libbpf: add NULL checks to bpf_object__{prev_map,next_map}

[PATCH] libbpf: add NULL checks to bpf_object__{prev_map,next_map}

Posted by Andreas Ziegler 1 year, 7 months ago

In the current state, an erroneous call to
bpf_object__find_map_by_name(NULL, ...) leads to a segmentation fault
through the following call chain:

bpf_object__find_map_by_name(obj = NULL, ...)
-> bpf_object__for_each_map(pos, obj = NULL)
-> bpf_object__next_map((obj = NULL), NULL)
-> return (obj = NULL)->maps

While calling bpf_object__find_map_by_name with obj = NULL is
obviously incorrect, this should not lead to a segmentation
fault but rather be handled gracefully.

As __bpf_map__iter already handles this situation correctly,
we can delegate the check for the regular case there and only
add a check in case the prev or next parameter is NULL.

Signed-off-by: Andreas Ziegler <ziegler.andreas@siemens.com>
---
 tools/lib/bpf/libbpf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 4a28fac4908a..30f121754d83 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10375,7 +10375,7 @@ __bpf_map__iter(const struct bpf_map *m, const struct bpf_object *obj, int i)
 struct bpf_map *
 bpf_object__next_map(const struct bpf_object *obj, const struct bpf_map *prev)
 {
-	if (prev == NULL)
+	if (prev == NULL && obj != NULL)
 		return obj->maps;
 
 	return __bpf_map__iter(prev, obj, 1);
@@ -10384,7 +10384,7 @@ bpf_object__next_map(const struct bpf_object *obj, const struct bpf_map *prev)
 struct bpf_map *
 bpf_object__prev_map(const struct bpf_object *obj, const struct bpf_map *next)
 {
-	if (next == NULL) {
+	if (next == NULL && obj != NULL) {
 		if (!obj->nr_maps)
 			return NULL;
 		return obj->maps + obj->nr_maps - 1;
-- 
2.39.2

Re: [PATCH] libbpf: add NULL checks to bpf_object__{prev_map,next_map}

Posted by Andrii Nakryiko 1 year, 7 months ago

On Wed, Jul 3, 2024 at 1:35 AM Andreas Ziegler
<ziegler.andreas@siemens.com> wrote:
>
> In the current state, an erroneous call to
> bpf_object__find_map_by_name(NULL, ...) leads to a segmentation fault
> through the following call chain:
>
> bpf_object__find_map_by_name(obj = NULL, ...)
> -> bpf_object__for_each_map(pos, obj = NULL)
> -> bpf_object__next_map((obj = NULL), NULL)
> -> return (obj = NULL)->maps
>
> While calling bpf_object__find_map_by_name with obj = NULL is
> obviously incorrect, this should not lead to a segmentation
> fault but rather be handled gracefully.
>
> As __bpf_map__iter already handles this situation correctly,
> we can delegate the check for the regular case there and only
> add a check in case the prev or next parameter is NULL.
>
> Signed-off-by: Andreas Ziegler <ziegler.andreas@siemens.com>
> ---
>  tools/lib/bpf/libbpf.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>

Generally speaking libbpf's APIs don't check non-optional parameters
for NULL. We historically did check that in some APIs and didn't in
others, it wasn't consistent. But since a long while ago we decided on
not checking arguments for NULL defensively. So I don't think this
patch is necessary.

pw-bot: cr


> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 4a28fac4908a..30f121754d83 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10375,7 +10375,7 @@ __bpf_map__iter(const struct bpf_map *m, const struct bpf_object *obj, int i)
>  struct bpf_map *
>  bpf_object__next_map(const struct bpf_object *obj, const struct bpf_map *prev)
>  {
> -       if (prev == NULL)
> +       if (prev == NULL && obj != NULL)
>                 return obj->maps;
>
>         return __bpf_map__iter(prev, obj, 1);
> @@ -10384,7 +10384,7 @@ bpf_object__next_map(const struct bpf_object *obj, const struct bpf_map *prev)
>  struct bpf_map *
>  bpf_object__prev_map(const struct bpf_object *obj, const struct bpf_map *next)
>  {
> -       if (next == NULL) {
> +       if (next == NULL && obj != NULL) {
>                 if (!obj->nr_maps)
>                         return NULL;
>                 return obj->maps + obj->nr_maps - 1;
> --
> 2.39.2
>