[libglnx: 1/2] fdio: Add glnx_tmpfile_reopen_rdonly()

[Colin Walters <walters src gnome org>]

commit 0cf0955acc5d27c86428e1dddae7652f267c2442
Author: Colin Walters <walters verbum org>
Date:   Thu Oct 31 12:20:00 2019 +0000

    fdio: Add glnx_tmpfile_reopen_rdonly()
    
    For fs-verity.

 glnx-fdio.c | 42 ++++++++++++++++++++++++++++++++++++++++--
 glnx-fdio.h |  4 ++++
 2 files changed, 44 insertions(+), 2 deletions(-)
---
diff --git a/glnx-fdio.c b/glnx-fdio.c
index 1dc5ffd..12879cd 100644
--- a/glnx-fdio.c
+++ b/glnx-fdio.c
@@ -363,8 +363,7 @@ glnx_link_tmpfile_at (GLnxTmpfile *tmpf,
     {
       /* This case we have O_TMPFILE, so our reference to it is via /proc/self/fd */
       char proc_fd_path[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(tmpf->fd) + 1];
-
-      sprintf (proc_fd_path, "/proc/self/fd/%i", tmpf->fd);
+      snprintf (proc_fd_path, sizeof (proc_fd_path), "/proc/self/fd/%i", tmpf->fd);
 
       if (replace)
         {
@@ -424,6 +423,45 @@ glnx_link_tmpfile_at (GLnxTmpfile *tmpf,
   return TRUE;
 }
 
+/* glnx_tmpfile_reopen_rdonly:
+ * @tmpf: tmpfile
+ * @error: Error
+ *
+ * Give up write access to the file descriptior.  One use
+ * case for this is fs-verity, which requires a read-only fd.
+ * It could also be useful to allocate an anonymous tmpfile
+ * write some sort of caching/indexing data to it, then reopen it
+ * read-only thereafter.
+ **/
+gboolean
+glnx_tmpfile_reopen_rdonly (GLnxTmpfile *tmpf,
+                            GError **error)
+{
+  g_return_val_if_fail (tmpf->fd >= 0, FALSE);
+  g_return_val_if_fail (tmpf->src_dfd == AT_FDCWD || tmpf->src_dfd >= 0, FALSE);
+
+  glnx_fd_close int rdonly_fd = -1;
+
+  if (tmpf->path)
+    {
+      if (!glnx_openat_rdonly (tmpf->src_dfd, tmpf->path, FALSE, &rdonly_fd, error))
+        return FALSE;
+    }
+  else
+    {
+      /* This case we have O_TMPFILE, so our reference to it is via /proc/self/fd */
+      char proc_fd_path[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(tmpf->fd) + 1];
+      snprintf (proc_fd_path, sizeof (proc_fd_path), "/proc/self/fd/%i", tmpf->fd);
+
+      if (!glnx_openat_rdonly (AT_FDCWD, proc_fd_path, TRUE, &rdonly_fd, error))
+        return FALSE;
+    }
+
+  glnx_close_fd (&tmpf->fd);
+  tmpf->fd = glnx_steal_fd (&rdonly_fd);
+  return TRUE;
+}
+
 /**
  * glnx_openat_rdonly:
  * @dfd: File descriptor for origin directory
diff --git a/glnx-fdio.h b/glnx-fdio.h
index d97ea36..9c57dc5 100644
--- a/glnx-fdio.h
+++ b/glnx-fdio.h
@@ -110,6 +110,10 @@ glnx_link_tmpfile_at (GLnxTmpfile *tmpf,
                       const char *target,
                       GError **error);
 
+gboolean
+glnx_tmpfile_reopen_rdonly (GLnxTmpfile *tmpf,
+                            GError **error);
+
 gboolean
 glnx_openat_rdonly (int             dfd,
                     const char     *path,