llvmpipe: Work around WSL 1 missing support for memfd_create()

WSL version 1 has a known limitation that the kernel does not support
the memfd_create() syscall, and it always returns -1 (see
https://github.com/microsoft/WSL/issues/3542). This results in
lavapipe/llvmpipe failing to create the anonymous file it needs for
allocations from the pipe_screen object, which in turn results in
results in failed application calls to Vulkan APIs (in the case I
observed, vkMapMemory returned an invalid (0xffffffff) pointer along
with VK_SUCCESS, leading to the application segfaulting).

This issue can be reproduced by simply running `vkcube` (or, presumably
any other simple Vulkan application) inside WSL 1, e.g.:

xvfb-run -s '-screen 0 1024x768x24' vkcube --c 300

This patch addresses the issue with several changes:

1. llvmpipe_create_screen() now checks for errors from the
   os_create_anonymous_file function and errors out early, making it
   easier to track down issues similar to this. Previously, the invalid
   fd of -1 would be stored in the pipe_screen struct and the problems
   would only appear later.
2. os_create_anonymous_file() now attempts to handle the case where
   memfd_create() fails, by falling back to creating a file in a
   temporary directory. This fallback is the same as is already done for
   builds that don't have memfd_create available at build time - note
   that the difference here is that this is a _runtime_ fallback, as is
   needed for the case of WSL 1.
3. The fallback logic previously relied on the XDG_RUNTIME_DIR
   environment variable, which might not be set when running inside WSL
   because there is unlikely to be a desktop environment configured.
   This patch adds a fallback of creating a new directory inside /tmp in
   this case.

Reviewed-by: Lucas Fryzek <lfryzek@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37679>

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -1031,6 +1031,11 @@ llvmpipe_create_screen(struct sw_winsys *winsys)
    util_vma_heap_init(&screen->mem_heap, alignment, UINT64_MAX - alignment);
    screen->mem_heap.alloc_high = false;
    screen->fd_mem_alloc = os_create_anonymous_file(0, "allocation fd");
+   if (screen->fd_mem_alloc == -1) {
+      mesa_loge("Failed to create anonymous file for memory allocations\n");
+      llvmpipe_destroy_screen(&screen->base);
+      return NULL;
+   }
 #endif
 
    snprintf(screen->renderer_string, sizeof(screen->renderer_string),

src/util/anon_file.c

@@ -1,5 +1,6 @@
 /*
  * Copyright © 2012 Collabora, Ltd.
+ * Copyright (C) 2025 Arm Ltd.
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
@@ -42,11 +43,16 @@
 #elif DETECT_OS_ANDROID
 #include <sys/syscall.h>
 #include <linux/memfd.h>
-#else
+#endif
+
+#if !(defined(__FreeBSD__) || DETECT_OS_ANDROID)
+#include "log.h"
+#include "os_misc.h"
+#include <sys/stat.h>
 #include <stdio.h>
 #endif
 
-#if !(defined(__FreeBSD__) || defined(HAVE_MEMFD_CREATE) || defined(HAVE_MKOSTEMP) || DETECT_OS_ANDROID)
+#if !(defined(__FreeBSD__) || defined(HAVE_MKOSTEMP) || DETECT_OS_ANDROID)
 static int
 set_cloexec_or_close(int fd)
 {
@@ -70,7 +76,7 @@ err:
 }
 #endif
 
-#if !(defined(__FreeBSD__) || defined(HAVE_MEMFD_CREATE) || DETECT_OS_ANDROID)
+#if !(defined(__FreeBSD__) || DETECT_OS_ANDROID)
 static int
 create_tmpfile_cloexec(char *tmpname)
 {
@@ -95,6 +101,61 @@ create_tmpfile_cloexec(char *tmpname)
 }
 #endif
 
+#if !(defined(__FreeBSD__) || DETECT_OS_ANDROID)
+/*
+ * Gets the path to a suitable temporary directory for the current user.
+ * Prefers using the environment variable `XDG_RUNTIME_DIR` if set,
+ * otherwise falls back to creating or re-using a folder in `/tmp`.
+ * Copies the path into the given `buf` of length `len` and also returns
+ * a pointer to the same buffer for convenience.
+ * Returns NULL if no suitable directory can found or created.
+ */
+static char*
+get_or_create_user_temp_dir(char* buf, size_t len) {
+    const char* env;
+    struct stat st;
+    int uid = getuid();
+
+    env = os_get_option("XDG_RUNTIME_DIR");
+    if (env && env[0] != '\0') {
+        snprintf(buf, len, "%s", env);
+        return buf;
+    }
+
+    snprintf(buf, len, "/tmp/xdg-runtime-mesa-%ld", (long)getuid());
+    mesa_logd("%s: XDG_RUNTIME_DIR not set; falling back to temp dir %s",
+        __func__, buf);
+    if (stat(buf, &st) == 0) {
+        /* If already exists, confirm the owner/permissions */
+        if (!S_ISDIR(st.st_mode)) {
+            mesa_loge(
+                "%s: %s exists but is not a directory", __func__, buf);
+            return NULL;
+        }
+        if (st.st_uid != uid) {
+            mesa_loge(
+                "%s: %s exists but has wrong owner", __func__, buf);
+            return NULL;
+        }
+
+        return buf;
+    } else if (errno == ENOENT) {
+        /* Doesn't exist, try to create it */
+        if (mkdir(buf, 0700) != 0) {
+            mesa_loge("%s: mkdir %s failed: %s", __func__, buf,
+                strerror(errno));
+            return NULL;
+        }
+
+        return buf;
+    } else {
+        mesa_loge("%s: stat %s failed: %s", __func__, buf, 
+            strerror(errno));
+        return NULL;
+    }
+}
+#endif
+
 /*
  * Create a new, unique, anonymous file of the given size, and
  * return the file descriptor for it. The file descriptor is set
@@ -116,7 +177,8 @@ create_tmpfile_cloexec(char *tmpname)
 int
 os_create_anonymous_file(int64_t size, const char *debug_name)
 {
-   int fd, ret;
+   int fd = -1, ret;
+   /* First try using preferred APIs */
 #if defined(HAVE_MEMFD_CREATE)
    if (!debug_name)
       debug_name = "mesa-shared";
@@ -132,26 +194,33 @@ os_create_anonymous_file(int64_t size, const char *debug_name)
    fd = shm_mkstemp(template);
    if (fd != -1)
       shm_unlink(template);
-#else
-   const char *path;
-   char *name;
+#endif
 
-   path = getenv("XDG_RUNTIME_DIR");
-   if (!path) {
-      errno = ENOENT;
-      return -1;
-   }
+    /*
+     * If preferred API failed (or not included in this build),
+     * fall back to using a file in a temporary dir
+     */
+#if !(defined(__FreeBSD__) || DETECT_OS_ANDROID)
+    if (fd == -1) {
+        char path[PATH_MAX];
+        char *name;
 
-   if (debug_name)
-      asprintf(&name, "%s/mesa-shared-%s-XXXXXX", path, debug_name);
-   else
-      asprintf(&name, "%s/mesa-shared-XXXXXX", path);
-   if (!name)
-      return -1;
+        if (!get_or_create_user_temp_dir(path, sizeof(path))) {
+            errno = ENOENT;
+            return -1;
+        }
 
-   fd = create_tmpfile_cloexec(name);
+        if (debug_name)
+            asprintf(&name, "%s/mesa-shared-%s-XXXXXX", path, debug_name);
+        else
+            asprintf(&name, "%s/mesa-shared-XXXXXX", path);
+        if (!name)
+            return -1;
 
-   free(name);
+        fd = create_tmpfile_cloexec(name);
+
+        free(name);
+    }
 #endif
 
    if (fd < 0)