[PATCH v4 1/7] util: Introduce qemu_get_runtime_dir()

[Akihiko Odaki <akihiko.odaki@daynix.com>]

qemu_get_runtime_dir() returns a dynamically allocated directory path
that is appropriate for storing runtime files. It corresponds to "run"
directory in Unix.

With a tree-wide search, it was found that there are several cases
where such a functionality is implemented so let's have one as a common
utlity function.

A notable feature of qemu_get_runtime_dir() is that it uses
$XDG_RUNTIME_DIR if available. While the function is often called by
executables which requires root privileges, it is still possible that
they are called from a user without privilege to write the system
runtime directory. In fact, I decided to write this patch when I ran
virtiofsd in a Linux namespace created by a normal user and realized
it tries to write the system runtime directory, not writable in this
case. $XDG_RUNTIME_DIR should provide a writable directory in such
cases.

This function does not use qemu_get_local_state_dir() or its logic
for Windows. Actually the implementation of qemu_get_local_state_dir()
for Windows seems not right as it calls g_get_system_data_dirs(),
which refers to $XDG_DATA_DIRS. In Unix terminology, it is basically
"/usr/share", not "/var", which qemu_get_local_state_dir() is intended
to provide. Instead, this function try to use the following in order:
- $XDG_RUNTIME_DIR
- LocalAppData folder
- get_relocated_path(CONFIG_QEMU_LOCALSTATEDIR "/run")

This function does not use g_get_user_runtime_dir() either as it
falls back to g_get_user_cache_dir() when $XDG_DATA_DIRS is not
available. In the case, we rather use:
get_relocated_path(CONFIG_QEMU_LOCALSTATEDIR "/run")

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230921075425.16738-2-akihiko.odaki@daynix.com>
---
 include/qemu/osdep.h | 12 ++++++++++++
 util/oslib-posix.c   | 11 +++++++++++
 util/oslib-win32.c   | 26 ++++++++++++++++++++++++++
 3 files changed, 49 insertions(+)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 191916f38e6d..fe8609fc1375 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -670,6 +670,18 @@ void qemu_set_cloexec(int fd);
  */
 char *qemu_get_local_state_dir(void);
 
+/**
+ * qemu_get_runtime_dir:
+ *
+ * Return a dynamically allocated directory path that is appropriate for storing
+ * runtime files. It corresponds to "run" directory in Unix, and uses
+ * $XDG_RUNTIME_DIR if available.
+ *
+ * The caller is responsible for releasing the value returned with g_free()
+ * after use.
+ */
+char *qemu_get_runtime_dir(void);
+
 /**
  * qemu_getauxval:
  * @type: the auxiliary vector key to lookup
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index e76441695bdc..9599509a9aa7 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -278,6 +278,17 @@ qemu_get_local_state_dir(void)
     return get_relocated_path(CONFIG_QEMU_LOCALSTATEDIR);
 }
 
+char *
+qemu_get_runtime_dir(void)
+{
+    char *env = getenv("XDG_RUNTIME_DIR");
+    if (env) {
+        return g_strdup(env);
+    }
+
+    return get_relocated_path(CONFIG_QEMU_LOCALSTATEDIR "/run");
+}
+
 void qemu_set_tty_echo(int fd, bool echo)
 {
     struct termios tty;
diff --git a/util/oslib-win32.c b/util/oslib-win32.c
index b623830d624f..8c5a02ee881d 100644
--- a/util/oslib-win32.c
+++ b/util/oslib-win32.c
@@ -27,6 +27,8 @@
  */
 
 #include "qemu/osdep.h"
+#include <shlobj.h>
+#include <wchar.h>
 #include <windows.h>
 #include "qapi/error.h"
 #include "qemu/main-loop.h"
@@ -237,6 +239,30 @@ qemu_get_local_state_dir(void)
     return g_strdup(data_dirs[0]);
 }
 
+char *
+qemu_get_runtime_dir(void)
+{
+    size_t size = GetEnvironmentVariableA("XDG_RUNTIME_DIR", NULL, 0);
+    if (size) {
+        char *env = g_malloc(size);
+        GetEnvironmentVariableA("XDG_RUNTIME_DIR", env, size);
+        return env;
+    }
+
+    PWSTR wpath;
+    const wchar_t *cwpath;
+    if (!SHGetKnownFolderPath(&FOLDERID_LocalAppData, KF_FLAG_DEFAULT, NULL, &wpath)) {
+        cwpath = wpath;
+        size = wcsrtombs(NULL, &cwpath, 0, &(mbstate_t){0}) + 1;
+        char *path = g_malloc(size);
+        wcsrtombs(path, &cwpath, size, &(mbstate_t){0});
+        CoTaskMemFree(wpath);
+        return path;
+    }
+
+    return get_relocated_path(CONFIG_QEMU_LOCALSTATEDIR "/run");
+}
+
 void qemu_set_tty_echo(int fd, bool echo)
 {
     HANDLE handle = (HANDLE)_get_osfhandle(fd);

-- 
2.45.2