puranjay@kernel.org>]

.jumptables sections generated by LLVM store byte offsets from the start
of the section to the jump's target instruction. On arm64 builds of
LLVM, it generates a rel section to add the section address to each
entry in the jump table, therefore storing the absolute address of the
jump target into the table. For BPF usage of .jumptables, we only care
about the instruction offsets and calculate the final address much later
after the JIT has finished, and called bpf_prog_update_insn_ptrs()

Without the change in this commit, compilation fails with:

libbpf: relocation against STT_SECTION in non-exec section is not
supported!  Error: failed to link
'tools/testing/selftests/bpf/cpuv4/bpf_gotox.bpf.o': Invalid argument (22)

This is due to the presence of a relocation section for .jumptables
(non-exec) against syscall (STT_SECTION):

Relocation section '.rel.jumptables' at offset 0x5b50 contains 263 entries:
  Offset        Info             Type         Symbol's Value  Symbol's Name
000000000000  0000000300000002 R_BPF_64_ABS64  0000000000000000 syscall
000000000008  0000000300000002 R_BPF_64_ABS64  0000000000000000 syscall
[...]

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 tools/lib/bpf/linker.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index f4403e3cf994..f4295962983b 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -634,6 +634,10 @@ static bool is_ignored_sec(struct src_sec *sec)
 		if (strcmp(name, BTF_ELF_SEC) == 0 ||
 		    strcmp(name, BTF_EXT_ELF_SEC) == 0)
 			return true;
+
+		/* .jumptables don't need relocations */
+		if (strcmp(name, JUMPTABLES_SEC) == 0)
+			return true;
 	}
 
 	return false;
-- 
2.47.3