[PATCH v3 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 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: Michael Roitzsch <reactorcontrol@icloud.com>
[Will Cohen: - Adjust coding style
             - Replace clang references with gcc]
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 2bfff79b12..3a8bdf05d5 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 cdb4c9e24c..62a88ce4f9 100644
--- a/hw/9pfs/9p-util-darwin.c
+++ b/hw/9pfs/9p-util-darwin.c
@@ -62,3 +62,36 @@ int fsetxattrat_nofollow(int dirfd, const char *filename, 
const char *name,
     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 gcc diagnostic push
+#pragma gcc diagnostic ignored "-Wdeprecated-declarations"
+    return syscall(SYS___pthread_fchdir, fd);
+#pragma gcc 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 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 38ef8b289d..bff9b3022c 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -97,4 +97,6 @@ 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
-- 
2.34.1