pkgs/sandbox: Factor out setup routine

This leaves sandbox.c with only the main() function and nothing else, so
that whenever we have a lot of binaries to generate, the compilation
time should be much lower now.

The change doesn't change anything in functionality.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

6 files changed, 609 insertions, 573 deletions

diff --git a/pkgs/games/build-support/build-sandbox/default.nix b/pkgs/games/build-support/build-sandbox/default.nix
index 3bb41093..337456e0 100644
--- a/pkgs/games/build-support/build-sandbox/default.nix
+++ b/pkgs/games/build-support/build-sandbox/default.nix
@@ -30,7 +30,8 @@ stdenv.mkDerivation ({
       bcdown
     ' ../sandbox-closure | sort -u)"
 
-    echo 'static bool setup_app_paths(void) {' > params.c
+    echo '#include "setup.h"' > params.c
+    echo 'bool setup_app_paths(void) {' >> params.c
 
     for dep in $runtimeDeps; do
       echo 'if (!bind_mount("'"$dep"'", true, true)) return false;' >> params.c
diff --git a/pkgs/games/build-support/build-sandbox/src/Makefile b/pkgs/games/build-support/build-sandbox/src/Makefile
index 57c732af..49b266fe 100644
--- a/pkgs/games/build-support/build-sandbox/src/Makefile
+++ b/pkgs/games/build-support/build-sandbox/src/Makefile
@@ -1,15 +1,17 @@
 BINARIES = $(wildcard $(BINDIR)/*)
 WRAPPERS = $(subst $(BINDIR),$(out)/bin,$(BINARIES))
 
+OBJECTS = get-closure.o params.o setup.o
+
 CFLAGS = -Wall -std=gnu11 -DFS_ROOT_DIR=\"$(out)\"
 CXXFLAGS = -Wall -std=c++14 `pkg-config --cflags nix-main`
 LDFLAGS = `pkg-config --libs nix-main`
 
-all: get-closure.o
+all: $(OBJECTS)
 
 $(out)/bin/%: CFLAGS += -DWRAPPED_PROGNAME=\"$(@F)\"
 $(out)/bin/%: CFLAGS += -DWRAPPED_PATH=\"$(BINDIR)/$(@F)\"
-$(out)/bin/%: get-closure.o
+$(out)/bin/%: $(OBJECTS)
 	mkdir -p $(out)/bin
 	$(CC) -o $@ $(CFLAGS) $(LDFLAGS) $? sandbox.c
 
diff --git a/pkgs/games/build-support/build-sandbox/src/params.h b/pkgs/games/build-support/build-sandbox/src/params.h
new file mode 100644
index 00000000..ea33872d
--- /dev/null
+++ b/pkgs/games/build-support/build-sandbox/src/params.h
@@ -0,0 +1,8 @@
+#ifndef _PARAMS_H
+#define _PARAMS_H
+
+#include <stdbool.h>
+
+bool setup_app_paths(void);
+
+#endif
diff --git a/pkgs/games/build-support/build-sandbox/src/sandbox.c b/pkgs/games/build-support/build-sandbox/src/sandbox.c
index dd0fe845..e2aa47cf 100644
--- a/pkgs/games/build-support/build-sandbox/src/sandbox.c
+++ b/pkgs/games/build-support/build-sandbox/src/sandbox.c
@@ -1,581 +1,13 @@
-#define _GNU_SOURCE
-#define _POSIX_C_SOURCE 200809L
-
-#include <sys/mount.h>
-#include <sys/stat.h>
-#include <sys/types.h>
-#include <sys/wait.h>
-
 #include <errno.h>
-#include <fcntl.h>
-#include <libgen.h>
-#include <limits.h>
-#include <malloc.h>
-#include <sched.h>
-#include <stdbool.h>
 #include <stdio.h>
-#include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 
-static bool write_proc(int proc_pid_fd, const char *fname, const char *buf,
-                       size_t buflen, bool ignore_errors)
-{
-    int fd;
-
-    if ((fd = openat(proc_pid_fd, fname, O_WRONLY)) == -1) {
-        fprintf(stderr, "open %s: %s\n", fname, strerror(errno));
-        return false;
-    }
-
-    if (write(fd, buf, buflen) == -1) {
-        if (!ignore_errors)
-            fprintf(stderr, "write %s: %s\n", fname, strerror(errno));
-        close(fd);
-        return ignore_errors;
-    }
-
-    close(fd);
-    return true;
-}
-
-#define WRITE_IDMAP(file, value) \
-    buflen = snprintf(buf, 100, "%1$lu %1$lu 1", (unsigned long)value); \
-    if (buflen >= 100) { \
-        fputs("Unable to write buffer for " file ".\n", stderr); \
-        close(proc_pid_fd); \
-        return false; \
-    } else if (buflen < 0) { \
-        perror("snprintf " file " buffer"); \
-        close(proc_pid_fd); \
-        return false; \
-    } \
-    if (!write_proc(proc_pid_fd, file, buf, buflen, false)) { \
-        close(proc_pid_fd); \
-        return false; \
-    }
-
-static bool write_maps(pid_t parent_pid)
-{
-    int proc_pid_fd;
-    size_t buflen;
-    char buf[100];
-
-    buflen = snprintf(buf, 100, "/proc/%lu", (unsigned long)parent_pid);
-    if (buflen >= 100) {
-        fputs("Unable to write buffer for child pid proc path.\n", stderr);
-        return false;
-    } else if (buflen < 0) {
-        perror("snprintf child pid proc path");
-        return false;
-    }
-
-    if ((proc_pid_fd = open(buf, O_RDONLY | O_DIRECTORY)) == -1) {
-        fprintf(stderr, "open %s: %s\n", buf, strerror(errno));
-        return false;
-    }
-
-    WRITE_IDMAP("uid_map", geteuid());
-
-    // Kernels prior to Linux 3.19 which do not impose setgroups()
-    // restrictions won't have this file, so ignore failure.
-    write_proc(proc_pid_fd, "setgroups", "deny", 4, true);
-
-    WRITE_IDMAP("gid_map", getegid());
-
-    return true;
-}
-
-static bool makedirs(const char *path)
-{
-    char *tmp, *segment;
-
-    if ((tmp = strdup(path)) == NULL) {
-        fprintf(stderr, "strdup of %s: %s\n", path, strerror(errno));
-        return false;
-    }
-
-    segment = dirname(tmp);
-
-    if (!(segment[0] == '/' && segment[1] == '\0')) {
-        if (!makedirs(segment)) {
-            free(tmp);
-            return false;
-        }
-    }
-
-    (void)mkdir(path, 0755);
-    free(tmp);
-    return true;
-}
-
-static char *get_mount_target(const char *path)
-{
-    size_t pathlen = strlen(path), rootdir_len = strlen(FS_ROOT_DIR);
-    char *target;
-
-    if ((target = malloc(rootdir_len + pathlen + 1)) == NULL) {
-        perror("malloc mount target");
-        return NULL;
-    }
-
-    memcpy(target, FS_ROOT_DIR, rootdir_len);
-    memcpy(target + rootdir_len, path, pathlen + 1);
-    return target;
-}
-
-static bool bind_mount(const char *path, bool restricted, bool resolve)
-{
-    int mflags = MS_BIND | MS_REC;
-    char src[PATH_MAX], *target;
-
-    if (restricted)
-        mflags |= MS_NOSUID | MS_NODEV | MS_NOATIME;
-
-    if (resolve) {
-        if (realpath(path, src) == NULL) {
-            fprintf(stderr, "realpath of %s: %s\n", path, strerror(errno));
-            return false;
-        }
-    }
-
-    if ((target = get_mount_target(resolve ? src : path)) == NULL)
-        return false;
-
-    if (!makedirs(target)) {
-        free(target);
-        return false;
-    }
-
-    if (mount(resolve ? src : path, target, "", mflags, NULL) == -1) {
-        fprintf(stderr, "mount %s to %s: %s\n",
-                resolve ? src : path, target, strerror(errno));
-        free(target);
-        return false;
-    }
-
-    free(target);
-    return true;
-}
-
-static bool bind_file(const char *path)
-{
-    char *target, *tmp;
-
-    if ((target = get_mount_target(path)) == NULL)
-        return false;
-
-    if ((tmp = strdup(target)) == NULL) {
-        perror("strdup bind file target path");
-        free(target);
-        return false;
-    }
-
-    if (!makedirs(dirname(tmp))) {
-        free(target);
-        free(tmp);
-        return false;
-    }
-
-    free(tmp);
-
-    if (creat(target, 0666) == -1) {
-        fprintf(stderr, "unable to create %s: %s\n", target, strerror(errno));
-        free(target);
-        return false;
-    }
-
-    if (mount(path, target, "", MS_BIND, NULL) == -1) {
-        fprintf(stderr, "mount file %s to %s: %s\n",
-                path, target, strerror(errno));
-        free(target);
-        return false;
-    }
-
-    return true;
-}
-
-struct envar_offset {
-    int start;
-    int length;
-    int var_start;
-    int var_length;
-    struct envar_offset *next;
-};
-
-static struct envar_offset *alloc_offset(void)
-{
-    struct envar_offset *new_offset;
-    new_offset = malloc(sizeof(struct envar_offset));
-
-    if (new_offset == NULL) {
-        perror("malloc envar_offset");
-        return NULL;
-    }
-
-    new_offset->next = NULL;
-    return new_offset;
-}
-
-static struct envar_offset *push_offset(struct envar_offset *current,
-                                        struct envar_offset **base)
-{
-    if (current == NULL) {
-        if ((current = alloc_offset()) != NULL)
-            *base = current;
-        return current;
-    }
-
-    return current->next = alloc_offset();
-}
-
-static void free_offsets(struct envar_offset *base)
-{
-    struct envar_offset *next;
-    if (base == NULL)
-        return;
-    next = base->next;
-    free(base);
-    if (next != NULL)
-        free_offsets(next);
-}
-
-static char *expand_xdg_fallback(const char *xdg_var)
-{
-    static char *home = NULL;
-    static size_t homelen;
-    char *result;
-
-    if (home == NULL) {
-        if ((home = getenv("HOME")) == NULL) {
-            fputs("Unable find $HOME.\n", stderr);
-            return NULL;
-        }
-        homelen = strlen(home);
-    }
-
-    if (strcmp(xdg_var, "XDG_DATA_HOME") == 0) {
-        result = malloc(homelen + 14);
-        if (result == NULL) {
-            perror("malloc XDG_DATA_HOME");
-            return NULL;
-        }
-        memcpy(result, home, homelen);
-        memcpy(result + homelen, "/.local/share", 14);
-        return result;
-    } else if (strcmp(xdg_var, "XDG_CONFIG_HOME") == 0) {
-        result = malloc(homelen + 9);
-        if (result == NULL) {
-            perror("malloc XDG_CONFIG_HOME");
-            return NULL;
-        }
-        memcpy(result, home, homelen);
-        memcpy(result + homelen, "/.config", 9);
-        return result;
-    }
-
-    return NULL;
-}
-
-static char *get_offset_var(struct envar_offset *offset, const char *haystack)
-{
-    char *tmp, *result;
-
-    tmp = strndup(haystack + offset->var_start, offset->var_length);
-
-    if (tmp == NULL) {
-        perror("strndup");
-        return NULL;
-    }
-
-    result = getenv(tmp);
-    if (result == NULL) {
-        if ((result = expand_xdg_fallback(tmp)) == NULL) {
-            fprintf(stderr, "Unable find variable %s in %s\n", tmp, haystack);
-            free(tmp);
-            return NULL;
-        }
-        free(tmp);
-        return result;
-    }
-    free(tmp);
-    return strdup(result);
-}
-
-static char *replace_env_offset_free(const char *path,
-                                     struct envar_offset *offset)
-{
-    struct envar_offset *tmp_offset;
-    size_t buflen, pathlen, varlen, tmplen;
-    int inpos = 0, outpos = 0;
-    char *buf, *curvar;
-
-    buflen = pathlen = strlen(path);
-
-    if ((buf = malloc(buflen + 1)) == NULL) {
-        perror("malloc replace_env buffer");
-        return NULL;
-    }
-
-    while (offset != NULL) {
-        if ((curvar = get_offset_var(offset, path)) == NULL) {
-            free(buf);
-            free_offsets(offset);
-            return NULL;
-        }
-
-        varlen = strlen(curvar);
-        tmplen = varlen + (buflen - offset->length);
-
-        if (tmplen > buflen) {
-            if ((buf = realloc(buf, (buflen = tmplen) + 1)) == NULL) {
-                perror("realloc replace_env buffer");
-                free(buf);
-                free(curvar);
-                free_offsets(offset);
-                return NULL;
-            }
-        }
-
-        memcpy(buf + outpos, path + inpos, offset->start - inpos);
-        outpos += offset->start - inpos;
-        inpos = offset->start;
-
-        memcpy(buf + outpos, curvar, varlen);
-        outpos += varlen;
-        inpos += offset->length;
-
-        free(curvar);
-
-        tmp_offset = offset;
-        offset = offset->next;
-        free(tmp_offset);
-    }
-
-    memcpy(buf + outpos, path + inpos, pathlen - inpos);
-    *(buf + outpos + (pathlen - inpos)) = '\0';
-
-    return buf;
-}
-
-static char *replace_env(const char *path)
-{
-    int i = 0, start = 0, var_start = 0;
-    size_t pathlen;
-    bool in_var = false, curly = false;
-    struct envar_offset *base = NULL, *offset = NULL;
-
-    pathlen = strlen(path);
-
-    while (i < pathlen) {
-        if (path[i] == '$' && !curly && !in_var) {
-            if (i + 1 >= pathlen)
-                break;
-
-            start = i;
-
-            if (path[i + 1] == '{') {
-                curly = true;
-                var_start = i + 2;
-                ++i;
-            } else {
-                in_var = true;
-                var_start = i + 1;
-            }
-        } else if (in_var) {
-            if (!(path[i] >= 'a' && path[i] <= 'z') &&
-                !(path[i] >= 'A' && path[i] <= 'Z') &&
-                !(path[i] >= '0' && path[i] <= '9') &&
-                path[i] != '_'
-            ) {
-                in_var = false;
-
-                if ((offset = push_offset(offset, &base)) == NULL) {
-                    free_offsets(base);
-                    return NULL;
-                }
-
-                offset->start = start;
-                offset->length = i - start;
-                offset->var_start = var_start;
-                offset->var_length = i - var_start;
-                continue;
-            }
-        } else if (curly) {
-            if (path[i] == '}') {
-                curly = false;
-
-                if ((offset = push_offset(offset, &base)) == NULL) {
-                    free_offsets(base);
-                    return NULL;
-                }
-
-                offset->start = start;
-                offset->length = (i + 1) - offset->start;
-                offset->var_start = var_start;
-                offset->var_length = i - offset->var_start;
-            }
-        }
-
-        ++i;
-    }
-
-    if (in_var) {
-        if ((offset = push_offset(offset, &base)) == NULL) {
-            free_offsets(base);
-            return NULL;
-        }
-
-        offset->start = start;
-        offset->length = i - start;
-        offset->var_start = var_start;
-        offset->var_length = i - var_start;
-    }
-
-    return replace_env_offset_free(path, base);
-}
-
-static bool extra_mount(const char *path)
-{
-    char *expanded;
-    if ((expanded = replace_env(path)) == NULL)
-        return false;
-
-    if (!bind_mount(expanded, true, true)) {
-        free(expanded);
-        return false;
-    }
-
-    free(expanded);
-    return true;
-}
-
-static bool setup_xauthority(void)
-{
-    char *xauth, *home;
-    bool result;
-    size_t homelen;
-
-    if ((xauth = getenv("XAUTHORITY")) != NULL)
-        return bind_file(xauth);
-
-    if ((home = getenv("HOME")) == NULL) {
-        fputs("Unable find $HOME.\n", stderr);
-        return false;
-    }
-
-    homelen = strlen(home);
-
-    if ((xauth = malloc(homelen + 13)) == NULL) {
-        perror("malloc xauth file path");
-        return false;
-    }
-
-    memcpy(xauth, home, homelen);
-    memcpy(xauth + homelen, "/.Xauthority", 13);
-
-    result = bind_file(xauth);
-    free(xauth);
-    return result;
-}
-
-#include "params.c"
-
-static bool setup_chroot(void)
-{
-    int mflags;
-
-    mflags = MS_NOEXEC | MS_NOSUID | MS_NODEV | MS_NOATIME;
-
-    if (mount("none", FS_ROOT_DIR, "tmpfs", mflags, NULL) == -1) {
-        perror("mount rootfs");
-        return false;
-    }
-
-    if (!bind_mount("/etc", true, false))
-        return false;
-
-    if (!bind_mount("/dev", false, false))
-        return false;
-
-    if (!bind_mount("/proc", false, false))
-        return false;
-
-    if (!bind_mount("/sys", false, false))
-        return false;
-
-    if (!bind_mount("/run", false, false))
-        return false;
-
-    if (!bind_mount("/var/run", false, false))
-        return false;
-
-    if (!bind_mount("/tmp", true, false))
-        return false;
-
-    if (!setup_app_paths())
-        return false;
-
-    if (!setup_xauthority())
-        return false;
-
-    if (chroot(FS_ROOT_DIR) == -1) {
-        perror("chroot");
-        return false;
-    }
-
-    if (chdir("/") == -1) {
-        perror("chdir rootfs");
-        return false;
-    }
-
-    return true;
-}
+#include "setup.h"
 
 int main(int argc, char **argv)
 {
-    int sync_pipe[2];
-    char sync_status = '.';
-    int child_status;
-    pid_t pid, parent_pid;
-
-    if (pipe(sync_pipe) == -1) {
-        perror("pipe");
-        return 1;
-    }
-
-    parent_pid = getpid();
-
-    switch (pid = fork()) {
-        case -1:
-            perror("fork");
-            return 1;
-        case 0:
-            close(sync_pipe[1]);
-            if (read(sync_pipe[0], &sync_status, 1) == -1) {
-                perror("read pipe from parent");
-                _exit(1);
-            } else if (sync_status == 'X')
-                _exit(1);
-            close(sync_pipe[0]);
-            _exit(write_maps(parent_pid) ? 0 : 1);
-        default:
-            if (unshare(CLONE_NEWNS | CLONE_NEWUSER) == -1) {
-                perror("unshare");
-                if (write(sync_pipe[1], "X", 1) == -1)
-                    perror("signal child exit");
-                waitpid(pid, NULL, 0);
-                return 1;
-            }
-
-            close(sync_pipe[1]);
-            waitpid(pid, &child_status, 0);
-            if (WIFEXITED(child_status) && WEXITSTATUS(child_status) == 0)
-                break;
-            return 1;
-    }
-
-    if (!setup_chroot())
+    if (!setup_sandbox())
         return 1;
 
     argv[0] = WRAPPED_PROGNAME;
diff --git a/pkgs/games/build-support/build-sandbox/src/setup.c b/pkgs/games/build-support/build-sandbox/src/setup.c
new file mode 100644
index 00000000..72b2f80e
--- /dev/null
+++ b/pkgs/games/build-support/build-sandbox/src/setup.c
@@ -0,0 +1,581 @@
+#define _GNU_SOURCE
+
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <limits.h>
+#include <malloc.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "params.h"
+
+static bool write_proc(int proc_pid_fd, const char *fname, const char *buf,
+                       size_t buflen, bool ignore_errors)
+{
+    int fd;
+
+    if ((fd = openat(proc_pid_fd, fname, O_WRONLY)) == -1) {
+        fprintf(stderr, "open %s: %s\n", fname, strerror(errno));
+        return false;
+    }
+
+    if (write(fd, buf, buflen) == -1) {
+        if (!ignore_errors)
+            fprintf(stderr, "write %s: %s\n", fname, strerror(errno));
+        close(fd);
+        return ignore_errors;
+    }
+
+    close(fd);
+    return true;
+}
+
+#define WRITE_IDMAP(file, value) \
+    buflen = snprintf(buf, 100, "%1$lu %1$lu 1", (unsigned long)value); \
+    if (buflen >= 100) { \
+        fputs("Unable to write buffer for " file ".\n", stderr); \
+        close(proc_pid_fd); \
+        return false; \
+    } else if (buflen < 0) { \
+        perror("snprintf " file " buffer"); \
+        close(proc_pid_fd); \
+        return false; \
+    } \
+    if (!write_proc(proc_pid_fd, file, buf, buflen, false)) { \
+        close(proc_pid_fd); \
+        return false; \
+    }
+
+bool write_maps(pid_t parent_pid)
+{
+    int proc_pid_fd;
+    size_t buflen;
+    char buf[100];
+
+    buflen = snprintf(buf, 100, "/proc/%lu", (unsigned long)parent_pid);
+    if (buflen >= 100) {
+        fputs("Unable to write buffer for child pid proc path.\n", stderr);
+        return false;
+    } else if (buflen < 0) {
+        perror("snprintf child pid proc path");
+        return false;
+    }
+
+    if ((proc_pid_fd = open(buf, O_RDONLY | O_DIRECTORY)) == -1) {
+        fprintf(stderr, "open %s: %s\n", buf, strerror(errno));
+        return false;
+    }
+
+    WRITE_IDMAP("uid_map", geteuid());
+
+    // Kernels prior to Linux 3.19 which do not impose setgroups()
+    // restrictions won't have this file, so ignore failure.
+    write_proc(proc_pid_fd, "setgroups", "deny", 4, true);
+
+    WRITE_IDMAP("gid_map", getegid());
+
+    return true;
+}
+
+static bool makedirs(const char *path)
+{
+    char *tmp, *segment;
+
+    if ((tmp = strdup(path)) == NULL) {
+        fprintf(stderr, "strdup of %s: %s\n", path, strerror(errno));
+        return false;
+    }
+
+    segment = dirname(tmp);
+
+    if (!(segment[0] == '/' && segment[1] == '\0')) {
+        if (!makedirs(segment)) {
+            free(tmp);
+            return false;
+        }
+    }
+
+    (void)mkdir(path, 0755);
+    free(tmp);
+    return true;
+}
+
+static char *get_mount_target(const char *path)
+{
+    size_t pathlen = strlen(path), rootdir_len = strlen(FS_ROOT_DIR);
+    char *target;
+
+    if ((target = malloc(rootdir_len + pathlen + 1)) == NULL) {
+        perror("malloc mount target");
+        return NULL;
+    }
+
+    memcpy(target, FS_ROOT_DIR, rootdir_len);
+    memcpy(target + rootdir_len, path, pathlen + 1);
+    return target;
+}
+
+bool bind_mount(const char *path, bool restricted, bool resolve)
+{
+    int mflags = MS_BIND | MS_REC;
+    char src[PATH_MAX], *target;
+
+    if (restricted)
+        mflags |= MS_NOSUID | MS_NODEV | MS_NOATIME;
+
+    if (resolve) {
+        if (realpath(path, src) == NULL) {
+            fprintf(stderr, "realpath of %s: %s\n", path, strerror(errno));
+            return false;
+        }
+    }
+
+    if ((target = get_mount_target(resolve ? src : path)) == NULL)
+        return false;
+
+    if (!makedirs(target)) {
+        free(target);
+        return false;
+    }
+
+    if (mount(resolve ? src : path, target, "", mflags, NULL) == -1) {
+        fprintf(stderr, "mount %s to %s: %s\n",
+                resolve ? src : path, target, strerror(errno));
+        free(target);
+        return false;
+    }
+
+    free(target);
+    return true;
+}
+
+static bool bind_file(const char *path)
+{
+    char *target, *tmp;
+
+    if ((target = get_mount_target(path)) == NULL)
+        return false;
+
+    if ((tmp = strdup(target)) == NULL) {
+        perror("strdup bind file target path");
+        free(target);
+        return false;
+    }
+
+    if (!makedirs(dirname(tmp))) {
+        free(target);
+        free(tmp);
+        return false;
+    }
+
+    free(tmp);
+
+    if (creat(target, 0666) == -1) {
+        fprintf(stderr, "unable to create %s: %s\n", target, strerror(errno));
+        free(target);
+        return false;
+    }
+
+    if (mount(path, target, "", MS_BIND, NULL) == -1) {
+        fprintf(stderr, "mount file %s to %s: %s\n",
+                path, target, strerror(errno));
+        free(target);
+        return false;
+    }
+
+    return true;
+}
+
+struct envar_offset {
+    int start;
+    int length;
+    int var_start;
+    int var_length;
+    struct envar_offset *next;
+};
+
+static struct envar_offset *alloc_offset(void)
+{
+    struct envar_offset *new_offset;
+    new_offset = malloc(sizeof(struct envar_offset));
+
+    if (new_offset == NULL) {
+        perror("malloc envar_offset");
+        return NULL;
+    }
+
+    new_offset->next = NULL;
+    return new_offset;
+}
+
+static struct envar_offset *push_offset(struct envar_offset *current,
+                                        struct envar_offset **base)
+{
+    if (current == NULL) {
+        if ((current = alloc_offset()) != NULL)
+            *base = current;
+        return current;
+    }
+
+    return current->next = alloc_offset();
+}
+
+static void free_offsets(struct envar_offset *base)
+{
+    struct envar_offset *next;
+    if (base == NULL)
+        return;
+    next = base->next;
+    free(base);
+    if (next != NULL)
+        free_offsets(next);
+}
+
+static char *expand_xdg_fallback(const char *xdg_var)
+{
+    static char *home = NULL;
+    static size_t homelen;
+    char *result;
+
+    if (home == NULL) {
+        if ((home = getenv("HOME")) == NULL) {
+            fputs("Unable find $HOME.\n", stderr);
+            return NULL;
+        }
+        homelen = strlen(home);
+    }
+
+    if (strcmp(xdg_var, "XDG_DATA_HOME") == 0) {
+        result = malloc(homelen + 14);
+        if (result == NULL) {
+            perror("malloc XDG_DATA_HOME");
+            return NULL;
+        }
+        memcpy(result, home, homelen);
+        memcpy(result + homelen, "/.local/share", 14);
+        return result;
+    } else if (strcmp(xdg_var, "XDG_CONFIG_HOME") == 0) {
+        result = malloc(homelen + 9);
+        if (result == NULL) {
+            perror("malloc XDG_CONFIG_HOME");
+            return NULL;
+        }
+        memcpy(result, home, homelen);
+        memcpy(result + homelen, "/.config", 9);
+        return result;
+    }
+
+    return NULL;
+}
+
+static char *get_offset_var(struct envar_offset *offset, const char *haystack)
+{
+    char *tmp, *result;
+
+    tmp = strndup(haystack + offset->var_start, offset->var_length);
+
+    if (tmp == NULL) {
+        perror("strndup");
+        return NULL;
+    }
+
+    result = getenv(tmp);
+    if (result == NULL) {
+        if ((result = expand_xdg_fallback(tmp)) == NULL) {
+            fprintf(stderr, "Unable find variable %s in %s\n", tmp, haystack);
+            free(tmp);
+            return NULL;
+        }
+        free(tmp);
+        return result;
+    }
+    free(tmp);
+    return strdup(result);
+}
+
+static char *replace_env_offset_free(const char *path,
+                                     struct envar_offset *offset)
+{
+    struct envar_offset *tmp_offset;
+    size_t buflen, pathlen, varlen, tmplen;
+    int inpos = 0, outpos = 0;
+    char *buf, *curvar;
+
+    buflen = pathlen = strlen(path);
+
+    if ((buf = malloc(buflen + 1)) == NULL) {
+        perror("malloc replace_env buffer");
+        return NULL;
+    }
+
+    while (offset != NULL) {
+        if ((curvar = get_offset_var(offset, path)) == NULL) {
+            free(buf);
+            free_offsets(offset);
+            return NULL;
+        }
+
+        varlen = strlen(curvar);
+        tmplen = varlen + (buflen - offset->length);
+
+        if (tmplen > buflen) {
+            if ((buf = realloc(buf, (buflen = tmplen) + 1)) == NULL) {
+                perror("realloc replace_env buffer");
+                free(buf);
+                free(curvar);
+                free_offsets(offset);
+                return NULL;
+            }
+        }
+
+        memcpy(buf + outpos, path + inpos, offset->start - inpos);
+        outpos += offset->start - inpos;
+        inpos = offset->start;
+
+        memcpy(buf + outpos, curvar, varlen);
+        outpos += varlen;
+        inpos += offset->length;
+
+        free(curvar);
+
+        tmp_offset = offset;
+        offset = offset->next;
+        free(tmp_offset);
+    }
+
+    memcpy(buf + outpos, path + inpos, pathlen - inpos);
+    *(buf + outpos + (pathlen - inpos)) = '\0';
+
+    return buf;
+}
+
+static char *replace_env(const char *path)
+{
+    int i = 0, start = 0, var_start = 0;
+    size_t pathlen;
+    bool in_var = false, curly = false;
+    struct envar_offset *base = NULL, *offset = NULL;
+
+    pathlen = strlen(path);
+
+    while (i < pathlen) {
+        if (path[i] == '$' && !curly && !in_var) {
+            if (i + 1 >= pathlen)
+                break;
+
+            start = i;
+
+            if (path[i + 1] == '{') {
+                curly = true;
+                var_start = i + 2;
+                ++i;
+            } else {
+                in_var = true;
+                var_start = i + 1;
+            }
+        } else if (in_var) {
+            if (!(path[i] >= 'a' && path[i] <= 'z') &&
+                !(path[i] >= 'A' && path[i] <= 'Z') &&
+                !(path[i] >= '0' && path[i] <= '9') &&
+                path[i] != '_'
+            ) {
+                in_var = false;
+
+                if ((offset = push_offset(offset, &base)) == NULL) {
+                    free_offsets(base);
+                    return NULL;
+                }
+
+                offset->start = start;
+                offset->length = i - start;
+                offset->var_start = var_start;
+                offset->var_length = i - var_start;
+                continue;
+            }
+        } else if (curly) {
+            if (path[i] == '}') {
+                curly = false;
+
+                if ((offset = push_offset(offset, &base)) == NULL) {
+                    free_offsets(base);
+                    return NULL;
+                }
+
+                offset->start = start;
+                offset->length = (i + 1) - offset->start;
+                offset->var_start = var_start;
+                offset->var_length = i - offset->var_start;
+            }
+        }
+
+        ++i;
+    }
+
+    if (in_var) {
+        if ((offset = push_offset(offset, &base)) == NULL) {
+            free_offsets(base);
+            return NULL;
+        }
+
+        offset->start = start;
+        offset->length = i - start;
+        offset->var_start = var_start;
+        offset->var_length = i - var_start;
+    }
+
+    return replace_env_offset_free(path, base);
+}
+
+bool extra_mount(const char *path)
+{
+    char *expanded;
+    if ((expanded = replace_env(path)) == NULL)
+        return false;
+
+    if (!bind_mount(expanded, true, true)) {
+        free(expanded);
+        return false;
+    }
+
+    free(expanded);
+    return true;
+}
+
+static bool setup_xauthority(void)
+{
+    char *xauth, *home;
+    bool result;
+    size_t homelen;
+
+    if ((xauth = getenv("XAUTHORITY")) != NULL)
+        return bind_file(xauth);
+
+    if ((home = getenv("HOME")) == NULL) {
+        fputs("Unable find $HOME.\n", stderr);
+        return false;
+    }
+
+    homelen = strlen(home);
+
+    if ((xauth = malloc(homelen + 13)) == NULL) {
+        perror("malloc xauth file path");
+        return false;
+    }
+
+    memcpy(xauth, home, homelen);
+    memcpy(xauth + homelen, "/.Xauthority", 13);
+
+    result = bind_file(xauth);
+    free(xauth);
+    return result;
+}
+
+static bool setup_chroot(void)
+{
+    int mflags;
+
+    mflags = MS_NOEXEC | MS_NOSUID | MS_NODEV | MS_NOATIME;
+
+    if (mount("none", FS_ROOT_DIR, "tmpfs", mflags, NULL) == -1) {
+        perror("mount rootfs");
+        return false;
+    }
+
+    if (!bind_mount("/etc", true, false))
+        return false;
+
+    if (!bind_mount("/dev", false, false))
+        return false;
+
+    if (!bind_mount("/proc", false, false))
+        return false;
+
+    if (!bind_mount("/sys", false, false))
+        return false;
+
+    if (!bind_mount("/run", false, false))
+        return false;
+
+    if (!bind_mount("/var/run", false, false))
+        return false;
+
+    if (!bind_mount("/tmp", true, false))
+        return false;
+
+    if (!setup_app_paths())
+        return false;
+
+    if (!setup_xauthority())
+        return false;
+
+    if (chroot(FS_ROOT_DIR) == -1) {
+        perror("chroot");
+        return false;
+    }
+
+    if (chdir("/") == -1) {
+        perror("chdir rootfs");
+        return false;
+    }
+
+    return true;
+}
+
+bool setup_sandbox(void)
+{
+    int sync_pipe[2];
+    char sync_status = '.';
+    int child_status;
+    pid_t pid, parent_pid;
+
+    if (pipe(sync_pipe) == -1) {
+        perror("pipe");
+        return false;
+    }
+
+    parent_pid = getpid();
+
+    switch (pid = fork()) {
+        case -1:
+            perror("fork");
+            return false;
+        case 0:
+            close(sync_pipe[1]);
+            if (read(sync_pipe[0], &sync_status, 1) == -1) {
+                perror("read pipe from parent");
+                _exit(1);
+            } else if (sync_status == 'X')
+                _exit(1);
+            close(sync_pipe[0]);
+            _exit(write_maps(parent_pid) ? 0 : 1);
+        default:
+            if (unshare(CLONE_NEWNS | CLONE_NEWUSER) == -1) {
+                perror("unshare");
+                if (write(sync_pipe[1], "X", 1) == -1)
+                    perror("signal child exit");
+                waitpid(pid, NULL, 0);
+                return false;
+            }
+
+            close(sync_pipe[1]);
+            waitpid(pid, &child_status, 0);
+            if (WIFEXITED(child_status) && WEXITSTATUS(child_status) == 0)
+                break;
+            return false;
+    }
+
+    if (!setup_chroot())
+        return false;
+
+    return true;
+}
diff --git a/pkgs/games/build-support/build-sandbox/src/setup.h b/pkgs/games/build-support/build-sandbox/src/setup.h
new file mode 100644
index 00000000..19cc6ca2
--- /dev/null
+++ b/pkgs/games/build-support/build-sandbox/src/setup.h
@@ -0,0 +1,12 @@
+#ifndef _SETUP_H
+#define _SETUP_H
+
+#include <stdbool.h>
+#include <sys/types.h>
+
+bool write_maps(pid_t parent_pid);
+bool bind_mount(const char *path, bool restricted, bool resolve);
+bool extra_mount(const char *path);
+bool setup_sandbox(void);
+
+#endif