Re: [PATCH 04/17] mount_service: use the new mount api for the mount service

[Bernd Schubert <bschubert@ddn.com>]

On 3/27/26 02:25, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Use the new fsopen/fsmount system calls to mount the filesystem so that
> we get somewhat better diagnostics if something gets screwed up.
> 
> Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> ---
>  meson.build          |   15 +++
>  util/mount_service.c |  299 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 314 insertions(+)
> 
> 
> diff --git a/meson.build b/meson.build
> index d6ba8740effd5c..b0988548bf806a 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -130,6 +130,21 @@ special_funcs = {
>  	int main(int argc, char *argv[]) {
>            return SD_LISTEN_FDS_START;
>  	}
> +    ''',
> +    'new_mount_api': '''
> +       #define _GNU_SOURCE
> +       #include <sys/mount.h>
> +       #include <linux/mount.h>
> +       #include <unistd.h>
> +       #include <fcntl.h>
> +
> +       int main(void) {
> +           int fsfd = fsopen("fuse", FSOPEN_CLOEXEC);
> +           int res = fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "test", 0);
> +           int mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
> +           res = move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> +           return 0;
> +       }
>      '''
>  }
>  
> diff --git a/util/mount_service.c b/util/mount_service.c
> index ae078e537dc560..4a2c1111b66b91 100644
> --- a/util/mount_service.c
> +++ b/util/mount_service.c
> @@ -65,6 +65,9 @@ struct mount_service {
>  
>  	/* O_PATH fd for mount point */
>  	int mountfd;
> +
> +	/* fd for fsopen */
> +	int fsopenfd;
>  };
>  
>  /* Filter out the subtype of the filesystem (e.g. fuse.Y -> Y) */
> @@ -87,6 +90,7 @@ static int mount_service_init(struct mount_service *mo, int argc, char *argv[])
>  	mo->argvfd = -1;
>  	mo->fusedevfd = -1;
>  	mo->mountfd = -1;
> +	mo->fsopenfd = -1;
>  
>  	for (i = 0; i < argc; i++) {
>  		if (!strcmp(argv[i], "-t") && i + 1 < argc) {
> @@ -690,9 +694,50 @@ static int mount_service_handle_fsopen_cmd(struct mount_service *mo,
>  		return mount_service_send_reply(mo, error);
>  	}
>  
> +#ifdef HAVE_NEW_MOUNT_API
> +	/* If this fails we fall back on mount() */
> +	mo->fsopenfd = fsopen(oc->value, FSOPEN_CLOEXEC);
> +#endif
> +
>  	return mount_service_send_reply(mo, 0);
>  }
>  
> +#ifdef HAVE_NEW_MOUNT_API
> +/* callers must preserve errno */
> +static void emit_fsconfig_messages(const struct mount_service *mo)
> +{
> +	uint8_t buf[BUFSIZ];
> +	ssize_t sz;
> +
> +	while ((sz = read(mo->fsopenfd, buf, sizeof(buf) - 1)) != -1) {
> +		if (sz <= 0)
> +			continue;
> +		if (buf[sz - 1] == '\n')
> +			buf[--sz] = '\0';
> +		else
> +			buf[sz] = '\0';
> +
> +		if (!*buf)
> +			continue;
> +
> +		switch (buf[0]) {
> +		case 'e':
> +			fprintf(stderr, "Error: %s\n", buf + 2);
> +			break;
> +		case 'w':
> +			fprintf(stderr, "Warning: %s\n", buf + 2);
> +			break;
> +		case 'i':
> +			fprintf(stderr, "Info: %s\n", buf + 2);
> +			break;
> +		default:
> +			fprintf(stderr, " %s\n", buf);
> +			break;
> +		}
> +	}
> +}
> +#endif

How about unifying that with my series and then use log_fsconfig_kmsg()?
We can also do it independently, but should not forget to use a common
function later on. That applies to all these functions for the new mount
API. Part of the headache I have with my series is that fusermount.c
duplicates quite some code.
For me it looks a bit like we can unify most of util/mount_service.c and
lib/mount_fsmount.c.

> +
>  static int mount_service_handle_source_cmd(struct mount_service *mo,
>  					   const struct fuse_service_packet *p)
>  {
> @@ -714,6 +759,21 @@ static int mount_service_handle_source_cmd(struct mount_service *mo,
>  		return mount_service_send_reply(mo, error);
>  	}
>  
> +#ifdef HAVE_NEW_MOUNT_API
> +	if (mo->fsopenfd >= 0) {
> +		int ret = fsconfig(mo->fsopenfd, FSCONFIG_SET_STRING, "source",
> +			       oc->value, 0);
> +		if (ret) {
> +			int error = errno;
> +
> +			fprintf(stderr, "%s: fsconfig source: %s\n",
> +				mo->msgtag, strerror(error));
> +			emit_fsconfig_messages(mo);
> +			return mount_service_send_reply(mo, error);
> +		}
> +	}
> +#endif
> +
>  	return mount_service_send_reply(mo, 0);
>  }
>  
> @@ -722,6 +782,8 @@ static int mount_service_handle_mntopts_cmd(struct mount_service *mo,
>  {
>  	struct fuse_service_string_command *oc =
>  			container_of(p, struct fuse_service_string_command, p);
> +	char *tokstr = oc->value;
> +	char *tok, *savetok;
>  
>  	if (mo->mntopts) {
>  		fprintf(stderr, "%s: mount options respecified!\n",
> @@ -738,6 +800,45 @@ static int mount_service_handle_mntopts_cmd(struct mount_service *mo,
>  		return mount_service_send_reply(mo, error);
>  	}
>  
> +	/* strtok_r mutates tokstr aka oc->value */
> +	while ((tok = strtok_r(tokstr, ",", &savetok)) != NULL) {
> +		char *equals = strchr(tok, '=');
> +		char oldchar = 0;
> +
> +		if (equals) {
> +			oldchar = *equals;
> +			*equals = 0;
> +		}
> +
> +#ifdef HAVE_NEW_MOUNT_API
> +		if (mo->fsopenfd >= 0) {
> +			int ret;
> +
> +			if (equals)
> +				ret = fsconfig(mo->fsopenfd,
> +					       FSCONFIG_SET_STRING, tok,
> +					       equals + 1, 0);
> +			else
> +				ret = fsconfig(mo->fsopenfd,
> +					       FSCONFIG_SET_FLAG, tok,
> +					       NULL, 0);
> +			if (ret) {
> +				int error = errno;
> +
> +				fprintf(stderr, "%s: set mount option: %s\n",
> +					mo->msgtag, strerror(error));
> +				emit_fsconfig_messages(mo);
> +				return mount_service_send_reply(mo, error);
> +			}
> +		}
> +#endif
> +
> +		if (equals)
> +			*equals = oldchar;
> +
> +		tokstr = NULL;
> +	}
> +
>  	return mount_service_send_reply(mo, 0);
>  }
>  
> @@ -875,6 +976,196 @@ static int mount_service_regular_mount(struct mount_service *mo,
>  	return mount_service_send_reply(mo, 0);
>  }
>  
> +#ifdef HAVE_NEW_MOUNT_API
> +struct ms_to_mount_map {
> +	unsigned long ms_flag;
> +	unsigned int mount_attr_flag;
> +};
> +
> +static const struct ms_to_mount_map attrs[] = {
> +	{ MS_RDONLY,		MOUNT_ATTR_RDONLY },
> +	{ MS_NOSUID,		MOUNT_ATTR_NOSUID },
> +	{ MS_NODEV,		MOUNT_ATTR_NODEV },
> +	{ MS_NOEXEC,		MOUNT_ATTR_NOEXEC },
> +	{ MS_RELATIME,		MOUNT_ATTR_RELATIME },
> +	{ MS_NOATIME,		MOUNT_ATTR_NOATIME },
> +	{ MS_STRICTATIME,	MOUNT_ATTR_STRICTATIME },
> +	{ MS_NODIRATIME,	MOUNT_ATTR_NODIRATIME },
> +#ifdef MOUNT_ATTR_NOSYMFOLLOW
> +	{ MS_NOSYMFOLLOW,	MOUNT_ATTR_NOSYMFOLLOW },
> +#endif
> +	{ 0, 0 },
> +};
> +
> +static void get_mount_attr_flags(const struct fuse_service_mount_command *oc,
> +				 unsigned int *attr_flags,
> +				 unsigned long *leftover_ms_flags)
> +{
> +	const struct ms_to_mount_map *i;
> +	unsigned int ms_flags = ntohl(oc->ms_flags);
> +	unsigned int mount_attr_flags = 0;
> +
> +	for (i = attrs; i->ms_flag != 0; i++) {
> +		if (ms_flags & i->ms_flag)
> +			mount_attr_flags |= i->mount_attr_flag;
> +		ms_flags &= ~i->ms_flag;
> +	}
> +
> +	*leftover_ms_flags = ms_flags;
> +	*attr_flags = mount_attr_flags;
> +}
> +
> +struct ms_to_str_map {
> +	unsigned long ms_flag;
> +	const char *string;
> +};
> +
> +static const struct ms_to_str_map strflags[] = {
> +	{ MS_SYNCHRONOUS,	"sync" },
> +	{ MS_DIRSYNC,		"dirsync" },
> +	{ MS_LAZYTIME,		"lazytime" },
> +	{ 0, 0 },
> +};
> +
> +static int set_ms_flags(struct mount_service *mo, unsigned long ms_flags)
> +{
> +	const struct ms_to_str_map *i;
> +	int ret;
> +
> +	for (i = strflags; i->ms_flag != 0; i++) {
> +		if (!(ms_flags & i->ms_flag))
> +			continue;
> +
> +		ret = fsconfig(mo->fsopenfd, FSCONFIG_SET_FLAG, i->string,
> +			       NULL, 0);
> +		if (ret) {
> +			int error = errno;
> +
> +			fprintf(stderr, "%s: set %s option: %s\n",
> +				mo->msgtag, i->string, strerror(error));
> +			emit_fsconfig_messages(mo);
> +
> +			errno = error;
> +			return -1;
> +		}
> +		ms_flags &= ~i->ms_flag;
> +	}
> +
> +	/*
> +	 * We can't translate all the supplied MS_ flags into MOUNT_ATTR_ flags
> +	 * or string flags!  Return a magic code so the caller will fall back
> +	 * to regular mount(2).
> +	 */
> +	return ms_flags ? -2 : 0;
> +}

Let's please not forget later on to use the new define.


Thanks,
Bernd