From 36e7985160782bc683001afe09e33a288435def0 Mon Sep 17 00:00:00 2001 From: Andrii Nakryiko Date: Thu, 25 Mar 2021 21:30:36 -0700 Subject: [PATCH] libbpf: Preserve empty DATASEC BTFs during static linking Ensure that BPF static linker preserves all DATASEC BTF types, even if some of them might not have any variable information at all. This may happen if the compiler promotes local initialized variable contents into .rodata section and there are no global or static functions in the program. For example, $ cat t.c struct t { char a; char b; char c; }; void bar(struct t*); void find() { struct t tmp = {1, 2, 3}; bar(&tmp); } $ clang -target bpf -O2 -g -S t.c .long 104 # BTF_KIND_DATASEC(id = 8) .long 251658240 # 0xf000000 .long 0 .ascii ".rodata" # string offset=104 $ clang -target bpf -O2 -g -c t.c $ readelf -S t.o | grep data [ 4] .rodata PROGBITS 0000000000000000 00000090 Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support") Reported-by: Alexei Starovoitov Signed-off-by: Andrii Nakryiko Signed-off-by: Daniel Borkmann Acked-by: Yonghong Song Link: https://lore.kernel.org/bpf/20210326043036.3081011-1-andrii@kernel.org --- tools/lib/bpf/linker.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c index 5e0aa2f2c0ca4..a29d62ff8041c 100644 --- a/tools/lib/bpf/linker.c +++ b/tools/lib/bpf/linker.c @@ -94,6 +94,7 @@ struct dst_sec { int sec_sym_idx; /* section's DATASEC variable info, emitted on BTF finalization */ + bool has_btf; int sec_var_cnt; struct btf_var_secinfo *sec_vars; @@ -1436,6 +1437,16 @@ static int linker_append_btf(struct bpf_linker *linker, struct src_obj *obj) continue; dst_sec = &linker->secs[src_sec->dst_id]; + /* Mark section as having BTF regardless of the presence of + * variables. In some cases compiler might generate empty BTF + * with no variables information. E.g., when promoting local + * array/structure variable initial values and BPF object + * file otherwise has no read-only static variables in + * .rodata. We need to preserve such empty BTF and just set + * correct section size. + */ + dst_sec->has_btf = true; + t = btf__type_by_id(obj->btf, src_sec->sec_type_id); src_var = btf_var_secinfos(t); n = btf_vlen(t); @@ -1717,7 +1728,7 @@ static int finalize_btf(struct bpf_linker *linker) for (i = 1; i < linker->sec_cnt; i++) { struct dst_sec *sec = &linker->secs[i]; - if (!sec->sec_var_cnt) + if (!sec->has_btf) continue; id = btf__add_datasec(btf, sec->sec_name, sec->sec_sz); -- 2.30.2