Re: [PATCH v5 4/9] ovl: Create ovl_casefold() to support casefolded strncmp()

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Aug 14, 2025 at 7:22 PM André Almeida <andrealmeid@igalia.com> wrote:
>
> To add overlayfs support casefold layers, create a new function
> ovl_casefold(), to be able to do case-insensitive strncmp().
>
> ovl_casefold() allocates a new buffer and stores the casefolded version
> of the string on it. If the allocation or the casefold operation fails,
> fallback to use the original string.
>
> The case-insentive name is then used in the rb-tree search/insertion
> operation. If the name is found in the rb-tree, the name can be
> discarded and the buffer is freed. If the name isn't found, it's then
> stored at struct ovl_cache_entry to be used later.
>
> Signed-off-by: André Almeida <andrealmeid@igalia.com>
> ---
> Changes from v4:
>  - Move the consumer/free buffer logic out to the caller
>  - s/aux/c_name
>
> Changes from v3:
>  - Improve commit message text
>  - s/OVL_NAME_LEN/NAME_MAX
>  - drop #ifdef in favor of if(IS_ENABLED)
>  - use new helper sb_encoding
>  - merged patch "Store casefold name..." and "Create ovl_casefold()..."
>  - Guard all the casefolding inside of IS_ENABLED(UNICODE)
>
> Changes from v2:
> - Refactor the patch to do a single kmalloc() per rb_tree operation
> - Instead of casefolding the cache entry name everytime per strncmp(),
>   casefold it once and reuse it for every strncmp().
> ---
>  fs/overlayfs/readdir.c | 115 +++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 97 insertions(+), 18 deletions(-)
>
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index b65cdfce31ce27172d28d879559f1008b9c87320..803ac6a7516d0156ae7793ee1ff884dbbf2e20b0 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -27,6 +27,8 @@ struct ovl_cache_entry {
>         bool is_upper;
>         bool is_whiteout;
>         bool check_xwhiteout;
> +       const char *cf_name;
> +       int cf_len;
>         char name[];
>  };
>
> @@ -45,6 +47,7 @@ struct ovl_readdir_data {
>         struct list_head *list;
>         struct list_head middle;
>         struct ovl_cache_entry *first_maybe_whiteout;
> +       struct unicode_map *map;
>         int count;
>         int err;
>         bool is_upper;
> @@ -66,6 +69,27 @@ static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
>         return rb_entry(n, struct ovl_cache_entry, node);
>  }
>
> +static int ovl_casefold(struct unicode_map *map, const char *str, int len, char **dst)
> +{
> +       const struct qstr qstr = { .name = str, .len = len };
> +       int cf_len;
> +
> +       if (!IS_ENABLED(CONFIG_UNICODE) || !map || is_dot_dotdot(str, len))
> +               return 0;
> +
> +       *dst = kmalloc(NAME_MAX, GFP_KERNEL);
> +
> +       if (dst) {
> +               cf_len = utf8_casefold(map, &qstr, *dst, NAME_MAX);
> +
> +               if (cf_len > 0)
> +                       return cf_len;
> +       }
> +
> +       kfree(*dst);
> +       return 0;
> +}
> +
>  static bool ovl_cache_entry_find_link(const char *name, int len,
>                                       struct rb_node ***link,
>                                       struct rb_node **parent)
> @@ -79,7 +103,7 @@ 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 = strncmp(name, tmp->cf_name, tmp->cf_len);
>                 if (cmp > 0)
>                         newp = &tmp->node.rb_right;
>                 else if (cmp < 0 || len < tmp->len)
> @@ -101,7 +125,7 @@ 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 = strncmp(name, p->cf_name, p->cf_len);
>                 if (cmp > 0)
>                         node = p->node.rb_right;
>                 else if (cmp < 0 || len < p->len)
> @@ -145,13 +169,16 @@ static bool ovl_calc_d_ino(struct ovl_readdir_data *rdd,
>
>  static struct ovl_cache_entry *ovl_cache_entry_new(struct ovl_readdir_data *rdd,
>                                                    const char *name, int len,
> +                                                  const char *cf_name, int cf_len,
>                                                    u64 ino, unsigned int d_type)
>  {
>         struct ovl_cache_entry *p;
>
>         p = kmalloc(struct_size(p, name, len + 1), GFP_KERNEL);
> -       if (!p)
> +       if (!p) {
> +               kfree(cf_name);

Not needed.
Caller will get -ENOMEM and will free c_name

No need to repost just for this I can fix on commit.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Thanks,
Amir.