[PATCH v4 09/11] 9p: darwin: Implement compatibility for mknodat

[Will Cohen]

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 tihs 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]
Signed-off-by: Will Cohen <wwcohen@gmail.com>
---
 hw/9pfs/9p-local.c       |  5 +++--
 hw/9pfs/9p-util-darwin.c | 27 +++++++++++++++++++++++++++
 hw/9pfs/9p-util-linux.c  |  5 +++++
 hw/9pfs/9p-util.h        |  2 ++
 4 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index ea3ded5fca..0569a8b4de 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;
         }
@@ -710,6 +710,7 @@ static int local_mknod(FsContext *fs_ctx, V9fsPath 
*dir_path,
 
 err_end:
     unlinkat_preserve_errno(dirfd, name, 0);
+
 out:
     close_preserve_errno(dirfd);
     return err;
diff --git a/hw/9pfs/9p-util-darwin.c b/hw/9pfs/9p-util-darwin.c
index cdb4c9e24c..128b6d87e8 100644
--- a/hw/9pfs/9p-util-darwin.c
+++ b/hw/9pfs/9p-util-darwin.c
@@ -5,6 +5,7 @@
  * See the COPYING file in the top-level directory.
  */
 
+#include <os/availability.h>
 #include "qemu/osdep.h"
 #include "qemu/xattr.h"
 #include "9p-util.h"
@@ -62,3 +63,29 @@ int fsetxattrat_nofollow(int dirfd, const char *filename, 
const char *name,
     close_preserve_errno(fd);
     return ret;
 }
+
+/*
+ * 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)
+ */
+
+int pthread_fchdir_np(int fd) API_AVAILABLE(macosx(10.12));
+
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
+{
+    int preserved_errno, err;
+    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;
+}
diff --git a/hw/9pfs/9p-util-linux.c b/hw/9pfs/9p-util-linux.c
index 398614a5d0..66e0ab1865 100644
--- a/hw/9pfs/9p-util-linux.c
+++ b/hw/9pfs/9p-util-linux.c
@@ -62,3 +62,8 @@ int fsetxattrat_nofollow(int dirfd, const char *filename, 
const char *name,
     g_free(proc_path);
     return ret;
 }
+
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
+{
+    return mknodat(dirfd, filename, mode, dev);
+}
diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index 8e610ad224..f6fed963bf 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -97,6 +97,8 @@ ssize_t flistxattrat_nofollow(int dirfd, const char *filename,
 ssize_t fremovexattrat_nofollow(int dirfd, const char *filename,
                                 const char *name);
 
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev);
+
 #endif
 
 
-- 
2.32.0 (Apple Git-132)