precompiles: Validate and preprocess expmod input

Decompose the expmod input into separate bytes_views for base, exp, mod.
Handle arguments padding. Handle mod being zero.

Simplify and vastly reduce the expmod stub by using the preprocessed
arguments and handling trivial cases before doing the result lookup.

Author: chfast

Diff for: test/state/precompiles.cpp

@@ -317,19 +317,61 @@ ExecutionResult expmod_execute(
     const uint8_t* input, size_t input_size, uint8_t* output, size_t output_size) noexcept
 {
     static constexpr auto LEN_SIZE = sizeof(intx::uint256);
+    static constexpr auto HEADER_SIZE = 3 * LEN_SIZE;
+    static constexpr auto LEN32_OFF = LEN_SIZE - sizeof(uint32_t);
 
     // The output size equal to the modulus size.
+    const auto mod_len = output_size;
+
     // Handle short incomplete input up front. The answer is 0 of the length of the modulus.
-    if (output_size == 0 || input_size <= 3 * LEN_SIZE) [[unlikely]]
+    if (input_size <= HEADER_SIZE) [[unlikely]]
+    {
+        std::fill_n(output, output_size, 0);
+        return {EVMC_SUCCESS, output_size};
+    }
+
+    const auto base_len = intx::be::unsafe::load<uint32_t>(&input[LEN32_OFF]);
+    const auto exp_len = intx::be::unsafe::load<uint32_t>(&input[LEN_SIZE + LEN32_OFF]);
+    assert(intx::be::unsafe::load<uint32_t>(&input[2 * LEN_SIZE + LEN32_OFF]) == mod_len);
+
+    const bytes_view payload{input + HEADER_SIZE, input_size - HEADER_SIZE};
+    const size_t mod_off = base_len + exp_len;  // Cannot overflow if gas cost computed before.
+    const auto mod_explicit = payload.substr(std::min(mod_off, payload.size()), mod_len);
+
+    // Handle the mod being zero early.
+    // This serves two purposes:
+    // - bigint libraries don't like zero modulus because division by 0 is not well-defined,
+    // - having non-zero modulus guarantees that base and exp aren't out-of-bounds.
+    if (mod_explicit.find_first_not_of(uint8_t{0}) == bytes_view::npos) [[unlikely]]
     {
+        // The modulus is zero, so the result is zero.
         std::fill_n(output, output_size, 0);
         return {EVMC_SUCCESS, output_size};
     }
 
+    const auto mod_requires_padding = mod_explicit.size() != mod_len;
+    if (mod_requires_padding) [[unlikely]]
+    {
+        // The modulus is the last argument and some of its bytes may be missing and be implicitly
+        // zero. In this case, copy the explict modulus bytes to the output buffer and pad the rest
+        // with zeroes. The output buffer is guaranteed to have exactly the modulus size.
+        const auto [_, output_p] = std::ranges::copy(mod_explicit, output);
+        std::fill(output_p, output + output_size, 0);
+    }
+
+    const auto base = payload.substr(0, base_len);
+    const auto exp = payload.substr(base_len, exp_len);
+    const auto mod = mod_requires_padding ? bytes_view{output, mod_len} : mod_explicit;
+
 #ifdef EVMONE_PRECOMPILES_SILKPRE
+    (void)base;
+    (void)exp;
+    (void)mod;
+    // For Silkpre use the raw input for compatibility.
     return silkpre_expmod_execute(input, input_size, output, output_size);
 #else
-    return expmod_stub(input, input_size, output, output_size);
+    expmod_stub(base, exp, mod, output);
+    return {EVMC_SUCCESS, mod.size()};
 #endif
 }