libbpf: Support flexible arrays in CO-RE

Some data stuctures in kernel are defined with either zero-sized array or
flexible (dimensionless) array at the end of a struct. Actual data of such
array follows in memory immediately after the end of that struct, forming its
variable-sized "body" of elements. Support such access pattern in CO-RE
relocation handling.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191215070844.1014385-2-andriin@fb.com

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index c9f90bf..a2cc731 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -3108,6 +3108,21 @@ static bool str_is_empty(const char *s)
        return !s || !s[0];
 }
 
+static bool is_flex_arr(const struct btf *btf,
+                       const struct bpf_core_accessor *acc,
+                       const struct btf_array *arr)
+{
+       const struct btf_type *t;
+
+       /* not a flexible array, if not inside a struct or has non-zero size */
+       if (!acc->name || arr->nelems > 0)
+               return false;
+
+       /* has to be the last member of enclosing struct */
+       t = btf__type_by_id(btf, acc->type_id);
+       return acc->idx == btf_vlen(t) - 1;
+}
+
 /*
  * Turn bpf_field_reloc into a low- and high-level spec representation,
  * validating correctness along the way, as well as calculating resulting
@@ -3145,6 +3160,7 @@ static int bpf_core_spec_parse(const struct btf *btf,
                               struct bpf_core_spec *spec)
 {
        int access_idx, parsed_len, i;
+       struct bpf_core_accessor *acc;
        const struct btf_type *t;
        const char *name;
        __u32 id;
@@ -3192,6 +3208,7 @@ static int bpf_core_spec_parse(const struct btf *btf,
                        return -EINVAL;
 
                access_idx = spec->raw_spec[i];
+               acc = &spec->spec[spec->len];
 
                if (btf_is_composite(t)) {
                        const struct btf_member *m;
@@ -3209,18 +3226,23 @@ static int bpf_core_spec_parse(const struct btf *btf,
                                if (str_is_empty(name))
                                        return -EINVAL;
 
-                               spec->spec[spec->len].type_id = id;
-                               spec->spec[spec->len].idx = access_idx;
-                               spec->spec[spec->len].name = name;
+                               acc->type_id = id;
+                               acc->idx = access_idx;
+                               acc->name = name;
                                spec->len++;
                        }
 
                        id = m->type;
                } else if (btf_is_array(t)) {
                        const struct btf_array *a = btf_array(t);
+                       bool flex;
 
                        t = skip_mods_and_typedefs(btf, a->type, &id);
-                       if (!t || access_idx >= a->nelems)
+                       if (!t)
+                               return -EINVAL;
+
+                       flex = is_flex_arr(btf, acc - 1, a);
+                       if (!flex && access_idx >= a->nelems)
                                return -EINVAL;
 
                        spec->spec[spec->len].type_id = id;
@@ -3525,12 +3547,14 @@ static int bpf_core_spec_match(struct bpf_core_spec *local_spec,
                         */
                        if (i > 0) {
                                const struct btf_array *a;
+                               bool flex;
 
                                if (!btf_is_array(targ_type))
                                        return 0;
 
                                a = btf_array(targ_type);
-                               if (local_acc->idx >= a->nelems)
+                               flex = is_flex_arr(targ_btf, targ_acc - 1, a);
+                               if (!flex && local_acc->idx >= a->nelems)
                                        return 0;
                                if (!skip_mods_and_typedefs(targ_btf, a->type,
                                                            &targ_id))