qemu: Add binfmt preserve-argv[0] wrapper

pkgs/applications/virtualization/qemu/binfmt-p-wrapper.c

···
+// This is a tiny wrapper that converts the extra arv[0] argument
+// from binfmt-misc with the P flag enabled to QEMU parameters.
+// It also prevents LD_* environment variables from being applied
+// to QEMU itself.
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#ifndef TARGET_QEMU
+#error "Define TARGET_QEMU to be the path to the qemu-user binary (e.g., -DTARGET_QEMU=\"/full/path/to/qemu-riscv64\")"
+#endif
+
+extern char **environ;
+
+int main(int argc, char *argv[]) {
+    if (argc < 3) {
+        fprintf(stderr, "%s: This should be run as the binfmt interpreter with the P flag\n", argv[0]);
+        fprintf(stderr, "%s: My preconfigured qemu-user binary: %s\n", argv[0], TARGET_QEMU);
+        return 1;
+    }
+
+    size_t environ_count = 0;
+    for (char **cur = environ; *cur != NULL; ++cur) {
+        environ_count++;
+    }
+
+    size_t new_argc = 3;
+    size_t new_argv_alloc = argc + 2 * environ_count + 2; // [ "-E", env ] for each LD_* env + [ "-0", argv0 ]
+    char **new_argv = (char**)malloc((new_argv_alloc + 1) * sizeof(char*));
+    if (!new_argv) {
+        fprintf(stderr, "FATAL: Failed to allocate new argv array\n");
+        abort();
+    }
+
+    new_argv[0] = TARGET_QEMU;
+    new_argv[1] = "-0";
+    new_argv[2] = argv[2];
+
+    // Pass all LD_ env variables as -E and strip them in `new_environ`
+    size_t new_environc = 0;
+    char **new_environ = (char**)malloc((environ_count + 1) * sizeof(char*));
+    if (!new_environ) {
+        fprintf(stderr, "FATAL: Failed to allocate new environ array\n");
+        abort();
+    }
+
+    for (char **cur = environ; *cur != NULL; ++cur) {
+        if (strncmp("LD_", *cur, 3) == 0) {
+            new_argv[new_argc++] = "-E";
+            new_argv[new_argc++] = *cur;
+        } else {
+            new_environ[new_environc++] = *cur;
+        }
+    }
+    new_environ[new_environc] = NULL;
+
+    size_t new_arg_start = new_argc;
+    new_argc += argc - 3 + 2; // [ "--", full_binary_path ]
+
+    if (argc > 3) {
+        memcpy(&new_argv[new_arg_start + 2], &argv[3], (argc - 3) * sizeof(char**));
+    }
+
+    new_argv[new_arg_start] = "--";
+    new_argv[new_arg_start + 1] = argv[1];
+    new_argv[new_argc] = NULL;
+
+#ifdef DEBUG
+    for (size_t i = 0; i < new_argc; ++i) {
+        fprintf(stderr, "argv[%zu] = %s\n", i, new_argv[i]);
+    }
+#endif
+
+    return execve(new_argv[0], new_argv, new_environ);
+}
+
+// vim: et:ts=4:sw=4

pkgs/applications/virtualization/qemu/binfmt-p-wrapper.nix

···
+# binfmt preserve-argv[0] wrapper
+#
+# More details in binfmt-p-wrapper.c
+#
+# The wrapper has to be static so LD_* environment variables
+# cannot affect the execution of the wrapper itself.
+
+{ lib, stdenv, pkgsStatic, enableDebug ? false }:
+
+name: emulator:
+
+pkgsStatic.stdenv.mkDerivation {
+  inherit name;
+
+  src = ./binfmt-p-wrapper.c;
+
+  dontUnpack = true;
+  dontInstall = true;
+
+  buildPhase = ''
+    runHook preBuild
+
+    mkdir -p $out/bin
+    $CC -o $out/bin/${name} -static -std=c99 -O2 \
+        -DTARGET_QEMU=\"${emulator}\" \
+        ${lib.optionalString enableDebug "-DDEBUG"} \
+        $src
+
+    runHook postBuild
+  '';
+}

pkgs/top-level/all-packages.nix

···
 
   qemu-utils = callPackage ../applications/virtualization/qemu/utils.nix {};
 
+  wrapQemuBinfmtP = callPackage ../applications/virtualization/qemu/binfmt-p-wrapper.nix { };
+
   qgis-unwrapped = libsForQt5.callPackage ../applications/gis/qgis/unwrapped.nix {
     withGrass = false;
   };