[PATCH v6 09/11] 9p: darwin: Implement compatibility for mknodat

[Will Cohen <wwcohen@gmail.com>]

From: Keno Fischer <keno@juliacomputing.com>

Darwin does not support mknodat. However, to avoid race conditions
with later setting the permissions, we must avoid using mknod on
the full path instead. We could try to fchdir, but that would cause
problems if multiple threads try to call mknodat at the same time.
However, luckily there is a solution: Darwin includes a function
that sets the cwd for the current thread only.
This should suffice to use mknod safely.

This function (pthread_fchdir_np) is protected by a check in
meson in a patch later in this series.

Signed-off-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Michael Roitzsch <reactorcontrol@icloud.com>
[Will Cohen: - Adjust coding style
             - Replace clang references with gcc
             - Note radar filed with Apple for missing syscall
             - Replace direct syscall with pthread_fchdir_np and
               adjust patch notes accordingly
             - Move qemu_mknodat from 9p-util to osdep and os-posix
             - Move pthread_fchdir_np declaration only to osdep
             - Declare pthread_fchdir_np with
             - __attribute__((weak_import)) to allow checking for
               its presence before usage
             - Move declarations above cplusplus guard]
Signed-off-by: Will Cohen <wwcohen@gmail.com>
---
 hw/9pfs/9p-local.c   |  4 ++--
 include/qemu/osdep.h | 12 ++++++++++++
 os-posix.c           | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index a0d08e5216..d42ce6d8b8 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -682,7 +682,7 @@ static int local_mknod(FsContext *fs_ctx, V9fsPath *dir_path,
 
     if (fs_ctx->export_flags & V9FS_SM_MAPPED ||
         fs_ctx->export_flags & V9FS_SM_MAPPED_FILE) {
-        err = mknodat(dirfd, name, fs_ctx->fmode | S_IFREG, 0);
+        err = qemu_mknodat(dirfd, name, fs_ctx->fmode | S_IFREG, 0);
         if (err == -1) {
             goto out;
         }
@@ -697,7 +697,7 @@ static int local_mknod(FsContext *fs_ctx, V9fsPath *dir_path,
         }
     } else if (fs_ctx->export_flags & V9FS_SM_PASSTHROUGH ||
                fs_ctx->export_flags & V9FS_SM_NONE) {
-        err = mknodat(dirfd, name, credp->fc_mode, credp->fc_rdev);
+        err = qemu_mknodat(dirfd, name, credp->fc_mode, credp->fc_rdev);
         if (err == -1) {
             goto out;
         }
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index d1660d67fa..d8cc7240ed 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -805,6 +805,18 @@ static inline int platform_does_not_support_system(const char *command)
 }
 #endif /* !HAVE_SYSTEM_FUNCTION */
 
+/*
+ * As long as mknodat is not available on macOS, this workaround
+ * using pthread_fchdir_np is needed. qemu_mknodat is defined in
+ * os-posix.c. pthread_fchdir_np is weakly linked here as a guard
+ * in case it disappears in future macOS versions, because it is
+ * is a private API.
+ */
+#ifdef CONFIG_DARWIN
+int pthread_fchdir_np(int fd) __attribute__((weak_import));
+#endif
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/os-posix.c b/os-posix.c
index ae6c9f2a5e..28a294b8b3 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -332,3 +332,38 @@ int os_mlock(void)
     return -ENOSYS;
 #endif
 }
+
+/*
+ * As long as mknodat is not available on macOS, this workaround
+ * using pthread_fchdir_np is needed.
+ *
+ * Radar filed with Apple for implementing mknodat:
+ * rdar://FB9862426 (https://openradar.appspot.com/FB9862426)
+ */
+#ifdef CONFIG_DARWIN
+
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
+{
+    int preserved_errno, err;
+    if (!pthread_fchdir_np) {
+        error_report_once("pthread_fchdir_np() is not available on this version of macOS");
+        return -ENOTSUP;
+    }
+    if (pthread_fchdir_np(dirfd) < 0) {
+        return -1;
+    }
+    err = mknod(filename, mode, dev);
+    preserved_errno = errno;
+    /* Stop using the thread-local cwd */
+    pthread_fchdir_np(-1);
+    if (err < 0) {
+        errno = preserved_errno;
+    }
+    return err;
+}
+#else
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
+{
+    return mknodat(dirfd, filename, mode, dev);
+}
+#endif
-- 
2.34.1