Re: [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting with error checking

On Thu, Apr 22, 2021 at 9:50 AM Yonghong Song [off-list ref] wrote:

On 4/16/21 1:23 PM, Andrii Nakryiko wrote:

quoted

It should never fail, but if it does, it's better to know about this rather
than end up with nonsensical type IDs.

So this is defensive programming. Maybe do another round of
audit of the callers and if you didn't find any issue, you
do not need to check not-happening condition here?

It's far from obvious that this will never happen, because we do a
decently complicated BTF processing (we skip some types altogether
believing that they are not used, for example) and it will only get
more complicated with time. Just as there are "verifier bug" checks in
kernel, this prevents things from going wild if non-trivial bugs will
inevitably happen.

quoted

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
  tools/lib/bpf/linker.c | 9 +++++++++
  1 file changed, 9 insertions(+)

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 283249df9831..d5dc1d401f57 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c

@@ -1423,6 +1423,15 @@ static int linker_fixup_btf(struct src_obj *obj)
  static int remap_type_id(__u32 *type_id, void *ctx)
  {
      int *id_map = ctx;
+     int new_id = id_map[*type_id];
+
+     if (*type_id == 0)
+             return 0;
+
+     if (new_id == 0) {
+             pr_warn("failed to find new ID mapping for original BTF type ID %u\n", *type_id);
+             return -EINVAL;
+     }

      *type_id = id_map[*type_id];