Merge pull request #388 from cwshugg/no_include_main_msvc

Add new build option to support Windows DLLs

src/options.rs

@@ -165,6 +165,20 @@ pub struct BuildOptions {
     /// and the fuzzer can store an input to the corpus at each condition that it passes;
     /// giving it a better chance of producing an input that reaches `res = 2;`.
     pub disable_branch_folding: Option<bool>,
+
+    #[arg(long)]
+    /// Disable the inclusion of the `/include:main` MSVC linker argument
+    ///
+    /// The purpose of `/include:main` is to force the MSVC linker to include an
+    /// external reference to the symbol `main`, such that fuzzing targets built
+    /// on Windows are able to find LibFuzzer's `main` function.
+    ///
+    /// In certain corner cases, users may prefer to *not* build with this
+    /// argument. One such example: if a user is intending to build and fuzz a
+    /// Windows DLL, they would likely choose to enable this flag, to prevent
+    /// the DLL from having an extern `main` reference added to it. (DLLs/shared
+    /// libraries should not have any reference to `main`.)
+    pub no_include_main_msvc: bool,
 }
 
 impl stdfmt::Display for BuildOptions {
@@ -265,6 +279,7 @@ mod test {
             no_cfg_fuzzing: false,
             no_trace_compares: false,
             disable_branch_folding: None,
+            no_include_main_msvc: false,
         };
 
         let opts = vec![

src/project.rs

@@ -233,8 +233,23 @@ impl FuzzProject {
         if !build.release || build.debug_assertions || build.careful_mode {
             rustflags.push_str(" -Cdebug-assertions");
         }
-        if build.triple.contains("-msvc") {
-            // The entrypoint is in the bundled libfuzzer rlib, this gets the linker to find it.
+        if build.triple.contains("-msvc") && !build.no_include_main_msvc {
+            // This forces the MSVC linker (which runs on Windows systems) to
+            // find the entry point (i.e. the `main` function) within the
+            // LibFuzzer `.rlib` file produced during the build.
+            //
+            // The `--no-include-main-msvc` argument disables the addition of
+            // this linker argument. In certain situations, a user may not want
+            // this argument included as part of the MSVC invocation.
+            //
+            // For example, if the user is attempting to build and fuzz a
+            // Windows DLL (shared library), adding `/include:main` will force
+            // the DLL to compile with an external reference to `main`.
+            // DLLs/shared libraries are designed to be built as a separate
+            // object file, intentionally left *without* knowledge of the entry
+            // point. So, forcing a DLL to include `main` will cause linking to
+            // fail. Using `--no-include-main-msvc` will allow the DLL to be
+            // built without issue.
             rustflags.push_str(" -Clink-arg=/include:main");
         }