grub-mount: Add support for libfuse3

The 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 behavior 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>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

3 files changed, 30 insertions, 15 deletions

diff --git a/Makefile.util.def b/Makefile.util.def
index b098d5bba..d919c562c 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 50f7bc6a0..b0b2fd283 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1797,17 +1797,11 @@ 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 ef00fdded..1c35b6a6e 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 +145,14 @@ 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 __attribute__ ((unused)))
+#endif
 {
   struct fuse_getattr_ctx ctx;
   char *pathname, *path2;
@@ -241,8 +246,11 @@ 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 +338,24 @@ 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 __attribute__ ((unused)))
+#endif
 {
   struct fuse_readdir_ctx ctx = {
     .path = path,