9p: darwin: Implement compatibility for mknodat

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
             - Declare pthread_fchdir_np with
             - __attribute__((weak_import)) to allow checking for
               its presence before usage
             - Move declarations above cplusplus guard
             - Add CONFIG_PTHREAD_FCHDIR_NP to meson and check for
               presence in 9p-util
             - Rebase to apply cleanly on top of the 2022-02-10
               changes to 9pfs
             - Fix line over 90 characters formatting error]
Signed-off-by: Will Cohen <wwcohen@gmail.com>
Message-Id: <20220227223522.91937-10-wwcohen@gmail.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index a0d08e5216024955507375022010b5289e945222..d42ce6d8b8227e7f2e708e8906380f57b94a9052 100644 (file)
--- 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/hw/9pfs/9p-util-darwin.c b/hw/9pfs/9p-util-darwin.c
index cdb4c9e24c271d5e0ac023ca139b99b6bacbd101..bec0253474db9b52dc16ca1c2dbc88567f57e412 100644 (file)
--- a/hw/9pfs/9p-util-darwin.c
+++ b/hw/9pfs/9p-util-darwin.c
@@ -7,6 +7,8 @@
 
 #include "qemu/osdep.h"
 #include "qemu/xattr.h"
+#include "qapi/error.h"
+#include "qemu/error-report.h"
 #include "9p-util.h"
 
 ssize_t fgetxattrat_nofollow(int dirfd, const char *filename, const char *name,
@@ -62,3 +64,34 @@ 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)
+ */
+#if defined CONFIG_PTHREAD_FCHDIR_NP
+
+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() 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;
+}
+
+#endif

diff --git a/hw/9pfs/9p-util-linux.c b/hw/9pfs/9p-util-linux.c
index 398614a5d015dc0201ca5a67f9e73e5c4612f24a..db451b0784805a3e028ecff6b7bbbaf9d4f3d4ec 100644 (file)
--- a/hw/9pfs/9p-util-linux.c
+++ b/hw/9pfs/9p-util-linux.c
@@ -61,4 +61,10 @@ int fsetxattrat_nofollow(int dirfd, const char *filename, const char *name,
     ret = lsetxattr(proc_path, name, value, size, flags);
     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 be8dc1a43a1b0b18a6b93aa5f28760b44eda4691..73b08c5561d02f1021a019765d0107b73f20766d 100644 (file)
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -114,5 +114,16 @@ static inline off_t qemu_dirent_off(struct dirent *dent)
 #endif
 }
 
+/*
+ * 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.
+ */
+#if defined CONFIG_DARWIN && defined CONFIG_PTHREAD_FCHDIR_NP
+int pthread_fchdir_np(int fd) __attribute__((weak_import));
+#endif
+int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev);
 
 #endif

diff --git a/meson.build b/meson.build
index 038502714ab942cf4b75277360a7a28c6bc9829a..40598630e6557164a3771f832cd2518b038eed00 100644 (file)
--- a/meson.build
+++ b/meson.build
@@ -1622,6 +1622,7 @@ config_host_data.set('CONFIG_POSIX_FALLOCATE', cc.has_function('posix_fallocate'
 config_host_data.set('CONFIG_POSIX_MEMALIGN', cc.has_function('posix_memalign'))
 config_host_data.set('CONFIG_PPOLL', cc.has_function('ppoll'))
 config_host_data.set('CONFIG_PREADV', cc.has_function('preadv', prefix: '#include <sys/uio.h>'))
+config_host_data.set('CONFIG_PTHREAD_FCHDIR_NP', cc.has_function('pthread_fchdir_np'))
 config_host_data.set('CONFIG_SEM_TIMEDWAIT', cc.has_function('sem_timedwait', dependencies: threads))
 config_host_data.set('CONFIG_SENDFILE', cc.has_function('sendfile'))
 config_host_data.set('CONFIG_SETNS', cc.has_function('setns') and cc.has_function('unshare'))