revert reader changes, handle align better

Author: bkietz

CMakeLists.txt

@@ -79,12 +79,7 @@ option (FLATCC_TRACE_VERIFY
 
 # Some producers allow empty vectors to be misaligned.
 # The following setting will cause the verifier to check for an
-# empty vector before checking alignment. This option will also
-# ensure accesses to a vector field never materializes misaligned
-# pointers.
-#
-# NOTE: enabling this means empty vectors will not have distinct
-# pointer identity.
+# empty vector before checking alignment of the vector's elements.
 option (FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS
     "don't fail verification if empty vectors are misaligned" OFF)
 

include/flatcc/flatcc_flatbuffers.h

@@ -48,23 +48,6 @@ extern "C" {
 #define FLATBUFFERS_WRAP_NAMESPACE(ns, x) ns ## _ ## x
 #endif
 
-#include <stddef.h>
-
-/*
- * Produce a pointer with maximum alignment to use as
- * a placeholder when FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS
- * is specified.
- */
-static inline const void* max_aligned_ptr(void)
-{
-#ifdef __cplusplus
-    static const max_align_t m{};
-#else
-    static const max_align_t m;
-#endif
-    return &m;
-}
-
 #endif /* flatcc_flatbuffers_defined */
 
 #ifdef __cplusplus

include/flatcc/flatcc_rtconfig.h

@@ -66,12 +66,7 @@ extern "C" {
 /*
  * Some producers allow empty vectors to be misaligned.
  * The following setting will cause the verifier to check for an
- * empty vector before checking alignment. This option will also
- * ensure accesses to a vector field never materializes misaligned
- * pointers.
- *
- * NOTE: enabling this means empty vectors will not have distinct
- * pointer identity.
+ * empty vector before checking alignment of the vector's elements.
  */
 #if !defined(FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS)
 #define FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS 0

src/compiler/codegen_c_reader.c

@@ -5,8 +5,6 @@
 #include "codegen_c.h"
 #include "codegen_c_sort.h"
 
-#include "flatcc/flatcc_rtconfig.h"
-
 static inline int match_kw_identifier(fb_symbol_t *sym)
 {
     return (sym->ident->len == 10 &&
@@ -620,34 +618,10 @@ static void gen_helpers(fb_output_t *out)
         "    return 0;\\\n"
         "}\n",
         nsc, nsc, nsc, nsc, nsc);
-#if FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS
-    fprintf(out->fp,
-        "#include <stdio.h>\n"
-        "#define __%svector_field(T, ID, t, r)\\\n"
-        "{\\\n"
-        "    T out__tmp;\\\n"
-        "    __%sread_vt(ID, offset__tmp, t)\\\n"
-        "    if (offset__tmp) {\\\n"
-        "        offset__tmp += __%suoffset_read_from_pe((uint8_t *)(t) + offset__tmp);\\\n"
-        "        if (__%suoffset_read_from_pe((uint8_t *)(t) + offset__tmp) == 0) {\\\n"
-        "            printf(\"Falling back for empty vector...\");\\\n"
-        "            return (T)max_aligned_ptr();\\\n"
-        "        }\\\n"
-        "        offset__tmp += sizeof(%suoffset_t);\\\n"
-        "        return (T)((uint8_t *)(t) + offset__tmp);\\\n"
-        "    }\\\n"
-        "    FLATCC_ASSERT(!(r) && \"required field missing\");\\\n"
-        "    return 0;\\\n"
-        "}\n",
-        nsc, nsc, nsc, nsc, nsc);
-#else
-    fprintf(out->fp,
-        "#define __%svector_field(T, ID, t, r) __%soffset_field(T, ID, t, r, sizeof(%suoffset_t))\n",
-        nsc, nsc, nsc);
-#endif
     fprintf(out->fp,
+        "#define __%svector_field(T, ID, t, r) __%soffset_field(T, ID, t, r, sizeof(%suoffset_t))\n"
         "#define __%stable_field(T, ID, t, r) __%soffset_field(T, ID, t, r, 0)\n",
-        nsc, nsc);
+        nsc, nsc, nsc, nsc, nsc);
     fprintf(out->fp,
         "#define __%sdefine_struct_field(ID, N, NK, T, r)\\\n"
         "static inline T N ## _ ## NK ## _get(N ## _table_t t__tmp)\\\n"

src/runtime/verifier.c

@@ -135,20 +135,6 @@ static inline int check_header(uoffset_t end, uoffset_t base, uoffset_t offset)
     return k > base && k + offset_size <= end && !(k & (offset_size - 1));
 }
 
-static inline int check_aligned_header(uoffset_t end, uoffset_t base, uoffset_t offset, uint16_t align)
-{
-    uoffset_t k = base + offset;
-
-    if (uoffset_size <= voffset_size && k + offset_size < k) {
-        return 0;
-    }
-    /* Alignment refers to element 0 and header must also be aligned. */
-    align = align < uoffset_size ? uoffset_size : align;
-
-    /* Note to self: the builder can also use the mask OR trick to propagate `min_align`. */
-    return k > base && k + offset_size <= end && !((k + offset_size) & ((offset_size - 1) | (align - 1u)));
-}
-
 static inline int verify_struct(uoffset_t end, uoffset_t base, uoffset_t offset, uoffset_t size, uint16_t align)
 {
     /* Structs can have zero size so `end` is a valid value. */
@@ -276,22 +262,18 @@ static inline int verify_string(const void *buf, uoffset_t end, uoffset_t base,
  */
 static inline int verify_vector(const void *buf, uoffset_t end, uoffset_t base, uoffset_t offset, uoffset_t elem_size, uint16_t align, uoffset_t max_count)
 {
-    uoffset_t n;
+    verify(check_header(end, base, offset), flatcc_verify_error_vector_header_out_of_range_or_unaligned);
+    base += offset;
+
+    uoffset_t n = read_uoffset(buf, base);
+    base += offset_size;
 
 #if FLATCC_TOLERATE_MISALIGNED_EMPTY_VECTORS
-    base += offset;
-    verify(end - base >= sizeof(n), flatcc_verify_error_vector_header_out_of_range_or_unaligned);
-    n = read_uoffset(buf, base);
-    if (n == 0) {
-        return flatcc_verify_ok;
-    }
-    verify(check_aligned_header(end, base - offset, offset, align), flatcc_verify_error_vector_header_out_of_range_or_unaligned);
-#else
-    verify(check_aligned_header(end, base, offset, align), flatcc_verify_error_vector_header_out_of_range_or_unaligned);
-    base += offset;
-    n = read_uoffset(buf, base);
+    align = n == 0 ? uoffset_size : align;
 #endif
-    base += offset_size;
+    align = align < uoffset_size ? uoffset_size : align;
+    verify(!(base & (align - 1u)),flatcc_verify_error_vector_header_out_of_range_or_unaligned);
+
     /* `n * elem_size` can overflow uncontrollably otherwise. */
     verify(n <= max_count, flatcc_verify_error_vector_count_exceeds_representable_vector_size);
     verify(end - base >= n * elem_size, flatcc_verify_error_vector_out_of_range);

test/load_test/load_test.c

@@ -136,8 +136,6 @@ int main(int argc, char *argv[])
         create_root_monster(B);
         flatcc_builder_copy_buffer(B, buffer, size);
         mon = ns(Monster_as_root(buffer));
-        char const *name = ns(Monster_name(mon));
-    printf("Name was %s\n", name ? name : "<NULL>");
         ret |= strcmp(ns(Monster_name(mon)), "root_monster");
         assert(ret == 0);
         mv = ns(Monster_testarrayoftables(mon));