bpf: Fix modifier skipping logic

Fix the way modifiers are skipped while walking pointers. Otherwise second
level dereferences of 'const struct foo *' will be rejected by the verifier.

Fixes: 9e15db66136a ("bpf: Implement accurate raw_tp context access via BTF")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200201000314.261392-1-ast@kernel.org

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 8c9d8f2..805c43b 100644 (file)
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3931,6 +3931,7 @@ again:
 
                if (btf_type_is_ptr(mtype)) {
                        const struct btf_type *stype;
+                       u32 id;
 
                        if (msize != size || off != moff) {
                                bpf_log(log,
@@ -3939,12 +3940,9 @@ again:
                                return -EACCES;
                        }
 
-                       stype = btf_type_by_id(btf_vmlinux, mtype->type);
-                       /* skip modifiers */
-                       while (btf_type_is_modifier(stype))
-                               stype = btf_type_by_id(btf_vmlinux, stype->type);
+                       stype = btf_type_skip_modifiers(btf_vmlinux, mtype->type, &id);
                        if (btf_type_is_struct(stype)) {
-                               *next_btf_id = mtype->type;
+                               *next_btf_id = id;
                                return PTR_TO_BTF_ID;
                        }
                }