[PATCH] mountinfo: parse empty strings in source

[PATCH] mountinfo: parse empty strings in source

[Alban Crequy]

From: Alban Crequy <alban@kinvolk.io>

The source of a mount in /proc/self/mountinfo can unfortunately be an
empty string. Before this patch, 'mount' and 'mountpoint' fail as
following:

  $ sudo mount -t tmpfs "" /tmp/bb
  $ mount
  mount: /proc/self/mountinfo: parse error at line 64 -- ignored
  $ mountpoint /tmp/bb
  /tmp/bb is not a mountpoint

This patch fixes the parsing. It is unfortunately more complex because
sscanf() does not handle fields with empty strings easily.

Other projects have their own parser for mountinfo and have similar
issues. I know of runc and runtime-tools (I'll send a patch for those
two) but there are probably others.

Signed-off-by: Alban Crequy <alban@kinvolk.io>
---
 libmount/src/tab_parse.c | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/libmount/src/tab_parse.c b/libmount/src/tab_parse.c
index 3ed84ebc2..4944d5f8c 100644
--- a/libmount/src/tab_parse.c
+++ b/libmount/src/tab_parse.c
@@ -173,13 +173,36 @@ static int mnt_parse_mountinfo_line(struct libmnt_fs *fs, char *s)
 		fs->opt_fields = strndup(s + 1, p - s - 1);
 	s = p + 3;
 
-	rc += sscanf(s,	UL_SCNsA" "	/* (8) FS type */
-			UL_SCNsA" "	/* (9) source */
-			UL_SCNsA,	/* (10) fs options (fs specific) */
+	end = 0;
+	rc += sscanf(s,	UL_SCNsA"%n",	/* (8) FS type */
 
 			&fstype,
-			&src,
-			&fs->fs_optstr);
+			&end);
+
+	if (rc >= 8 && end > 0)
+		s += end;
+	if (s[0] == ' ')
+		s++;
+
+	/* (9) source can unfortunately be an empty string "" and scanf does
+	 * not work well with empty string. Test with:
+	 *     $ sudo mount -t tmpfs "" /tmp/bb
+	 *     $ mountpoint /tmp/bb
+	 * */
+	if (s[0] == ' ') {
+		src = strdup("");
+		s++;
+		rc++;
+		rc += sscanf(s,	UL_SCNsA,	/* (10) fs options (fs specific) */
+
+				&fs->fs_optstr);
+	} else {
+		rc += sscanf(s,	UL_SCNsA" "	/* (9) source */
+				UL_SCNsA,	/* (10) fs options (fs specific) */
+
+				&src,
+				&fs->fs_optstr);
+	}
 
 	if (rc >= 10) {
 		size_t sz;
-- 
2.14.3

Re: [PATCH] mountinfo: parse empty strings in source

[Karel Zak]

On Fri, Jun 22, 2018 at 01:54:25PM +0200, Alban Crequy wrote:
> From: Alban Crequy <alban@kinvolk.io>
> 
> The source of a mount in /proc/self/mountinfo can unfortunately be an
> empty string. Before this patch, 'mount' and 'mountpoint' fail as
> following:
> 
>   $ sudo mount -t tmpfs "" /tmp/bb
>   $ mount
>   mount: /proc/self/mountinfo: parse error at line 64 -- ignored
>   $ mountpoint /tmp/bb
>   /tmp/bb is not a mountpoint
> 
> This patch fixes the parsing. It is unfortunately more complex because
> sscanf() does not handle fields with empty strings easily.
> 
> Other projects have their own parser for mountinfo and have similar
> issues. I know of runc and runtime-tools (I'll send a patch for those
> two) but there are probably others.

The right place is to fix kernel rather than all places where we parse
the file in userspace. 

There is already a pending trivial kernel patch:
https://patchwork.kernel.org/patch/10349095/ ... but not sure about
the current status of the patch :-(

    Karel

> 
> Signed-off-by: Alban Crequy <alban@kinvolk.io>
> ---
>  libmount/src/tab_parse.c | 33 ++++++++++++++++++++++++++++-----
>  1 file changed, 28 insertions(+), 5 deletions(-)
> 
> diff --git a/libmount/src/tab_parse.c b/libmount/src/tab_parse.c
> index 3ed84ebc2..4944d5f8c 100644
> --- a/libmount/src/tab_parse.c
> +++ b/libmount/src/tab_parse.c
> @@ -173,13 +173,36 @@ static int mnt_parse_mountinfo_line(struct libmnt_fs *fs, char *s)
>  		fs->opt_fields = strndup(s + 1, p - s - 1);
>  	s = p + 3;
>  
> -	rc += sscanf(s,	UL_SCNsA" "	/* (8) FS type */
> -			UL_SCNsA" "	/* (9) source */
> -			UL_SCNsA,	/* (10) fs options (fs specific) */
> +	end = 0;
> +	rc += sscanf(s,	UL_SCNsA"%n",	/* (8) FS type */
>  
>  			&fstype,
> -			&src,
> -			&fs->fs_optstr);
> +			&end);
> +
> +	if (rc >= 8 && end > 0)
> +		s += end;
> +	if (s[0] == ' ')
> +		s++;
> +
> +	/* (9) source can unfortunately be an empty string "" and scanf does
> +	 * not work well with empty string. Test with:
> +	 *     $ sudo mount -t tmpfs "" /tmp/bb
> +	 *     $ mountpoint /tmp/bb
> +	 * */
> +	if (s[0] == ' ') {
> +		src = strdup("");
> +		s++;
> +		rc++;
> +		rc += sscanf(s,	UL_SCNsA,	/* (10) fs options (fs specific) */
> +
> +				&fs->fs_optstr);
> +	} else {
> +		rc += sscanf(s,	UL_SCNsA" "	/* (9) source */
> +				UL_SCNsA,	/* (10) fs options (fs specific) */
> +
> +				&src,
> +				&fs->fs_optstr);
> +	}
>  
>  	if (rc >= 10) {
>  		size_t sz;
> -- 
> 2.14.3
> 

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: [PATCH] mountinfo: parse empty strings in source

[Karel Zak]

On Fri, Jun 22, 2018 at 01:54:25PM +0200, Alban Crequy wrote:
>  libmount/src/tab_parse.c | 33 ++++++++++++++++++++++++++++-----
>  1 file changed, 28 insertions(+), 5 deletions(-)

Applied, thanks!

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com