[PATCH bpf-next v2 2/3] bpftool: Compute light skeleton ctx.sz without relying on links

[Thiébaud Weksteen]

When generating a light skeleton, gen_trace() unconditionally sets
skel->ctx.sz using (char *)&skel->links - (char *)skel. However, if a
BPF object has no programs and no struct_ops maps (prog_cnt +
attach_map_cnt == 0), do_skeleton() omits the links struct, causing the
generated skeleton header to fail compilation.

Compute skel->ctx.sz from the end of progs (if prog_cnt > 0), maps (if
map_cnt > 0), or ctx instead of relying on skel->links. Also remove the
unused opts.data_sz argument passed to codegen().

Fixes: d510296d331a ("bpftool: Use syscall/loader program in "prog load" and 
"gen skeleton" command.")
Signed-off-by: Thiébaud Weksteen <tw...@google.com>
---
 tools/bpf/bpftool/gen.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index e9a1a018f270..4a93c392857a 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -711,7 +711,7 @@ static void codegen_destroy(struct bpf_object *obj, const 
char *obj_name)
 }
 
 static int gen_trace(struct bpf_object *obj, const char *obj_name, const char 
*header_guard,
-                    const struct gen_loader_opts *opts)
+                    const struct gen_loader_opts *opts, size_t prog_cnt, 
size_t map_cnt)
 {
        struct bpf_load_and_run_opts sopts = {};
        char sig_buf[MAX_SIG_SIZE];
@@ -746,9 +746,16 @@ static int gen_trace(struct bpf_object *obj, const char 
*obj_name, const char *h
                        skel = (struct %1$s *)skel_alloc(sizeof(*skel));    \n\
                        if (!skel)                                          \n\
                                goto cleanup;                               \n\
-                       skel->ctx.sz = (char *)&skel->links - (char *)skel; \n\
                ",
-               obj_name, opts->data_sz);
+               obj_name);
+       if (prog_cnt)
+               printf("\tskel->ctx.sz = (char *)&skel->progs - (char *)skel\n"
+                      "\t\t       + sizeof(skel->progs);\n");
+       else if (map_cnt)
+               printf("\tskel->ctx.sz = (char *)&skel->maps - (char *)skel\n"
+                      "\t\t       + sizeof(skel->maps);\n");
+       else
+               printf("\tskel->ctx.sz = sizeof(skel->ctx);\n");
        bpf_object__for_each_map(map, obj) {
                const void *mmap_data = NULL;
                size_t mmap_size = 0;
@@ -1469,7 +1476,7 @@ static int do_skeleton(int argc, char **argv)
                        goto out;
        }
        if (use_loader) {
-               err = gen_trace(obj, obj_name, header_guard, &gen_opts);
+               err = gen_trace(obj, obj_name, header_guard, &gen_opts, 
prog_cnt, map_cnt);
                goto out;
        }
 
-- 
2.55.0.979.g7e5102b832-goog