[PATCH 10/11] 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 as an (unexposed in the
C library) system call that sets the cwd for the current thread only.
This should suffice to use mknod safely.

Signed-off-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Will Cohen <wwcohen@gmail.com>
---
 hw/9pfs/9p-local.c       |  5 +++--
 hw/9pfs/9p-util-darwin.c | 33 +++++++++++++++++++++++++++++++++
 hw/9pfs/9p-util-linux.c  |  5 +++++
 hw/9pfs/9p-util.h        |  2 ++
 4 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index 4268703d05..42b65e143b 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -673,7 +673,7 @@ 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;
         }
@@ -688,7 +688,7 @@ 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;
         }
@@ -701,6 +701,7 @@ 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 ac414bcbfd..25e67d5067 100644
--- a/hw/9pfs/9p-util-darwin.c
+++ b/hw/9pfs/9p-util-darwin.c
@@ -158,3 +158,36 @@ done:
     close_preserve_errno(fd);
     return ret;
 }
+
+#ifndef SYS___pthread_fchdir
+# define SYS___pthread_fchdir 349
+#endif
+
+/*
+ * This is an undocumented OS X syscall. It would be best to avoid it,
+ * but there doesn't seem to be another safe way to implement mknodat.
+ * Dear Apple, please implement mknodat before you remove this syscall.
+ */
+static int fchdir_thread_local(int fd)
+{
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+    return syscall(SYS___pthread_fchdir, fd);
+#pragma clang diagnostic pop
+}
+
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
+{
+    int preserved_errno, err;
+    if (fchdir_thread_local(dirfd) < 0) {
+        return -1;
+    }
+    err = mknod(filename, mode, dev);
+    preserved_errno = errno;
+    /* Stop using the thread-local cwd */
+    fchdir_thread_local(-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 d54bf57a59..4f57d8c047 100644
--- a/hw/9pfs/9p-util-linux.c
+++ b/hw/9pfs/9p-util-linux.c
@@ -68,3 +68,8 @@ int utimensat_nofollow(int dirfd, const char *filename,
 {
     return utimensat(dirfd, filename, times, AT_SYMLINK_NOFOLLOW);
 }
+
+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 1c477a0e66..cac682d335 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -105,4 +105,6 @@ ssize_t fremovexattrat_nofollow(int dirfd, const char *filename,
 int utimensat_nofollow(int dirfd, const char *filename,
                        const struct timespec times[2]);
 
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev);
+
 #endif
-- 
2.33.0