diff --git a/Sources/libreprl/libreprl-posix.c b/Sources/libreprl/libreprl-posix.c
index 34905d91d..f523e76eb 100644
--- a/Sources/libreprl/libreprl-posix.c
+++ b/Sources/libreprl/libreprl-posix.c
@@ -23,9 +23,11 @@
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <poll.h>
 #include <signal.h>
 #include <stdarg.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -33,19 +35,42 @@
 #include <sys/resource.h>
 #include <sys/time.h>
 #include <sys/types.h>
+#include <sys/utsname.h>
 #include <sys/wait.h>
 #include <time.h>
 #include <unistd.h>
 
+#ifdef __linux__
+#  ifdef __has_include
+#    if __has_include(<linux/close_range.h>)
+#      define HAS_CLOSE_RANGE_HEADERS
+#    endif
+#  elif defined(__FreeBSD__)
+#    ifdef CLOSE_RANGE_CLOEXEC
+#      define HAS_CLOSE_RANGE_HEADERS
+#    endif
+#  endif
+#endif
+
 // Well-known file descriptor numbers for reprl <-> child communication, child process side
+// Make sure you modify reprl_fds[] below if you change these.
 #define REPRL_CHILD_CTRL_IN 100
 #define REPRL_CHILD_CTRL_OUT 101
 #define REPRL_CHILD_DATA_IN 102
 #define REPRL_CHILD_DATA_OUT 103
 
+static const int reprl_fds[] = {
+    REPRL_CHILD_CTRL_IN,
+    REPRL_CHILD_CTRL_OUT,
+    REPRL_CHILD_DATA_IN,
+    REPRL_CHILD_DATA_OUT
+};
+
 /// Maximum timeout in microseconds. Mostly just limited by the fact that the timeout in milliseconds has to fit into a 32-bit integer.
 #define REPRL_MAX_TIMEOUT_IN_MICROSECONDS ((uint64_t)(INT_MAX) * 1000)
 
+#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
+
 static size_t min(size_t x, size_t y) {
   return x < y ? x : y;
 }
@@ -79,6 +104,101 @@ static void free_string_array(char** arr)
     free(arr);
 }
 
+static int getdtablesize_or_crash() {
+    const int tablesize = getdtablesize();
+    if (tablesize < 0) {
+        fprintf(stderr, "getdtablesize() failed: %s. This likely means the system is borked.\n",
+                strerror(errno));
+        abort();
+    }
+
+    return tablesize;
+}
+
+static bool system_supports_close_range() {
+#ifdef HAS_CLOSE_RANGE_HEADERS
+#  ifdef __linux__
+    struct utsname buffer;
+    int major, minor, patch;
+    (void)uname(&buffer); // Linux uname can only throw EFAULT if buf is invalid.
+
+    if (sscanf(buffer.release, "%d.%d.%d", &major, &minor, &patch) != 3) {
+        return false;
+    }
+
+    return major > 5 || (major == 5 && minor >= 9);
+#  elif defined(__FreeBSD__)
+    // TODO: Technically, FreeBSD does support close_range, but I don't need support for it right
+    //                    now, so leaving this unimplemented. Don't have a platform to test on.
+    // https://man.freebsd.org/cgi/man.cgi?close_range(2)
+    return false;
+#  else
+    return false;
+#  endif
+#else
+    return false;
+#endif
+}
+
+static int fd_qsort_compare(const void* a, const void* b) {
+    int fd_a = *(const int*)a;
+    int fd_b = *(const int*)b;
+    return (fd_a > fd_b) - (fd_a < fd_b);
+}
+
+/// Fast path which uses close_range() to close ranges of fds.
+static void close_all_non_reprl_fds_fast() {
+#ifdef HAS_CLOSE_RANGE_HEADERS
+    // Unfortunately we cannot trust the reprl_fds array to be sorted since an accidental edit to
+    // reprl_fds could have broken that assumption. So let's create a new sorted array. It's cheap
+    // anyways.
+    int sorted_reprl_fds[ARRAY_SIZE(reprl_fds) + 2];
+    sorted_reprl_fds[0] = 3; // Skip the well-known stdin, stdout, stderr fds.
+    memcpy(sorted_reprl_fds + 1, reprl_fds, sizeof(reprl_fds));
+    sorted_reprl_fds[ARRAY_SIZE(sorted_reprl_fds) - 1] = getdtablesize_or_crash();
+    qsort(sorted_reprl_fds, ARRAY_SIZE(sorted_reprl_fds), sizeof(int), fd_qsort_compare);
+
+    // Cool, now we will iterate the sorted fds in ranges and close everything in between.
+    int start_fd = 3, end_fd;
+    for (size_t i = 0; i < ARRAY_SIZE(sorted_reprl_fds); i++) {
+        end_fd = sorted_reprl_fds[i];
+        if (start_fd < end_fd) {
+            // Close the range [start_fd, end_fd)
+            close_range(start_fd, end_fd - 1, 0);
+        }
+        start_fd = end_fd + 1;
+    }
+#else
+    fprintf(stderr, "close_all_non_reprl_fds_fast() called on a system which does not support "
+            "close_range(). This is likely a programming bug.\n");
+    abort();
+#endif
+}
+
+/// Fallback path which makes a close() syscall for each non-REPRL fd.
+static void close_all_non_reprl_fds_slow() {
+    const int tablesize = getdtablesize_or_crash();
+
+    for (int i = 3; i < tablesize; i++) {
+        bool is_reprl_fd = false;
+        for (size_t j = 0; j < ARRAY_SIZE(reprl_fds); j++) {
+            if (i == reprl_fds[j]) {
+                is_reprl_fd = true;
+                break;
+            }
+        }
+
+        if (!is_reprl_fd) {
+            close(i);
+        }
+    }
+}
+
+/// Close all file descriptors except the well-known REPRL and stdio fds.
+static void close_all_non_reprl_fds() {
+    system_supports_close_range() ? close_all_non_reprl_fds_fast() : close_all_non_reprl_fds_slow();
+}
+
 // A unidirectional communication channel for larger amounts of data, up to a maximum size (REPRL_MAX_DATA_SIZE).
 // Implemented as a (RAM-backed) file for which the file descriptor is shared with the child process and which is mapped into our address space.
 struct data_channel {
@@ -250,13 +370,7 @@ static int reprl_spawn_child(struct reprl_context* ctx)
         close(devnull);
 
         // close all other FDs. We try to use FD_CLOEXEC everywhere, but let's be extra sure we don't leak any fds to the child.
-        int tablesize = getdtablesize();
-        for (int i = 3; i < tablesize; i++) {
-            if (i == REPRL_CHILD_CTRL_IN || i == REPRL_CHILD_CTRL_OUT || i == REPRL_CHILD_DATA_IN || i == REPRL_CHILD_DATA_OUT) {
-                continue;
-            }
-            close(i);
-        }
+        close_all_non_reprl_fds();
 
         execve(ctx->argv[0], ctx->argv, ctx->envp);