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

[Bernd Schubert <bernd@bsbernd.com>]

On 3/30/26 23:18, Darrick J. Wong wrote:
> On Mon, Mar 30, 2026 at 09:06:05PM +0000, Bernd Schubert wrote:
>> 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.
> 
> <nod> The mount option string parsing (i.e. all that MS_ -> MOUNT_ATTR
> translation) and error reporting parts look trivially shareable.
> fuse_kern_fsmount isn't so easy to share since the fuse server sends
> over all the relevant pieces in separate commands.
> 
>>> +
>>>  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.
> 
> Er... which new define?  HAVE_NEW_MOUNT_API?  I think we're still inside
> the #ifdef for that at this point.
> 
> <confused>


Sorry, I meant FUSE_MOUNT_FALLBACK_NEEDED.