Re: [Virtio-fs] [PATCH 3/2] fs: simplify get_filesystem_list / get_all_fs_names

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jun 22, 2021 at 10:12:17AM +0200, Christoph Hellwig wrote:
> Just output the '\0' separate list of supported file systems for block
> devices directly rather than going through a pointless round of string
> manipulation.
> 
> Based on an earlier patch from Al Viro <viro@zeniv.linux.org.uk>.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/filesystems.c   | 24 ++++++++++++++----------
>  include/linux/fs.h |  2 +-
>  init/do_mounts.c   | 20 +-------------------
>  3 files changed, 16 insertions(+), 30 deletions(-)
> 
> diff --git a/fs/filesystems.c b/fs/filesystems.c
> index 90b8d879fbaf..7c136251607a 100644
> --- a/fs/filesystems.c
> +++ b/fs/filesystems.c
> @@ -209,21 +209,25 @@ SYSCALL_DEFINE3(sysfs, int, option, unsigned long, arg1, unsigned long, arg2)
>  }
>  #endif
>  
> -int __init get_filesystem_list(char *buf)
> +void __init list_bdev_fs_names(char *buf, size_t size)
>  {
> -	int len = 0;
> -	struct file_system_type * tmp;
> +	struct file_system_type *p;
> +	size_t len;
>  
>  	read_lock(&file_systems_lock);
> -	tmp = file_systems;
> -	while (tmp && len < PAGE_SIZE - 80) {
> -		len += sprintf(buf+len, "%s\t%s\n",
> -			(tmp->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
> -			tmp->name);
> -		tmp = tmp->next;
> +	for (p = file_systems; p; p = p->next) {
> +		if (!(p->fs_flags & FS_REQUIRES_DEV))
> +			continue;
> +		len = strlen(p->name) + 1;
> +		if (len > size) {
> +			pr_warn("%s: truncating file system list\n", __func__);
> +			break;
> +		}
> +		memcpy(buf, p->name, len);
> +		buf += len;
> +		size -= len;
>  	}
>  	read_unlock(&file_systems_lock);
> -	return len;
>  }

I don't see the extra NUL terminator byte being added that's required by
the loop in mount_block_root()?

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