Re: [PATCH v2 02/20] fuse: Allow exporting BDSs via FUSE

[Kevin Wolf]

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 <mreitz@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 <mreitz@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 <mreitz@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