Fuzzers: Add a basic input shim when running standalone

Meta/Lagom/Fuzzers/CMakeLists.txt

@@ -12,6 +12,9 @@ function(add_simple_fuzzer name)
       PUBLIC ${ARGN} LagomCore
       PRIVATE $<$<CXX_COMPILER_ID:Clang>:-fsanitize=fuzzer>
       )
+  else()
+    target_sources(${name} PRIVATE "EntryShim.cpp")
+    target_link_libraries(${name} PUBLIC ${ARGN} LagomCore)
   endif()
 endfunction()
 

Meta/Lagom/Fuzzers/EntryShim.cpp

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <fcntl.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size);
+
+int fuzz_from_file(const char* filename)
+{
+    struct stat file_stats;
+
+    if (stat(filename, &file_stats) < 0) {
+        perror("EntryShim: Failed to stat the input file");
+        return 1;
+    }
+
+    size_t file_size = file_stats.st_size;
+
+    uint8_t* file_buffer = (uint8_t*)malloc(file_size);
+
+    if (!file_buffer) {
+        fprintf(stderr, "EntryShim: Failed to allocate file buffer\n");
+        return 1;
+    }
+
+    int fd = open(filename, O_RDONLY);
+
+    if (fd < 0) {
+        perror("EntryShim: Failed to open the input file");
+        return 1;
+    }
+
+    ssize_t bytes_read = read(fd, file_buffer, file_size);
+    if (bytes_read < 0) {
+        fprintf(stderr, "EntryShim: Failed to read the input file\n");
+        return 1;
+    }
+
+    LLVMFuzzerTestOneInput(file_buffer, bytes_read);
+    return 0;
+}
+
+int fuzz_from_stdin()
+{
+    size_t chunk_size = 4096;
+
+    uint8_t* file_buffer = nullptr;
+    size_t file_size = 0;
+
+    while (true) {
+        file_buffer = (uint8_t*)realloc(file_buffer, file_size + chunk_size);
+
+        if (!file_buffer) {
+            fprintf(stderr, "EntryShim: Failed to reallocate buffer to a size of %lu bytes\n", file_size + chunk_size);
+            return 1;
+        }
+
+        ssize_t bytes_read = read(STDIN_FILENO, file_buffer + file_size, chunk_size);
+
+        if (bytes_read < 0) {
+            perror("EntryShim: Failed to read from stdin");
+            return 1;
+        }
+
+        if (bytes_read == 0)
+            break;
+
+        file_size += bytes_read;
+    }
+
+    LLVMFuzzerTestOneInput(file_buffer, file_size);
+    return 0;
+}
+
+extern "C" int main(int argc, char** argv)
+{
+    if (argc > 1)
+        return fuzz_from_file(argv[1]);
+
+    return fuzz_from_stdin();
+}