fuse: Implement hole detection through lseek

This is a relatively new feature in libfuse (available since 3.8.0,
which was released in November 2019), so we have to add a dedicated
check whether it is available before making use of it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201027190600.192171-7-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/fuse.c

@@ -627,6 +627,80 @@ static void fuse_flush(fuse_req_t req, fuse_ino_t inode,
     fuse_fsync(req, inode, 1, fi);
 }
 
+#ifdef CONFIG_FUSE_LSEEK
+/**
+ * Let clients inquire allocation status.
+ */
+static void fuse_lseek(fuse_req_t req, fuse_ino_t inode, off_t offset,
+                       int whence, struct fuse_file_info *fi)
+{
+    FuseExport *exp = fuse_req_userdata(req);
+
+    if (whence != SEEK_HOLE && whence != SEEK_DATA) {
+        fuse_reply_err(req, EINVAL);
+        return;
+    }
+
+    while (true) {
+        int64_t pnum;
+        int ret;
+
+        ret = bdrv_block_status_above(blk_bs(exp->common.blk), NULL,
+                                      offset, INT64_MAX, &pnum, NULL, NULL);
+        if (ret < 0) {
+            fuse_reply_err(req, -ret);
+            return;
+        }
+
+        if (!pnum && (ret & BDRV_BLOCK_EOF)) {
+            int64_t blk_len;
+
+            /*
+             * If blk_getlength() rounds (e.g. by sectors), then the
+             * export length will be rounded, too.  However,
+             * bdrv_block_status_above() may return EOF at unaligned
+             * offsets.  We must not let this become visible and thus
+             * always simulate a hole between @offset (the real EOF)
+             * and @blk_len (the client-visible EOF).
+             */
+
+            blk_len = blk_getlength(exp->common.blk);
+            if (blk_len < 0) {
+                fuse_reply_err(req, -blk_len);
+                return;
+            }
+
+            if (offset > blk_len || whence == SEEK_DATA) {
+                fuse_reply_err(req, ENXIO);
+            } else {
+                fuse_reply_lseek(req, offset);
+            }
+            return;
+        }
+
+        if (ret & BDRV_BLOCK_DATA) {
+            if (whence == SEEK_DATA) {
+                fuse_reply_lseek(req, offset);
+                return;
+            }
+        } else {
+            if (whence == SEEK_HOLE) {
+                fuse_reply_lseek(req, offset);
+                return;
+            }
+        }
+
+        /* Safety check against infinite loops */
+        if (!pnum) {
+            fuse_reply_err(req, ENXIO);
+            return;
+        }
+
+        offset += pnum;
+    }
+}
+#endif
+
 static const struct fuse_lowlevel_ops fuse_ops = {
     .init       = fuse_init,
     .lookup     = fuse_lookup,
@@ -638,6 +712,9 @@ static const struct fuse_lowlevel_ops fuse_ops = {
     .fallocate  = fuse_fallocate,
     .flush      = fuse_flush,
     .fsync      = fuse_fsync,
+#ifdef CONFIG_FUSE_LSEEK
+    .lseek      = fuse_lseek,
+#endif
 };
 
 const BlockExportDriver blk_exp_fuse = {

configure

@@ -450,6 +450,7 @@ ninja=""
 skip_meson=no
 gettext=""
 fuse="auto"
+fuse_lseek="auto"
 
 bogus_os="no"
 malloc_trim="auto"
@@ -1530,6 +1531,10 @@ for opt do
   ;;
   --disable-fuse) fuse="disabled"
   ;;
+  --enable-fuse-lseek) fuse_lseek="enabled"
+  ;;
+  --disable-fuse-lseek) fuse_lseek="disabled"
+  ;;
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -1856,6 +1861,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   rng-none        dummy RNG, avoid using /dev/(u)random and getrandom()
   libdaxctl       libdaxctl support
   fuse            FUSE block device export
+  fuse-lseek      SEEK_HOLE/SEEK_DATA support for FUSE exports
 
 NOTE: The object files are built at the place where configure is launched
 EOF
@@ -7020,7 +7026,7 @@ NINJA=$ninja $meson setup \
         -Diconv=$iconv -Dcurses=$curses -Dlibudev=$libudev\
         -Ddocs=$docs -Dsphinx_build=$sphinx_build -Dinstall_blobs=$blobs \
         -Dvhost_user_blk_server=$vhost_user_blk_server \
-        -Dfuse=$fuse \
+        -Dfuse=$fuse -Dfuse_lseek=$fuse_lseek \
         $cross_arg \
         "$PWD" "$source_path"
 

meson.build

@@ -773,10 +773,28 @@ elif get_option('vhost_user_blk_server').disabled() or not have_system
     have_vhost_user_blk_server = false
 endif
 
+if get_option('fuse').disabled() and get_option('fuse_lseek').enabled()
+  error('Cannot enable fuse-lseek while fuse is disabled')
+endif
+
 fuse = dependency('fuse3', required: get_option('fuse'),
                   version: '>=3.1', method: 'pkg-config',
                   static: enable_static)
 
+fuse_lseek = not_found
+if not get_option('fuse_lseek').disabled()
+  if fuse.version().version_compare('>=3.8')
+    # Dummy dependency
+    fuse_lseek = declare_dependency()
+  elif get_option('fuse_lseek').enabled()
+    if fuse.found()
+      error('fuse-lseek requires libfuse >=3.8, found ' + fuse.version())
+    else
+      error('fuse-lseek requires libfuse, which was not found')
+    endif
+  endif
+endif
+
 #################
 # config-host.h #
 #################
@@ -812,6 +830,7 @@ config_host_data.set('CONFIG_GETTID', has_gettid)
 config_host_data.set('CONFIG_MALLOC_TRIM', has_malloc_trim)
 config_host_data.set('CONFIG_STATX', has_statx)
 config_host_data.set('CONFIG_FUSE', fuse.found())
+config_host_data.set('CONFIG_FUSE_LSEEK', fuse_lseek.found())
 config_host_data.set('QEMU_VERSION', '"@0@"'.format(meson.project_version()))
 config_host_data.set('QEMU_VERSION_MAJOR', meson.project_version().split('.')[0])
 config_host_data.set('QEMU_VERSION_MINOR', meson.project_version().split('.')[1])
@@ -2214,6 +2233,7 @@ summary_info += {'thread sanitizer':  config_host.has_key('CONFIG_TSAN')}
 summary_info += {'rng-none':          config_host.has_key('CONFIG_RNG_NONE')}
 summary_info += {'Linux keyring':     config_host.has_key('CONFIG_SECRET_KEYRING')}
 summary_info += {'FUSE exports':      fuse.found()}
+summary_info += {'FUSE lseek':        fuse_lseek.found()}
 summary(summary_info, bool_yn: true)
 
 if not supported_cpus.contains(cpu)

meson_options.txt

@@ -68,6 +68,8 @@ option('vhost_user_blk_server', type: 'feature', value: 'auto',
        description: 'build vhost-user-blk server')
 option('fuse', type: 'feature', value: 'auto',
        description: 'FUSE block device export')
+option('fuse_lseek', type : 'feature', value : 'auto',
+       description: 'SEEK_HOLE/SEEK_DATA support for FUSE exports')
 
 option('capstone', type: 'combo', value: 'auto',
        choices: ['disabled', 'enabled', 'auto', 'system', 'internal'],