grub-editenv: Make grub-editenv chase symlinks including those across devices

The grub-editenv create command will wrongly overwrite /boot/grub2/grubenv
with a regular file if grubenv is a symbolic link. But instead, it should
create a new file in the path the symlink points to.

This lets /boot/grub2/grubenv be a symlink to /boot/efi/EFI/fedora/grubenv
even when they're different mount points, which allows grub2-editenv to be
the same across platforms (i.e. UEFI vs BIOS).

For example, in Fedora the GRUB EFI builds have prefix set to /EFI/fedora
(on the EFI System Partition), but for BIOS machine it'll be /boot/grub2
(which may or may not be its own mountpoint).

With this patch, on EFI machines we can make /boot/grub2/grubenv a symlink
to /boot/efi/EFI/fedora/grubenv, and the same copy of grub-set-default will
work on both kinds of systems.

Windows doesn't implement a readlink primitive, so the current behaviour is
maintained for this operating system.

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Jonathan Lebon <jlebon@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Makefile.util.def

@@ -242,8 +242,19 @@ program = {
 
   common = util/grub-editenv.c;
   common = util/editenv.c;
+  common = util/grub-install-common.c;
   common = grub-core/osdep/init.c;
+  common = grub-core/osdep/compress.c;
+  extra_dist = grub-core/osdep/unix/compress.c;
+  extra_dist = grub-core/osdep/basic/compress.c;
+  common = util/mkimage.c;
+  common = util/grub-mkimage32.c;
+  common = util/grub-mkimage64.c;
+  common = grub-core/osdep/config.c;
+  common = util/config.c;
+  common = util/resolve.c;
 
+  ldadd = '$(LIBLZMA)';
   ldadd = libgrubmods.a;
   ldadd = libgrubgcry.a;
   ldadd = libgrubkern.a;

util/editenv.c

@@ -28,6 +28,9 @@
 
 #include <errno.h>
 #include <string.h>
+#if !defined(_WIN32)
+#include <libgen.h>
+#endif
 
 #define DEFAULT_ENVBLK_SIZE	1024
 #define GRUB_ENVBLK_MESSAGE	"# WARNING: Do not edit this file by tools other than "PACKAGE"-editenv!!!\n"
@@ -37,6 +40,11 @@ grub_util_create_envblk_file (const char *name)
 {
   FILE *fp;
   char *buf, *pbuf, *namenew;
+#if !defined(_WIN32)
+  ssize_t size = 1;
+  char *rename_target = xstrdup (name);
+  int rc;
+#endif
 
   buf = xmalloc (DEFAULT_ENVBLK_SIZE);
 
@@ -64,7 +72,64 @@ grub_util_create_envblk_file (const char *name)
   free (buf);
   fclose (fp);
 
+#if defined(_WIN32)
   if (grub_util_rename (namenew, name) < 0)
     grub_util_error (_("cannot rename the file %s to %s"), namenew, name);
+#else
+  while (1)
+    {
+      char *linkbuf;
+      ssize_t retsize;
+
+      linkbuf = xmalloc (size + 1);
+      retsize = grub_util_readlink (rename_target, linkbuf, size);
+      if (retsize < 0 && (errno == ENOENT || errno == EINVAL))
+        {
+          free (linkbuf);
+          break;
+        }
+      else if (retsize < 0)
+        {
+          free (linkbuf);
+          grub_util_error (_("cannot rename the file %s to %s: %m"), namenew, name);
+        }
+      else if (retsize == size)
+        {
+          free (linkbuf);
+          size += 128;
+          continue;
+        }
+
+      linkbuf[retsize] = '\0';
+      if (linkbuf[0] == '/')
+        {
+          free (rename_target);
+          rename_target = linkbuf;
+        }
+      else
+        {
+          char *dbuf = xstrdup (rename_target);
+          const char *dir = dirname (dbuf);
+
+          free (rename_target);
+          rename_target = xasprintf ("%s/%s", dir, linkbuf);
+          free (dbuf);
+          free (linkbuf);
+        }
+    }
+
+  rc = grub_util_rename (namenew, rename_target);
+  if (rc < 0 && errno == EXDEV)
+    {
+      rc = grub_install_copy_file (namenew, rename_target, 1);
+      grub_util_unlink (namenew);
+    }
+
+  free (rename_target);
+
+  if (rc < 0)
+    grub_util_error (_("cannot rename the file %s to %s: %m"), namenew, name);
+#endif
+
   free (namenew);
 }