On 3/24/21 10:11 PM, Andrii Nakryiko wrote:
Ensure that BPF static linker preserves all DATASEC BTF types, even if some of
them might not have any variable information at all. It's not completely clear
in which cases Clang chooses to not emit variable information, so adding
reliable repro is hard. But manual testing showed that this work correctly.
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
you will find:
.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
Reported-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support")
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Ack with a nit below.
Acked-by: Yonghong Song <yhs@xxxxxx>
---
tools/lib/bpf/linker.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 5e0aa2f2c0ca..2c43943da30c 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 +1437,15 @@ 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. It seems to happen sometimes when BPF object
+ * file has only static variables inside functions, not
+ * globally, that DATASEC BTF with zero variables will be
+ * emitted by Clang. We need to preserve such empty BTF and
Maybe give a more specific example here, e.g.,
For example, these static variables may be generated by the compiler
by promoting local array/structure variable initial values.
+ * 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 +1727,7 @@ 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);
