Am 22.09.2020 um 12:49 hat Max Reitz geschrieben: > block-export-add type=fuse allows mounting block graph nodes via FUSE on > some existing regular file. That file should then appears like a raw > disk image, and accesses to it result in accesses to the exported BDS. > > Right now, we only implement the necessary block export functions to set > it up and shut it down. We do not implement any access functions, so > accessing the mount point only results in errors. This will be > addressed by a followup patch. > > We keep a hash table of exported mount points, because we want to be > able to detect when users try to use a mount point twice. This is > because we invoke stat() to check whether the given mount point is a > regular file, but if that file is served by ourselves (because it is > already used as a mount point), then this stat() would have to be served > by ourselves, too, which is impossible to do while we (as the caller) > are waiting for it to settle. Therefore, keep track of mount point > paths to at least catch the most obvious instances of that problem. > > Signed-off-by: Max Reitz <mre...@redhat.com> > --- > qapi/block-export.json | 24 +++- > include/block/fuse.h | 30 +++++ > block.c | 1 + > block/export/export.c | 4 + > block/export/fuse.c | 253 +++++++++++++++++++++++++++++++++++++++ > block/export/meson.build | 1 + > 6 files changed, 311 insertions(+), 2 deletions(-) > create mode 100644 include/block/fuse.h > create mode 100644 block/export/fuse.c > > diff --git a/qapi/block-export.json b/qapi/block-export.json > index 0c045dfe7b..cb5bd54cbf 100644 > --- a/qapi/block-export.json > +++ b/qapi/block-export.json > @@ -174,6 +174,21 @@ > ## > { 'command': 'nbd-server-stop' } > > +## > +# @BlockExportOptionsFuse: > +# > +# Options for exporting a block graph node on some (file) mountpoint > +# as a raw image. > +# > +# @mountpoint: Path on which to export the block device via FUSE. > +# This must point to an existing regular file. > +# > +# Since: 5.2 > +## > +{ 'struct': 'BlockExportOptionsFuse', > + 'data': { 'mountpoint': 'str' }, > + 'if': 'defined(CONFIG_FUSE)' } > + > ## > # @BlockExportType: > # > @@ -181,10 +196,13 @@ > # > # @nbd: NBD export > # > +# @fuse: Fuse export (since: 5.2)
s/Fuse/FUSE/? > +# > # Since: 4.2 > ## > { 'enum': 'BlockExportType', > - 'data': [ 'nbd' ] } > + 'data': [ 'nbd', > + { 'name': 'fuse', 'if': 'defined(CONFIG_FUSE)' } ] } > > ## > # @BlockExportOptions: > @@ -213,7 +231,9 @@ > '*writethrough': 'bool' }, > 'discriminator': 'type', > 'data': { > - 'nbd': 'BlockExportOptionsNbd' > + 'nbd': 'BlockExportOptionsNbd', > + 'fuse': { 'type': 'BlockExportOptionsFuse', > + 'if': 'defined(CONFIG_FUSE)' } > } } > > ## > diff --git a/include/block/fuse.h b/include/block/fuse.h > new file mode 100644 > index 0000000000..ffa91fe364 > --- /dev/null > +++ b/include/block/fuse.h > @@ -0,0 +1,30 @@ > +/* > + * Present a block device as a raw image through FUSE > + * > + * Copyright (c) 2020 Max Reitz <mre...@redhat.com> > + * > + * This program is free software; you can redistribute it and/or modify > + * it under the terms of the GNU General Public License as published by > + * the Free Software Foundation; under version 2 or later of the License. > + * > + * This program is distributed in the hope that it will be useful, > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > + * GNU General Public License for more details. > + * > + * You should have received a copy of the GNU General Public License > + * along with this program; if not, see <http://www.gnu.org/licenses/>. > + */ > + > +#ifndef BLOCK_FUSE_H > +#define BLOCK_FUSE_H > + > +#ifdef CONFIG_FUSE > + > +#include "block/export.h" > + > +extern const BlockExportDriver blk_exp_fuse; > + > +#endif /* CONFIG_FUSE */ > + > +#endif > diff --git a/block.c b/block.c > index acde53f92a..7bf45f6ede 100644 > --- a/block.c > +++ b/block.c > @@ -26,6 +26,7 @@ > #include "block/trace.h" > #include "block/block_int.h" > #include "block/blockjob.h" > +#include "block/fuse.h" > #include "block/nbd.h" > #include "block/qdict.h" > #include "qemu/error-report.h" > diff --git a/block/export/export.c b/block/export/export.c > index 64d39a6934..ba866d6ba7 100644 > --- a/block/export/export.c > +++ b/block/export/export.c > @@ -16,6 +16,7 @@ > #include "block/block.h" > #include "sysemu/block-backend.h" > #include "block/export.h" > +#include "block/fuse.h" > #include "block/nbd.h" > #include "qapi/error.h" > #include "qapi/qapi-commands-block-export.h" > @@ -24,6 +25,9 @@ > > static const BlockExportDriver *blk_exp_drivers[] = { > &blk_exp_nbd, > +#ifdef CONFIG_FUSE > + &blk_exp_fuse, > +#endif > }; > > /* Only accessed from the main thread */ > diff --git a/block/export/fuse.c b/block/export/fuse.c > new file mode 100644 > index 0000000000..75f11d2514 > --- /dev/null > +++ b/block/export/fuse.c > @@ -0,0 +1,253 @@ > +/* > + * Present a block device as a raw image through FUSE > + * > + * Copyright (c) 2020 Max Reitz <mre...@redhat.com> > + * > + * This program is free software; you can redistribute it and/or modify > + * it under the terms of the GNU General Public License as published by > + * the Free Software Foundation; under version 2 or later of the License. > + * > + * This program is distributed in the hope that it will be useful, > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > + * GNU General Public License for more details. > + * > + * You should have received a copy of the GNU General Public License > + * along with this program; if not, see <http://www.gnu.org/licenses/>. > + */ > + > +#define FUSE_USE_VERSION 31 > + > +#include "qemu/osdep.h" > +#include "block/aio.h" > +#include "block/block.h" > +#include "block/export.h" > +#include "block/fuse.h" > +#include "block/qapi.h" > +#include "qapi/error.h" > +#include "qapi/qapi-commands-block.h" > +#include "sysemu/block-backend.h" > + > +#include <fuse.h> > +#include <fuse_lowlevel.h> > + > + > +typedef struct FuseExport { > + BlockExport common; > + > + struct fuse_session *fuse_session; > + struct fuse_buf fuse_buf; > + bool mounted, fd_handler_set_up; > + > + char *mountpoint; > + bool writable; > +} FuseExport; > + > +static GHashTable *exports; > +static const struct fuse_lowlevel_ops fuse_ops; > + > +static void fuse_export_shutdown(BlockExport *exp); > +static void fuse_export_delete(BlockExport *exp); > + > +static void init_fuse(void); > +static int setup_fuse_export(FuseExport *exp, const char *mountpoint, > + Error **errp); > +static void read_from_fuse_export(void *opaque); > + > +static bool is_regular_file(const char *path, Error **errp); > + > + > +static int fuse_export_create(BlockExport *blk_exp, > + BlockExportOptions *blk_exp_args, > + Error **errp) > +{ > + FuseExport *exp = container_of(blk_exp, FuseExport, common); > + BlockExportOptionsFuse *args = &blk_exp_args->u.fuse; > + int ret; > + > + assert(blk_exp_args->type == BLOCK_EXPORT_TYPE_FUSE); > + > + init_fuse(); > + > + /* > + * It is important to do this check before calling is_regular_file() -- > + * that function will do a stat(), which we would have to handle if we > + * already exported something on @mountpoint. But we cannot, because > + * we are currently caught up here. > + * (Note that ideally we would want to resolve relative paths here, > + * but bdrv_make_absolute_filename() might do the wrong thing for > + * paths that contain colons, and realpath() would resolve symlinks, > + * which we do not want: The mount point is not going to be the > + * symlink's destination, but the link itself.) > + * So this will not catch all potential clashes, but hopefully at > + * least the most common one of specifying exactly the same path > + * string twice. > + */ > + if (g_hash_table_contains(exports, args->mountpoint)) { > + error_setg(errp, "There already is a FUSE export on '%s'", > + args->mountpoint); > + ret = -EEXIST; > + goto fail; > + } I guess the proper solution would be to make block export creation asynchronous so that we can actually serve that request in the background. Something for another day... > + > + if (!is_regular_file(args->mountpoint, errp)) { > + ret = -EINVAL; > + goto fail; > + } > + > + exp->mountpoint = g_strdup(args->mountpoint); > + exp->writable = blk_exp_args->writable; > + > + ret = setup_fuse_export(exp, args->mountpoint, errp); > + if (ret < 0) { > + goto fail; > + } > + > + g_hash_table_insert(exports, g_strdup(args->mountpoint), NULL); > + return 0; > + > +fail: > + fuse_export_shutdown(blk_exp); > + fuse_export_delete(blk_exp); Why fuse_export_shutdown()? The only thing to possibly undo at this point is allocating exp->mountpoint, which is freed in fuse_export_delete(). Or can setup_fuse_export() fail without cleaning up? I hope not. > + return ret; > +} > + > +/** > + * Ensure that the global FUSE context is set up. > + */ > +static void init_fuse(void) > +{ > + if (exports) { > + return; > + } > + > + exports = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL); > +} > + > +/** > + * Create exp->fuse_session and mount it. > + */ > +static int setup_fuse_export(FuseExport *exp, const char *mountpoint, > + Error **errp) > +{ > + const char *fuse_argv[2]; > + struct fuse_args fuse_args; > + int ret; > + > + fuse_argv[0] = ""; /* Dummy program name */ > + fuse_argv[1] = NULL; > + fuse_args = (struct fuse_args)FUSE_ARGS_INIT(1, (char **)fuse_argv); > + > + exp->fuse_session = fuse_session_new(&fuse_args, &fuse_ops, > + sizeof(fuse_ops), exp); > + if (!exp->fuse_session) { > + error_setg(errp, "Failed to set up FUSE session"); > + return -EIO; > + } > + > + ret = fuse_session_mount(exp->fuse_session, mountpoint); > + if (ret < 0) { > + error_setg(errp, "Failed to mount FUSE session to export"); > + return -EIO; Oh, it can. :-( I think it would be better to call fuse_export_shutdown() here so that the function doesn't change the state when it returns failure. > + } > + exp->mounted = true; > + > + aio_set_fd_handler(exp->common.ctx, > + fuse_session_fd(exp->fuse_session), true, > + read_from_fuse_export, NULL, NULL, exp); > + exp->fd_handler_set_up = true; > + > + return 0; > +} > + > +/** > + * Callback to be invoked when the FUSE session FD can be read from. > + * (This is basically the FUSE event loop.) > + */ > +static void read_from_fuse_export(void *opaque) > +{ > + FuseExport *exp = opaque; > + int ret; > + > + blk_exp_ref(&exp->common); > + > + ret = fuse_session_receive_buf(exp->fuse_session, &exp->fuse_buf); The fuse_session_loop() implementation seems to imply that we should retry on EINTR. > + if (ret < 0) { > + goto out; > + } > + > + fuse_session_process_buf(exp->fuse_session, &exp->fuse_buf); > + > +out: > + blk_exp_unref(&exp->common); > +} > + > +static void fuse_export_shutdown(BlockExport *blk_exp) > +{ > + FuseExport *exp = container_of(blk_exp, FuseExport, common); > + > + if (exp->fuse_session) { > + fuse_session_exit(exp->fuse_session); > + > + if (exp->mounted) { > + fuse_session_unmount(exp->fuse_session); > + exp->mounted = false; > + } > + > + if (exp->fd_handler_set_up) { > + aio_set_fd_handler(exp->common.ctx, > + fuse_session_fd(exp->fuse_session), true, > + NULL, NULL, NULL, NULL); > + exp->fd_handler_set_up = false; > + } > + > + fuse_session_destroy(exp->fuse_session); > + exp->fuse_session = NULL; What happens if a request is still in flight? Oh, can't happen because the driver is fully synchronous after this series. Fair enough, making it asynchronous can come on top of it. However, I think we should move fuse_session_unmount() and fuse_session_destroy() to .delete instead of .shutdown to avoid setting up a trap for whoever introduces async requests to the FUSE export. > + } > + > + if (exp->mountpoint) { > + g_hash_table_remove(exports, exp->mountpoint); > + } > +} > + > +static void fuse_export_delete(BlockExport *blk_exp) > +{ > + FuseExport *exp = container_of(blk_exp, FuseExport, common); > + > + free(exp->fuse_buf.mem); > + g_free(exp->mountpoint); > +} > + > +/** > + * Check whether @path points to a regular file. If not, put an > + * appropriate message into *errp. > + */ > +static bool is_regular_file(const char *path, Error **errp) > +{ > + struct stat statbuf; > + int ret; > + > + ret = stat(path, &statbuf); > + if (ret < 0) { > + error_setg_errno(errp, errno, "Failed to stat '%s'", path); > + return false; > + } > + > + if (!S_ISREG(statbuf.st_mode)) { > + error_setg(errp, "'%s' is not a regular file", path); > + return false; > + } > + > + return true; > +} This is obviously racy (TOCTOU), but if I understand correctly, we use it only to provide a nicer error message than if fuse_session_mount() fails, so this is fine. > +static const struct fuse_lowlevel_ops fuse_ops = { > +}; > + > +const BlockExportDriver blk_exp_fuse = { > + .type = BLOCK_EXPORT_TYPE_FUSE, > + .instance_size = sizeof(FuseExport), > + .create = fuse_export_create, > + .delete = fuse_export_delete, > + .request_shutdown = fuse_export_shutdown, > +}; Kevin