When creating file bind mount destinations, check if they already exist

If they already exist as a regular file then we don't need to create
it, which is good because doing so would break if the filesystem
is readonly. I.e. that returns EROFS instead of EEXISTS in that case.

3 files changed, 24 insertions, 2 deletions

diff --git a/bubblewrap.c b/bubblewrap.c
index c016992..cbfe70d 100644
--- a/bubblewrap.c
+++ b/bubblewrap.c
@@ -550,7 +550,7 @@ setup_newroot (bool unshare_pid,
           }
         else
           {
-            if (create_file (dest, 0666, NULL) != 0 && errno != EEXIST)
+            if (ensure_file (dest, 0666) != 0)
               die_with_error ("Can't create file at %s", op->dest);
           }
 
@@ -617,7 +617,7 @@ setup_newroot (bool unshare_pid,
                     }
                   else
                     {
-                      if (create_file (dst_path, 0666, NULL) != 0)
+                      if (ensure_file (dst_path, 0666) != 0)
                         die_with_error ("Can't create file at %s", dst_path);
                     }
 
diff --git a/utils.c b/utils.c
index 55fd700..b459c63 100644
--- a/utils.c
+++ b/utils.c
@@ -330,6 +330,26 @@ create_file (const char *path,
   return res;
 }
 
+int
+ensure_file (const char *path,
+             mode_t      mode)
+{
+ struct stat buf;
+
+ /* We check this ahead of time, otherwise
+    the create file will fail in the read-only
+    case with EROFD instead of EEXIST */
+ if (stat (path, &buf) ==  0 &&
+     S_ISREG (buf.st_mode))
+   return 0;
+
+ if (create_file (path, mode, NULL) != 0 &&  errno != EEXIST)
+   return -1;
+
+ return 0;
+}
+
+
 #define BUFSIZE	8192
 /* Sets errno on error (!= 0), ENOSPC on short write */
 int
diff --git a/utils.h b/utils.h
index 915bed3..1cfe93a 100644
--- a/utils.h
+++ b/utils.h
@@ -89,6 +89,8 @@ int   copy_file      (const char  *src_path,
 int   create_file    (const char  *path,
                       mode_t       mode,
                       const char  *content);
+int   ensure_file    (const char  *path,
+                      mode_t       mode);
 int   get_file_mode  (const char *pathname);
 int   mkdir_with_parents (const char *pathname,
                           int         mode,