Re: [PATCH] tracing/cfi: Fix cmp_entries_* functions signature mismatch

[Sami Tolvanen]

Hi Kalesh,

On Thu, Sep 23, 2021 at 10:09 AM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
>
> If CONFIG_CFI_CLANG=y, attempting to read an event histogram will cause
> the kernel to panic due to failed CFI check.
>
>     1. echo 'hist:keys=common_pid' >> events/sched/sched_switch/trigger
>     2. cat >> events/sched/sched_switch/hist
>     3. kernel panices on attempting to read hist
>
> This happens because the sort() function expects a generic
> int (*)(const void *, const void *) pointer for the compare function.
> To prevent this CFI failure, change tracing map cmp_entries_* function
> signatures to match this.
>
> Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> ---
>  kernel/trace/tracing_map.c | 40 ++++++++++++++++++++++----------------
>  1 file changed, 23 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/trace/tracing_map.c b/kernel/trace/tracing_map.c
> index d6bddb157ef2..a8c80ebbf9da 100644
> --- a/kernel/trace/tracing_map.c
> +++ b/kernel/trace/tracing_map.c
> @@ -834,19 +834,21 @@ int tracing_map_init(struct tracing_map *map)
>         return err;
>  }
>
> -static int cmp_entries_dup(const struct tracing_map_sort_entry **a,
> -                          const struct tracing_map_sort_entry **b)
> +static int cmp_entries_dup(const void *__a, const void *__b)
>  {
>         int ret = 0;
> +       const struct tracing_map_sort_entry *a
> +               = *(const struct tracing_map_sort_entry **)__a;
> +       const struct tracing_map_sort_entry *b
> +               = *(const struct tracing_map_sort_entry **)__b;
>
> -       if (memcmp((*a)->key, (*b)->key, (*a)->elt->map->key_size))
> +       if (memcmp(a->key, b->key, a->elt->map->key_size))
>                 ret = 1;
>
>         return ret;
>  }
>
> -static int cmp_entries_sum(const struct tracing_map_sort_entry **a,
> -                          const struct tracing_map_sort_entry **b)
> +static int cmp_entries_sum(const void *__a, const void *__b)
>  {
>         const struct tracing_map_elt *elt_a, *elt_b;
>         struct tracing_map_sort_key *sort_key;
> @@ -854,9 +856,13 @@ static int cmp_entries_sum(const struct tracing_map_sort_entry **a,
>         tracing_map_cmp_fn_t cmp_fn;
>         void *val_a, *val_b;
>         int ret = 0;
> +       const struct tracing_map_sort_entry *a
> +               = *(const struct tracing_map_sort_entry **)__a;
> +       const struct tracing_map_sort_entry *b
> +               = *(const struct tracing_map_sort_entry **)__b;
>
> -       elt_a = (*a)->elt;
> -       elt_b = (*b)->elt;
> +       elt_a = a->elt;
> +       elt_b = b->elt;
>
>         sort_key = &elt_a->map->sort_key;
>
> @@ -873,8 +879,7 @@ static int cmp_entries_sum(const struct tracing_map_sort_entry **a,
>         return ret;
>  }
>
> -static int cmp_entries_key(const struct tracing_map_sort_entry **a,
> -                          const struct tracing_map_sort_entry **b)
> +static int cmp_entries_key(const void *__a, const void *__b)
>  {
>         const struct tracing_map_elt *elt_a, *elt_b;
>         struct tracing_map_sort_key *sort_key;
> @@ -882,9 +887,13 @@ static int cmp_entries_key(const struct tracing_map_sort_entry **a,
>         tracing_map_cmp_fn_t cmp_fn;
>         void *val_a, *val_b;
>         int ret = 0;
> +       const struct tracing_map_sort_entry *a
> +               = *(const struct tracing_map_sort_entry **)__a;
> +       const struct tracing_map_sort_entry *b
> +               = *(const struct tracing_map_sort_entry **)__b;
>
> -       elt_a = (*a)->elt;
> -       elt_b = (*b)->elt;
> +       elt_a = a->elt;
> +       elt_b = b->elt;
>
>         sort_key = &elt_a->map->sort_key;
>
> @@ -989,10 +998,8 @@ static void sort_secondary(struct tracing_map *map,
>                            struct tracing_map_sort_key *primary_key,
>                            struct tracing_map_sort_key *secondary_key)
>  {
> -       int (*primary_fn)(const struct tracing_map_sort_entry **,
> -                         const struct tracing_map_sort_entry **);
> -       int (*secondary_fn)(const struct tracing_map_sort_entry **,
> -                           const struct tracing_map_sort_entry **);
> +       int (*primary_fn)(const void *, const void *);
> +       int (*secondary_fn)(const void *, const void *);
>         unsigned i, start = 0, n_sub = 1;
>
>         if (is_key(map, primary_key->field_idx))
> @@ -1061,8 +1068,7 @@ int tracing_map_sort_entries(struct tracing_map *map,
>                              unsigned int n_sort_keys,
>                              struct tracing_map_sort_entry ***sort_entries)
>  {
> -       int (*cmp_entries_fn)(const struct tracing_map_sort_entry **,
> -                             const struct tracing_map_sort_entry **);
> +       int (*cmp_entries_fn)(const void *, const void *);
>         struct tracing_map_sort_entry *sort_entry, **entries;
>         int i, n_entries, ret;
>
>

Thanks for the patch! This looks correct to me and fixes the function
type mismatch that trips CFI.

Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Sami