[PATCH] grub-mount: Support libfuse 3

[Fabian Vogt <fvogt@suse.de>]

libfuse 3.0.0 got released in 2016, with some API changes compared to 2.x.
This commit introduces support for 3.x while keeping it compatible with 2.6
as a fallback still.

To detect fuse3, switch configure over to use pkg-config, which is simpler yet
more reliable than looking for library and header manually. Also set
FUSE_USE_VERSION that way, as it depends on the used libfuse version.

Now that the CFLAGS are read from pkg-config, use just <fuse.h>, which works
with 2.x as well as 3.x and is recommended by libfuse upstream.

One behaviour change of libfuse3 is that FUSE_ATOMIC_O_TRUNC is set by default,
which means that open with O_TRUNC is passed as-is instead of calling the
truncate operation. With libfuse2, truncate failed with -ENOSYS and that was
returned to the application. To make O_TRUNC fail with libfuse3, return -EROFS
explicitly if writing was requested.

Signed-off-by: Fabian Vogt <fvogt@suse.de>
---
I tested building with libfuse 2.9.9 as well as 3.10.5, both successful.
It appears to work properly when mounting a btrfs partition or ext4 image in
some quick manual testing.

 Makefile.util.def |  4 +++-
 configure.ac      | 16 +++++-----------
 util/grub-mount.c | 24 +++++++++++++++++++++---
 3 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/Makefile.util.def b/Makefile.util.def
index f8b356cc1..e92c1f346 100644
--- a/Makefile.util.def
+++ b/Makefile.util.def
@@ -309,11 +309,13 @@ program = {
   common = grub-core/disk/host.c;
   common = grub-core/osdep/init.c;
 
+  cflags = '$(FUSE_CFLAGS)';
+
   ldadd = libgrubmods.a;
   ldadd = libgrubgcry.a;
   ldadd = libgrubkern.a;
   ldadd = grub-core/lib/gnulib/libgnu.a;
-  ldadd = '$(LIBINTL) $(LIBDEVMAPPER) $(LIBZFS) $(LIBNVPAIR) $(LIBGEOM) -lfuse';
+  ldadd = '$(LIBINTL) $(LIBDEVMAPPER) $(LIBZFS) $(LIBNVPAIR) $(LIBGEOM) $(FUSE_LIBS)';
   condition = COND_GRUB_MOUNT;
 };
 
diff --git a/configure.ac b/configure.ac
index 4f649edaf..1d40f9560 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1787,17 +1787,11 @@ if test x"$enable_grub_mount" = xno ; then
 fi
 
 if test x"$grub_mount_excuse" = x ; then
-  AC_CHECK_LIB([fuse], [fuse_main_real], [],
-               [grub_mount_excuse="need FUSE library"])
-fi
-
-if test x"$grub_mount_excuse" = x ; then
-  # Check for fuse headers.
-  SAVED_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="$CPPFLAGS -DFUSE_USE_VERSION=26"
-  AC_CHECK_HEADERS([fuse/fuse.h], [],
-  	[grub_mount_excuse=["need FUSE headers"]])
-  CPPFLAGS="$SAVED_CPPFLAGS"
+  PKG_CHECK_MODULES([FUSE], [fuse3], [FUSE_CFLAGS="$FUSE_CFLAGS -DFUSE_USE_VERSION=32"], [
+    PKG_CHECK_MODULES([FUSE], [fuse], [FUSE_CFLAGS="$FUSE_CFLAGS -DFUSE_USE_VERSION=26"], [
+      grub_mount_excuse="need fuse or fuse3 libraries"
+    ])
+  ])
 fi
 
 if test x"$enable_grub_mount" = xyes && test x"$grub_mount_excuse" != x ; then
diff --git a/util/grub-mount.c b/util/grub-mount.c
index d7be2a427..f983dd765 100644
--- a/util/grub-mount.c
+++ b/util/grub-mount.c
@@ -16,7 +16,6 @@
  *  You should have received a copy of the GNU General Public License
  *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
  */
-#define FUSE_USE_VERSION 26
 #include <config.h>
 #include <grub/types.h>
 #include <grub/emu/misc.h>
@@ -34,7 +33,7 @@
 #include <grub/command.h>
 #include <grub/zfs/zfs.h>
 #include <grub/i18n.h>
-#include <fuse/fuse.h>
+#include <fuse.h>
 
 #include <stdio.h>
 #include <unistd.h>
@@ -146,8 +145,13 @@ fuse_getattr_find_file (const char *cur_filename,
   return 0;
 }
 
+#if FUSE_USE_VERSION < 30
 static int
 fuse_getattr (const char *path, struct stat *st)
+#else
+static int
+fuse_getattr (const char *path, struct stat *st, struct fuse_file_info *fi)
+#endif
 {
   struct fuse_getattr_ctx ctx;
   char *pathname, *path2;
@@ -241,8 +245,11 @@ static grub_file_t files[65536];
 static int first_fd = 1;
 
 static int 
-fuse_open (const char *path, struct fuse_file_info *fi __attribute__ ((unused)))
+fuse_open (const char *path, struct fuse_file_info *fi)
 {
+  if ((fi->flags & O_ACCMODE) != O_RDONLY)
+    return -EROFS;
+
   grub_file_t file;
   file = grub_file_open (path, GRUB_FILE_TYPE_MOUNT);
   if (! file)
@@ -330,13 +337,24 @@ fuse_readdir_call_fill (const char *filename,
   st.st_blocks = (st.st_size + 511) >> 9;
   st.st_atime = st.st_mtime = st.st_ctime
     = info->mtimeset ? info->mtime : 0;
+#if FUSE_USE_VERSION < 30
   ctx->fill (ctx->buf, filename, &st, 0);
+#else
+  ctx->fill (ctx->buf, filename, &st, 0, 0);
+#endif
   return 0;
 }
 
+#if FUSE_USE_VERSION < 30
 static int 
 fuse_readdir (const char *path, void *buf,
 	      fuse_fill_dir_t fill, off_t off, struct fuse_file_info *fi)
+#else
+static int
+fuse_readdir (const char *path, void *buf,
+	      fuse_fill_dir_t fill, off_t off, struct fuse_file_info *fi,
+	      enum fuse_readdir_flags flags)
+#endif
 {
   struct fuse_readdir_ctx ctx = {
     .path = path,
-- 
2.33.1