cc-wrapper: include fortify-headers before libc includes for musl

pkgs/build-support/cc-wrapper/default.nix

@@ -51,6 +51,8 @@
 
 # the derivation at which the `-B` and `-L` flags added by `useCcForLibs` will point
 , gccForLibs ? if useCcForLibs then cc else null
+, fortify-headers ? null
+, includeFortifyHeaders ? null
 }:
 
 with lib;
@@ -65,6 +67,10 @@ let
   stdenv = stdenvNoCC;
   inherit (stdenv) hostPlatform targetPlatform;
 
+  includeFortifyHeaders' = if includeFortifyHeaders != null
+    then includeFortifyHeaders
+    else targetPlatform.libc == "musl";
+
   # Prefix for binaries. Customarily ends with a dash separator.
   #
   # TODO(@Ericson2314) Make unconditional, or optional but always true by
@@ -165,6 +171,8 @@ let
     stdenv.targetPlatform.darwinMinVersionVariable;
 in
 
+assert includeFortifyHeaders' -> fortify-headers != null;
+
 # Ensure bintools matches
 assert libc_bin == bintools.libc_bin;
 assert libc_dev == bintools.libc_dev;
@@ -414,6 +422,16 @@ stdenv.mkDerivation {
 
       echo "${libc_lib}" > $out/nix-support/orig-libc
       echo "${libc_dev}" > $out/nix-support/orig-libc-dev
+    ''
+    # fortify-headers is a set of wrapper headers that augment libc
+    # and use #include_next to pass through to libc's true
+    # implementations, so must appear before them in search order.
+    # in theory a correctly placed -idirafter could be used, but in
+    # practice the compiler may have been built with a --with-headers
+    # like option that forces the libc headers before all -idirafter,
+    # hence -isystem here.
+    + optionalString includeFortifyHeaders' ''
+      echo "-isystem ${fortify-headers}/include" >> $out/nix-support/libc-cflags
     '')
 
     ##

pkgs/stdenv/linux/default.nix

@@ -194,6 +194,7 @@ let
           inherit lib;
           inherit (prevStage) coreutils gnugrep;
           stdenvNoCC = prevStage.ccWrapperStdenv;
+          fortify-headers = prevStage.fortify-headers;
         }).overrideAttrs(a: lib.optionalAttrs (prevStage.gcc-unwrapped.passthru.isXgcc or false) {
           # This affects only `xgcc` (the compiler which compiles the final compiler).
           postFixup = (a.postFixup or "") + ''
@@ -568,6 +569,7 @@ in
         inherit lib;
         inherit (self) stdenvNoCC coreutils gnugrep;
         shell = self.bash + "/bin/bash";
+        fortify-headers = self.fortify-headers;
       };
     };
     extraNativeBuildInputs = [
@@ -645,6 +647,7 @@ in
         ++  [ linuxHeaders # propagated from .dev
             binutils gcc gcc.cc gcc.cc.lib gcc.expand-response-params gcc.cc.libgcc glibc.passthru.libgcc
           ]
+        ++ lib.optionals (localSystem.libc == "musl") [ fortify-headers ]
         ++ [ prevStage.updateAutotoolsGnuConfigScriptsHook prevStage.gnu-config ]
         ++ (with gcc-unwrapped.passthru; [
           gmp libmpc mpfr isl