bpf: Do not allow btf_ctx_access with __int128 types

To ensure btf_ctx_access() is safe the verifier checks that the BTF
arg type is an int, enum, or pointer. When the function does the
BTF arg lookup it uses the calculation 'arg = off / 8'  using the
fact that registers are 8B. This requires that the first arg is
in the first reg, the second in the second, and so on. However,
for __int128 the arg will consume two registers by default LLVM
implementation. So this will cause the arg layout assumed by the
'arg = off / 8' calculation to be incorrect.

Because __int128 is uncommon this patch applies the easiest fix and
will force int types to be sizeof(u64) or smaller so that they will
fit in a single register.

v2: remove unneeded parens per Andrii's feedback

Fixes: 9e15db66 ("bpf: Implement accurate raw_tp context access via BTF")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159303723962.11287.13309537171132420717.stgit@john-Precision-5820-Tower

include/linux/btf.h

@@ -82,6 +82,11 @@ static inline bool btf_type_is_int(const struct btf_type *t)
 	return BTF_INFO_KIND(t->info) == BTF_KIND_INT;
 }
 
+static inline bool btf_type_is_small_int(const struct btf_type *t)
+{
+	return btf_type_is_int(t) && t->size <= sizeof(u64);
+}
+
 static inline bool btf_type_is_enum(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_ENUM;

kernel/bpf/btf.c

@@ -3746,7 +3746,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 				return false;
 
 			t = btf_type_skip_modifiers(btf, t->type, NULL);
-			if (!btf_type_is_int(t)) {
+			if (!btf_type_is_small_int(t)) {
 				bpf_log(log,
 					"ret type %s not allowed for fmod_ret\n",
 					btf_kind_str[BTF_INFO_KIND(t->info)]);
@@ -3768,7 +3768,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	/* skip modifiers */
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);
-	if (btf_type_is_int(t) || btf_type_is_enum(t))
+	if (btf_type_is_small_int(t) || btf_type_is_enum(t))
 		/* accessing a scalar */
 		return true;
 	if (!btf_type_is_ptr(t)) {