linux-kernel - Re: [PATCH RFC v2 2/8] ovl: Create ovl

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Message-ID: <87o6stakb6.fsf@mailhost.krisman.be>
Date: Tue, 05 Aug 2025 10:56:45 -0400
From: Gabriel Krisman Bertazi <gabriel@...sman.be>
To: André Almeida <andrealmeid@...lia.com>
Cc: Miklos Szeredi <miklos@...redi.hu>,  Amir Goldstein
 <amir73il@...il.com>,  Theodore Tso <tytso@....edu>,
  linux-unionfs@...r.kernel.org,  linux-kernel@...r.kernel.org,
  linux-fsdevel@...r.kernel.org,  Alexander Viro <viro@...iv.linux.org.uk>,
  Christian Brauner <brauner@...nel.org>,  Jan Kara <jack@...e.cz>,
  kernel-dev@...lia.com
Subject: Re: [PATCH RFC v2 2/8] ovl: Create ovl_strcmp() with casefold support

André Almeida <andrealmeid@...lia.com> writes:

> To add overlayfs support casefold filesystems, create a new function
> ovl_strcmp() with support for casefold names.
>
> If the ovl_cache_entry have stored a casefold name, use it and create
> a casfold version of the name that is going to be compared to.
>
> For the casefold support, just comparing the strings does not work
> because we need the dentry enconding, so make this function find the
> equivalent dentry for a giving directory, if any.
>
> As this function is used for search and insertion in the red-black tree,
> that means that the tree node keys are going to be the casefolded
> version of the dentry's names. Otherwise, the search would not work for
> case-insensitive mount points.
>
> For the non-casefold names, nothing changes.
>
> Signed-off-by: André Almeida <andrealmeid@...lia.com>
> ---
> I wonder what should be done here if kmalloc fails, if the strcmp()
> should fail as well or just fallback to the normal name?
> ---
>  fs/overlayfs/readdir.c | 42 ++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 83bca1bcb0488461b08effa70b32ff2fefba134e..1b8eb10e72a229ade40d18795746d3c779797a06 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -72,6 +72,44 @@ static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
>  	return rb_entry(n, struct ovl_cache_entry, node);
>  }
>  
> +/*
> + * Compare a string with a cache entry, with support for casefold names.
> + */
> +static int ovl_strcmp(const char *str, struct ovl_cache_entry *p, int len)
> +{

Why do you need to re-casefold str on every call to ovl_strcmp?  Isn't
it done in a loop while walking the rbtree with a constant "str" (i.e.,
the name being added, see ovl_cache_entry_find)? Can't you do it once,
outside of ovl_strcmp? This way you don't repeatedly allocate/free
memory for each node of the tree (as Viro mentioned), and you don't have
to deal with kmalloc failures here.

> +
> +	const struct qstr qstr = { .name = str, .len = len };
> +	const char *p_name = p->name, *name = str;
> +	char *dst = NULL;
> +	int cmp, cf_len;
> +
> +	if (p->cf_name)
> +		p_name = p->cf_name;

This should check IS_ENABLED(CONFIG_UNICODE) so it can be
compiled out by anyone doing CONFIG_UNICODE=n

> +
> +	if (p->map && !is_dot_dotdot(str, len)) {
> +		dst = kmalloc(OVL_NAME_LEN, GFP_KERNEL);
> +
> +		/*
> +		 * strcmp can't fail, so we fallback to the use the original
> +		 * name
> +		 */
> +		if (dst) {
> +			cf_len = utf8_casefold(p->map, &qstr, dst, OVL_NAME_LEN);

utf8_casefold can fail, as you know and checked.  But if it does, a
negative cf_len is passed to strncmp and cast to a very high
value.

> +
> +			if (cf_len > 0) {
> +				name = dst;
> +				dst[cf_len] = '\0';
> +			}

utf8_casefold ensures the string is NULL-terminated on success already.

> +		}
> +	}
> +
> +	cmp = strncmp(name, p_name, cf_len);
> +
> +	kfree(dst);
> +
> +	return cmp;
> +}
> +
>  static bool ovl_cache_entry_find_link(const char *name, int len,
>  				      struct rb_node ***link,
>  				      struct rb_node **parent)
> @@ -85,7 +123,7 @@ static bool ovl_cache_entry_find_link(const char *name, int len,
>  
>  		*parent = *newp;
>  		tmp = ovl_cache_entry_from_node(*newp);
> -		cmp = strncmp(name, tmp->name, len);
> +		cmp = ovl_strcmp(name, tmp, len);
>  		if (cmp > 0)
>  			newp = &tmp->node.rb_right;
>  		else if (cmp < 0 || len < tmp->len)
> @@ -107,7 +145,7 @@ static struct ovl_cache_entry *ovl_cache_entry_find(struct rb_root *root,
>  	while (node) {
>  		struct ovl_cache_entry *p = ovl_cache_entry_from_node(node);
>  
> -		cmp = strncmp(name, p->name, len);
> +		cmp = ovl_strcmp(name, p, len);
>  		if (cmp > 0)
>  			node = p->node.rb_right;
>  		else if (cmp < 0 || len < p->len)

-- 
Gabriel Krisman Bertazi