Merge #1480: Get rid of untested sizeof(secp256k1_ge_storage) == 64 code path

ba5d72d assumptions: Use new STATIC_ASSERT macro (Tim Ruffing)
e53c2d9 Require that sizeof(secp256k1_ge_storage) == 64 (Tim Ruffing)
d0ba2ab util: Add STATIC_ASSERT macro (Tim Ruffing)

Pull request description:

  This gets rid of an untested code path. Resolves #1352.

  This is a bit opinionated in the sense that it adds a static assertion where it's needed in `secp256k1_pubkey_save` and `secp256k1_pubkey_load`. I think this is justified in this case. It helps the reviewer verify that these functions are correct.

  See individual commit messages.

ACKs for top commit:
  sipa:
    utACK ba5d72d
  jonasnick:
    ACK ba5d72d

Tree-SHA512: 2553c0610b62bcda6d4ef26eb26b5b2e07acf723bcd299691a2d02da57af22b8763f63c2d4adb17d30de8825b6157be6e4f0398147854fbabdf8b865fb0b5c88

Author: real-or-random

src/assumptions.h

@@ -19,65 +19,69 @@
    reduce the odds of experiencing an unwelcome surprise.
 */
 
-struct secp256k1_assumption_checker {
-    /* This uses a trick to implement a static assertion in C89: a type with an array of negative size is not
-       allowed. */
-    int dummy_array[(
-        /* Bytes are 8 bits. */
-        (CHAR_BIT == 8) &&
+#if defined(__has_attribute)
+# if __has_attribute(__unavailable__)
+__attribute__((__unavailable__("Don't call this function. It only exists because STATIC_ASSERT cannot be used outside a function.")))
+# endif
+#endif
+static void secp256k1_assumption_checker(void) {
+    /* Bytes are 8 bits. */
+    STATIC_ASSERT(CHAR_BIT == 8);
 
-        /* No integer promotion for uint32_t. This ensures that we can multiply uintXX_t values where XX >= 32
-           without signed overflow, which would be undefined behaviour. */
-        (UINT_MAX <= UINT32_MAX) &&
+    /* No integer promotion for uint32_t. This ensures that we can multiply uintXX_t values where XX >= 32
+       without signed overflow, which would be undefined behaviour. */
+    STATIC_ASSERT(UINT_MAX <= UINT32_MAX);
 
-        /* Conversions from unsigned to signed outside of the bounds of the signed type are
-           implementation-defined. Verify that they function as reinterpreting the lower
-           bits of the input in two's complement notation. Do this for conversions:
-           - from uint(N)_t to int(N)_t with negative result
-           - from uint(2N)_t to int(N)_t with negative result
-           - from int(2N)_t to int(N)_t with negative result
-           - from int(2N)_t to int(N)_t with positive result */
+    /* Conversions from unsigned to signed outside of the bounds of the signed type are
+       implementation-defined. Verify that they function as reinterpreting the lower
+       bits of the input in two's complement notation. Do this for conversions:
+       - from uint(N)_t to int(N)_t with negative result
+       - from uint(2N)_t to int(N)_t with negative result
+       - from int(2N)_t to int(N)_t with negative result
+       - from int(2N)_t to int(N)_t with positive result */
 
-        /* To int8_t. */
-        ((int8_t)(uint8_t)0xAB == (int8_t)-(int8_t)0x55) &&
-        ((int8_t)(uint16_t)0xABCD == (int8_t)-(int8_t)0x33) &&
-        ((int8_t)(int16_t)(uint16_t)0xCDEF == (int8_t)(uint8_t)0xEF) &&
-        ((int8_t)(int16_t)(uint16_t)0x9234 == (int8_t)(uint8_t)0x34) &&
+    /* To int8_t. */
+    STATIC_ASSERT(((int8_t)(uint8_t)0xAB == (int8_t)-(int8_t)0x55));
+    STATIC_ASSERT((int8_t)(uint16_t)0xABCD == (int8_t)-(int8_t)0x33);
+    STATIC_ASSERT((int8_t)(int16_t)(uint16_t)0xCDEF == (int8_t)(uint8_t)0xEF);
+    STATIC_ASSERT((int8_t)(int16_t)(uint16_t)0x9234 == (int8_t)(uint8_t)0x34);
 
-        /* To int16_t. */
-        ((int16_t)(uint16_t)0xBCDE == (int16_t)-(int16_t)0x4322) &&
-        ((int16_t)(uint32_t)0xA1B2C3D4 == (int16_t)-(int16_t)0x3C2C) &&
-        ((int16_t)(int32_t)(uint32_t)0xC1D2E3F4 == (int16_t)(uint16_t)0xE3F4) &&
-        ((int16_t)(int32_t)(uint32_t)0x92345678 == (int16_t)(uint16_t)0x5678) &&
+    /* To int16_t. */
+    STATIC_ASSERT((int16_t)(uint16_t)0xBCDE == (int16_t)-(int16_t)0x4322);
+    STATIC_ASSERT((int16_t)(uint32_t)0xA1B2C3D4 == (int16_t)-(int16_t)0x3C2C);
+    STATIC_ASSERT((int16_t)(int32_t)(uint32_t)0xC1D2E3F4 == (int16_t)(uint16_t)0xE3F4);
+    STATIC_ASSERT((int16_t)(int32_t)(uint32_t)0x92345678 == (int16_t)(uint16_t)0x5678);
 
-        /* To int32_t. */
-        ((int32_t)(uint32_t)0xB2C3D4E5 == (int32_t)-(int32_t)0x4D3C2B1B) &&
-        ((int32_t)(uint64_t)0xA123B456C789D012ULL == (int32_t)-(int32_t)0x38762FEE) &&
-        ((int32_t)(int64_t)(uint64_t)0xC1D2E3F4A5B6C7D8ULL == (int32_t)(uint32_t)0xA5B6C7D8) &&
-        ((int32_t)(int64_t)(uint64_t)0xABCDEF0123456789ULL == (int32_t)(uint32_t)0x23456789) &&
+    /* To int32_t. */
+    STATIC_ASSERT((int32_t)(uint32_t)0xB2C3D4E5 == (int32_t)-(int32_t)0x4D3C2B1B);
+    STATIC_ASSERT((int32_t)(uint64_t)0xA123B456C789D012ULL == (int32_t)-(int32_t)0x38762FEE);
+    STATIC_ASSERT((int32_t)(int64_t)(uint64_t)0xC1D2E3F4A5B6C7D8ULL == (int32_t)(uint32_t)0xA5B6C7D8);
+    STATIC_ASSERT((int32_t)(int64_t)(uint64_t)0xABCDEF0123456789ULL == (int32_t)(uint32_t)0x23456789);
 
-        /* To int64_t. */
-        ((int64_t)(uint64_t)0xB123C456D789E012ULL == (int64_t)-(int64_t)0x4EDC3BA928761FEEULL) &&
+    /* To int64_t. */
+    STATIC_ASSERT((int64_t)(uint64_t)0xB123C456D789E012ULL == (int64_t)-(int64_t)0x4EDC3BA928761FEEULL);
 #if defined(SECP256K1_INT128_NATIVE)
-        ((int64_t)(((uint128_t)0xA1234567B8901234ULL << 64) + 0xC5678901D2345678ULL) == (int64_t)-(int64_t)0x3A9876FE2DCBA988ULL) &&
-        (((int64_t)(int128_t)(((uint128_t)0xB1C2D3E4F5A6B7C8ULL << 64) + 0xD9E0F1A2B3C4D5E6ULL)) == (int64_t)(uint64_t)0xD9E0F1A2B3C4D5E6ULL) &&
-        (((int64_t)(int128_t)(((uint128_t)0xABCDEF0123456789ULL << 64) + 0x0123456789ABCDEFULL)) == (int64_t)(uint64_t)0x0123456789ABCDEFULL) &&
+    STATIC_ASSERT((int64_t)(((uint128_t)0xA1234567B8901234ULL << 64) + 0xC5678901D2345678ULL) == (int64_t)-(int64_t)0x3A9876FE2DCBA988ULL);
+    STATIC_ASSERT(((int64_t)(int128_t)(((uint128_t)0xB1C2D3E4F5A6B7C8ULL << 64) + 0xD9E0F1A2B3C4D5E6ULL)) == (int64_t)(uint64_t)0xD9E0F1A2B3C4D5E6ULL);
+    STATIC_ASSERT(((int64_t)(int128_t)(((uint128_t)0xABCDEF0123456789ULL << 64) + 0x0123456789ABCDEFULL)) == (int64_t)(uint64_t)0x0123456789ABCDEFULL);
 
-        /* To int128_t. */
-        ((int128_t)(((uint128_t)0xB1234567C8901234ULL << 64) + 0xD5678901E2345678ULL) == (int128_t)(-(int128_t)0x8E1648B3F50E80DCULL * 0x8E1648B3F50E80DDULL + 0x5EA688D5482F9464ULL)) &&
+    /* To int128_t. */
+    STATIC_ASSERT((int128_t)(((uint128_t)0xB1234567C8901234ULL << 64) + 0xD5678901E2345678ULL) == (int128_t)(-(int128_t)0x8E1648B3F50E80DCULL * 0x8E1648B3F50E80DDULL + 0x5EA688D5482F9464ULL));
 #endif
 
-        /* Right shift on negative signed values is implementation defined. Verify that it
-           acts as a right shift in two's complement with sign extension (i.e duplicating
-           the top bit into newly added bits). */
-        ((((int8_t)0xE8) >> 2) == (int8_t)(uint8_t)0xFA) &&
-        ((((int16_t)0xE9AC) >> 4) == (int16_t)(uint16_t)0xFE9A) &&
-        ((((int32_t)0x937C918A) >> 9) == (int32_t)(uint32_t)0xFFC9BE48) &&
-        ((((int64_t)0xA8B72231DF9CF4B9ULL) >> 19) == (int64_t)(uint64_t)0xFFFFF516E4463BF3ULL) &&
+    /* Right shift on negative signed values is implementation defined. Verify that it
+       acts as a right shift in two's complement with sign extension (i.e duplicating
+       the top bit into newly added bits). */
+    STATIC_ASSERT((((int8_t)0xE8) >> 2) == (int8_t)(uint8_t)0xFA);
+    STATIC_ASSERT((((int16_t)0xE9AC) >> 4) == (int16_t)(uint16_t)0xFE9A);
+    STATIC_ASSERT((((int32_t)0x937C918A) >> 9) == (int32_t)(uint32_t)0xFFC9BE48);
+    STATIC_ASSERT((((int64_t)0xA8B72231DF9CF4B9ULL) >> 19) == (int64_t)(uint64_t)0xFFFFF516E4463BF3ULL);
 #if defined(SECP256K1_INT128_NATIVE)
-        ((((int128_t)(((uint128_t)0xCD833A65684A0DBCULL << 64) + 0xB349312F71EA7637ULL)) >> 39) == (int128_t)(((uint128_t)0xFFFFFFFFFF9B0674ULL << 64) + 0xCAD0941B79669262ULL)) &&
+    STATIC_ASSERT((((int128_t)(((uint128_t)0xCD833A65684A0DBCULL << 64) + 0xB349312F71EA7637ULL)) >> 39) == (int128_t)(((uint128_t)0xFFFFFFFFFF9B0674ULL << 64) + 0xCAD0941B79669262ULL));
 #endif
-    1) * 2 - 1];
-};
+
+    /* This function is not supposed to be called. */
+    VERIFY_CHECK(0);
+}
 
 #endif /* SECP256K1_ASSUMPTIONS_H */

src/secp256k1.c

@@ -237,36 +237,25 @@ static SECP256K1_INLINE void secp256k1_declassify(const secp256k1_context* ctx,
 }
 
 static int secp256k1_pubkey_load(const secp256k1_context* ctx, secp256k1_ge* ge, const secp256k1_pubkey* pubkey) {
-    if (sizeof(secp256k1_ge_storage) == 64) {
-        /* When the secp256k1_ge_storage type is exactly 64 byte, use its
-         * representation inside secp256k1_pubkey, as conversion is very fast.
-         * Note that secp256k1_pubkey_save must use the same representation. */
-        secp256k1_ge_storage s;
-        memcpy(&s, &pubkey->data[0], sizeof(s));
-        secp256k1_ge_from_storage(ge, &s);
-    } else {
-        /* Otherwise, fall back to 32-byte big endian for X and Y. */
-        secp256k1_fe x, y;
-        ARG_CHECK(secp256k1_fe_set_b32_limit(&x, pubkey->data));
-        ARG_CHECK(secp256k1_fe_set_b32_limit(&y, pubkey->data + 32));
-        secp256k1_ge_set_xy(ge, &x, &y);
-    }
+    secp256k1_ge_storage s;
+
+    /* We require that the secp256k1_ge_storage type is exactly 64 bytes.
+     * This is formally not guaranteed by the C standard, but should hold on any
+     * sane compiler in the real world. */
+    STATIC_ASSERT(sizeof(secp256k1_ge_storage) == 64);
+    memcpy(&s, &pubkey->data[0], 64);
+    secp256k1_ge_from_storage(ge, &s);
     ARG_CHECK(!secp256k1_fe_is_zero(&ge->x));
     return 1;
 }
 
 static void secp256k1_pubkey_save(secp256k1_pubkey* pubkey, secp256k1_ge* ge) {
-    if (sizeof(secp256k1_ge_storage) == 64) {
-        secp256k1_ge_storage s;
-        secp256k1_ge_to_storage(&s, ge);
-        memcpy(&pubkey->data[0], &s, sizeof(s));
-    } else {
-        VERIFY_CHECK(!secp256k1_ge_is_infinity(ge));
-        secp256k1_fe_normalize_var(&ge->x);
-        secp256k1_fe_normalize_var(&ge->y);
-        secp256k1_fe_get_b32(pubkey->data, &ge->x);
-        secp256k1_fe_get_b32(pubkey->data + 32, &ge->y);
-    }
+    secp256k1_ge_storage s;
+
+    STATIC_ASSERT(sizeof(secp256k1_ge_storage) == 64);
+    VERIFY_CHECK(!secp256k1_ge_is_infinity(ge));
+    secp256k1_ge_to_storage(&s, ge);
+    memcpy(&pubkey->data[0], &s, 64);
 }
 
 int secp256k1_ec_pubkey_parse(const secp256k1_context* ctx, secp256k1_pubkey* pubkey, const unsigned char *input, size_t inputlen) {

src/util.h

@@ -51,13 +51,27 @@ static void print_buf_plain(const unsigned char *buf, size_t len) {
 #  define SECP256K1_INLINE inline
 # endif
 
+/** Assert statically that expr is true.
+ *
+ * This is a statement-like macro and can only be used inside functions.
+ */
+#define STATIC_ASSERT(expr) do { \
+    switch(0) { \
+        case 0: \
+        /* If expr evaluates to 0, we have two case labels "0", which is illegal. */ \
+        case /* ERROR: static assertion failed */ (expr): \
+        ; \
+    } \
+} while(0)
+
 /** Assert statically that expr is an integer constant expression, and run stmt.
  *
  * Useful for example to enforce that magnitude arguments are constant.
  */
 #define ASSERT_INT_CONST_AND_DO(expr, stmt) do { \
     switch(42) { \
-        case /* ERROR: integer argument is not constant */ expr: \
+        /* C allows only integer constant expressions as case labels. */ \
+        case /* ERROR: integer argument is not constant */ (expr): \
             break; \
         default: ; \
     } \