[bug?] Valgrind reports errors in WASM parser

[alarmfox]

Hello,
This might be a false positive, but running valgrind on a basic program like this:

#include <stdio.h>
#include <unistd.h>
#include <yara_x.h>

static const char rule[] = "rule test { condition: true }";

int main(void) {
  YRX_COMPILER *compiler = NULL;
  YRX_RESULT result = YRX_SUCCESS;
  YRX_RULES *rules = NULL;

  /* Create a compiler */
  result = yrx_compiler_create(0, &compiler);
  if (result != YRX_SUCCESS) {
    printf("[YARA] Failed to create compiler: %s\n", yrx_last_error());
    exit(1);
  }

  /* Add simple YARA rule */
  result = yrx_compiler_add_source(compiler, rule);
  if (result != YRX_SUCCESS) {
    yrx_compiler_destroy(compiler);
    printf("[YARA] Failed to create compiler: %s\n", yrx_last_error());
    exit(1);
  }

  rules = yrx_compiler_build(compiler);
  if (!rules) {
    yrx_compiler_destroy(compiler);
    printf("[YARA] Failed to build rules: %s\n", yrx_last_error());
    exit(1);
  }

  yrx_compiler_destroy(compiler);
  yrx_rules_destroy(rules);
  return 0;
}

I compiled with:

cc -O2 -g -Wall -Wextra -W -Werror -lyara_x_capi a.c

And executed valgrind with:

valgrind --tool=memcheck -s  \
--show-leak-kinds=all  \
--track-origins=yes \
--error-exitcode=1 \
--quiet \
./a.out

Returns this output:

==10770== Conditional jump or move depends on uninitialised value(s)
==10770==    at 0x5BDB652: wasmparser::parser::Parser::parse_reader (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x5BDC783: wasmparser::parser::Parser::parse (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x5AFC553: wasmtime_environ::compile::module_environ::ModuleEnvironment::translate (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x550465D: wasmtime::compile::build_artifacts (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x549F9FD: wasmtime::compile::runtime::<impl wasmtime::compile::code_builder::CodeBuilder>::compile_module (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x5506739: wasmtime::runtime::module::Module::from_binary (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x510A015: yara_x::compiler::Compiler::build (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x4EDFA99: yrx_compiler_build (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x400112B: main (a.c:29)
==10770==  Uninitialised value was created by a stack allocation
==10770==    at 0x5504538: wasmtime::compile::build_artifacts (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==
==10770== ERROR SUMMARY: 6 errors from 1 contexts (suppressed: 0 from 0)
==10770==
==10770== 6 errors in context 1 of 1:
==10770== Conditional jump or move depends on uninitialised value(s)
==10770==    at 0x5BDB652: wasmparser::parser::Parser::parse_reader (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x5BDC783: wasmparser::parser::Parser::parse (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x5AFC553: wasmtime_environ::compile::module_environ::ModuleEnvironment::translate (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x550465D: wasmtime::compile::build_artifacts (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x549F9FD: wasmtime::compile::runtime::<impl wasmtime::compile::code_builder::CodeBuilder>::compile_module (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x5506739: wasmtime::runtime::module::Module::from_binary (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x510A015: yara_x::compiler::Compiler::build (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x4EDFA99: yrx_compiler_build (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==    by 0x400112B: main (a.c:29)
==10770==  Uninitialised value was created by a stack allocation
==10770==    at 0x5504538: wasmtime::compile::build_artifacts (in /usr/local/lib/libyara_x_capi.so.1.10.0)
==10770==
==10770== ERROR SUMMARY: 6 errors from 1 contexts (suppressed: 0 from 0)

I built the library from the current main branch with cargo cinstall -p yara-x-capi --release

$ rustc --version
rustc 1.91.1 (ed61e7d7e 2025-11-07)

I am missing something on how to use the C API?

I leave the full track (--leak-check=full) valgrind.txt

[plusvic]

You are using the C API correctly. The warning is deep into wasmtime code, so this is not related to YARA-X itself. I would say that this a false positive, but I'm not familiar enough with wasmtime internals to verify it.