This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
creation and manipulation of struct bpf_iter_css in open-coded iterator
style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
css_iter can be used to:
1) iterating a sepcific cgroup tree with pre/post/up order
2) iterating cgroup_subsystem in BPF Prog, like
for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
parameters defining iteration order and starting css. Here we also reuse
BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
BPF_CGROUP_ITER_ANCESTORS_UP enums.
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
---
kernel/bpf/cgroup_iter.c | 57 +++++++++++++++++++
kernel/bpf/helpers.c | 3 +
.../testing/selftests/bpf/bpf_experimental.h | 6 ++
3 files changed, 66 insertions(+)
diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
index 810378f04fbc..ebc3d9471f52 100644
--- a/kernel/bpf/cgroup_iter.c
+++ b/kernel/bpf/cgroup_iter.c
@@ -294,3 +294,60 @@ static int __init bpf_cgroup_iter_init(void)
}
late_initcall(bpf_cgroup_iter_init);
+
+struct bpf_iter_css {
+ __u64 __opaque[2];
+ __u32 __opaque_int[1];
+} __attribute__((aligned(8)));
+
+struct bpf_iter_css_kern {
+ struct cgroup_subsys_state *start;
+ struct cgroup_subsys_state *pos;
+ int order;
+} __attribute__((aligned(8)));
+
+__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
+ struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order)
+{
+ struct bpf_iter_css_kern *kit = (void *)it;
+ kit->start = NULL;
+ BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
+ BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
+ switch (order) {
+ case BPF_CGROUP_ITER_DESCENDANTS_PRE:
+ case BPF_CGROUP_ITER_DESCENDANTS_POST:
+ case BPF_CGROUP_ITER_ANCESTORS_UP:
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ kit->start = start;
+ kit->pos = NULL;
+ kit->order = order;
+ return 0;
+}
+
+__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
+{
+ struct bpf_iter_css_kern *kit = (void *)it;
+ if (!kit->start)
+ return NULL;
+
+ switch (kit->order) {
+ case BPF_CGROUP_ITER_DESCENDANTS_PRE:
+ kit->pos = css_next_descendant_pre(kit->pos, kit->start);
+ break;
+ case BPF_CGROUP_ITER_DESCENDANTS_POST:
+ kit->pos = css_next_descendant_post(kit->pos, kit->start);
+ break;
+ default:
+ kit->pos = kit->pos ? kit->pos->parent : kit->start;
+ }
+
+ return kit->pos;
+}
+
+__bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
+{
+}
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 556262c27a75..9c3af36249a2 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2510,6 +2510,9 @@ BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
BTF_ID_FLAGS(func, bpf_iter_task_next, KF_ITER_NEXT | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_iter_task_destroy, KF_ITER_DESTROY)
+BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_iter_css_next, KF_ITER_NEXT | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
BTF_ID_FLAGS(func, bpf_dynptr_adjust)
BTF_ID_FLAGS(func, bpf_dynptr_is_null)
BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index d989775dbdb5..aa247d1d81d1 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -174,4 +174,10 @@ extern int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task,
extern struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it) __weak __ksym;
extern void bpf_iter_task_destroy(struct bpf_iter_task *it) __weak __ksym;
+struct bpf_iter_css;
+extern int bpf_iter_css_new(struct bpf_iter_css *it,
+ struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order) __weak __ksym;
+extern struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it) __weak __ksym;
+extern void bpf_iter_css_destroy(struct bpf_iter_css *it) __weak __ksym;
+
#endif
--
2.20.1On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>
> This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
> creation and manipulation of struct bpf_iter_css in open-coded iterator
> style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
> css_iter can be used to:
>
> 1) iterating a sepcific cgroup tree with pre/post/up order
>
> 2) iterating cgroup_subsystem in BPF Prog, like
> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
>
> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
> parameters defining iteration order and starting css. Here we also reuse
> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
> BPF_CGROUP_ITER_ANCESTORS_UP enums.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> ---
> kernel/bpf/cgroup_iter.c | 57 +++++++++++++++++++
> kernel/bpf/helpers.c | 3 +
> .../testing/selftests/bpf/bpf_experimental.h | 6 ++
> 3 files changed, 66 insertions(+)
>
> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> index 810378f04fbc..ebc3d9471f52 100644
> --- a/kernel/bpf/cgroup_iter.c
> +++ b/kernel/bpf/cgroup_iter.c
> @@ -294,3 +294,60 @@ static int __init bpf_cgroup_iter_init(void)
> }
>
> late_initcall(bpf_cgroup_iter_init);
> +
> +struct bpf_iter_css {
> + __u64 __opaque[2];
> + __u32 __opaque_int[1];
> +} __attribute__((aligned(8)));
> +
same as before, __opaque[3] only
> +struct bpf_iter_css_kern {
> + struct cgroup_subsys_state *start;
> + struct cgroup_subsys_state *pos;
> + int order;
> +} __attribute__((aligned(8)));
> +
> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
> + struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order)
Similarly, I wonder if we should go for a more generic "flags" argument?
> +{
> + struct bpf_iter_css_kern *kit = (void *)it;
empty line
> + kit->start = NULL;
> + BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
> + BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
please move this up before kit->start assignment, and separate by empty lines
> + switch (order) {
> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
> + case BPF_CGROUP_ITER_ANCESTORS_UP:
> + break;
> + default:
> + return -EINVAL;
> + }
> +
> + kit->start = start;
> + kit->pos = NULL;
> + kit->order = order;
> + return 0;
> +}
> +
> +__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
> +{
> + struct bpf_iter_css_kern *kit = (void *)it;
empty line
> + if (!kit->start)
> + return NULL;
> +
> + switch (kit->order) {
> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
> + kit->pos = css_next_descendant_pre(kit->pos, kit->start);
> + break;
> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
> + kit->pos = css_next_descendant_post(kit->pos, kit->start);
> + break;
> + default:
we know it's BPF_CGROUP_ITER_ANCESTORS_UP, so why not have that here explicitly?
> + kit->pos = kit->pos ? kit->pos->parent : kit->start;
> + }
> +
> + return kit->pos;
wouldn't this implementation never return the "start" css? is that intentional?
> +}
> +
> +__bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
> +{
> +}
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 556262c27a75..9c3af36249a2 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2510,6 +2510,9 @@ BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
> BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
> BTF_ID_FLAGS(func, bpf_iter_task_next, KF_ITER_NEXT | KF_RET_NULL)
> BTF_ID_FLAGS(func, bpf_iter_task_destroy, KF_ITER_DESTROY)
> +BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
> +BTF_ID_FLAGS(func, bpf_iter_css_next, KF_ITER_NEXT | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
> BTF_ID_FLAGS(func, bpf_dynptr_adjust)
> BTF_ID_FLAGS(func, bpf_dynptr_is_null)
> BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
> index d989775dbdb5..aa247d1d81d1 100644
> --- a/tools/testing/selftests/bpf/bpf_experimental.h
> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> @@ -174,4 +174,10 @@ extern int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task,
> extern struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it) __weak __ksym;
> extern void bpf_iter_task_destroy(struct bpf_iter_task *it) __weak __ksym;
>
> +struct bpf_iter_css;
> +extern int bpf_iter_css_new(struct bpf_iter_css *it,
> + struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order) __weak __ksym;
> +extern struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it) __weak __ksym;
> +extern void bpf_iter_css_destroy(struct bpf_iter_css *it) __weak __ksym;
> +
> #endif
> --
> 2.20.1
>
Hello,
在 2023/9/28 07:24, Andrii Nakryiko 写道:
> On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
>> creation and manipulation of struct bpf_iter_css in open-coded iterator
>> style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
>> css_iter can be used to:
>>
>> 1) iterating a sepcific cgroup tree with pre/post/up order
>>
>> 2) iterating cgroup_subsystem in BPF Prog, like
>> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
>>
>> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
>> parameters defining iteration order and starting css. Here we also reuse
>> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
>> BPF_CGROUP_ITER_ANCESTORS_UP enums.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>> ---
>> kernel/bpf/cgroup_iter.c | 57 +++++++++++++++++++
>> kernel/bpf/helpers.c | 3 +
>> .../testing/selftests/bpf/bpf_experimental.h | 6 ++
>> 3 files changed, 66 insertions(+)
>>
>> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
>> index 810378f04fbc..ebc3d9471f52 100644
>> --- a/kernel/bpf/cgroup_iter.c
>> +++ b/kernel/bpf/cgroup_iter.c
>> @@ -294,3 +294,60 @@ static int __init bpf_cgroup_iter_init(void)
>> }
>>
>> late_initcall(bpf_cgroup_iter_init);
>> +
>> +struct bpf_iter_css {
>> + __u64 __opaque[2];
>> + __u32 __opaque_int[1];
>> +} __attribute__((aligned(8)));
>> +
>
> same as before, __opaque[3] only
>
>
>> +struct bpf_iter_css_kern {
>> + struct cgroup_subsys_state *start;
>> + struct cgroup_subsys_state *pos;
>> + int order;
>> +} __attribute__((aligned(8)));
>> +
>> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
>> + struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order)
>
> Similarly, I wonder if we should go for a more generic "flags" argument?
>
>> +{
>> + struct bpf_iter_css_kern *kit = (void *)it;
>
> empty line
>
>> + kit->start = NULL;
>> + BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
>> + BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
>
> please move this up before kit->start assignment, and separate by empty lines
>
>> + switch (order) {
>> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
>> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
>> + case BPF_CGROUP_ITER_ANCESTORS_UP:
>> + break;
>> + default:
>> + return -EINVAL;
>> + }
>> +
>> + kit->start = start;
>> + kit->pos = NULL;
>> + kit->order = order;
>> + return 0;
>> +}
>> +
>> +__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
>> +{
>> + struct bpf_iter_css_kern *kit = (void *)it;
>
> empty line
>
>> + if (!kit->start)
>> + return NULL;
>> +
>> + switch (kit->order) {
>> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
>> + kit->pos = css_next_descendant_pre(kit->pos, kit->start);
>> + break;
>> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
>> + kit->pos = css_next_descendant_post(kit->pos, kit->start);
>> + break;
>> + default:
>
> we know it's BPF_CGROUP_ITER_ANCESTORS_UP, so why not have that here explicitly?
>
>> + kit->pos = kit->pos ? kit->pos->parent : kit->start;
>> + }
>> +
>> + return kit->pos;
>
> wouldn't this implementation never return the "start" css? is that intentional?
>
Thanks for the review.
This implementation actually would return the "start" css.
1. BPF_CGROUP_ITER_DESCENDANTS_PRE:
1.1 when we first call next(), css_next_descendant_pre(NULL, kit->start)
will return kit->start.
1.2 second call next(), css_next_descendant_pre(kit->start, kit->start)
would return a first valid child under kit->start with pre-order
1.3 third call next, css_next_descendant_pre(last_valid_child,
kit->start) would return the next valid child
...
util css_next_descendant_pre return a NULL pointer, which means we have
visited all valid child including "start" css itself.
The above logic is equal to macro 'css_for_each_descendant_pre' in kernel.
Same, BPF_CGROUP_ITER_DESCENDANTS_POST is equal to macro
'css_for_each_descendant_post' which would return 'start' css when we
have visited all valid child.
2. BPF_CGROUP_ITER_ANCESTORS_UP
2.1 when we fisrt call next(), kit->pos is NULL, and we would return
kit->start.
The selftest in patch7 whould check:
1. when we use BPF_CGROUP_ITER_DESCENDANTS_PRE to iterate a cgroup tree,
the first cgroup we visted should be root('start') cgroup.
2. when we use BPF_CGROUP_ITER_DESCENDANTS_POST to iterate a cgroup
tree, the last cgroup we visited should be root('start') cgroup.
Am I miss something important?
Thanks.
On Wed, Sep 27, 2023 at 7:51 PM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>
> Hello,
>
> 在 2023/9/28 07:24, Andrii Nakryiko 写道:
> > On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
> >>
> >> This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
> >> creation and manipulation of struct bpf_iter_css in open-coded iterator
> >> style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
> >> css_iter can be used to:
> >>
> >> 1) iterating a sepcific cgroup tree with pre/post/up order
> >>
> >> 2) iterating cgroup_subsystem in BPF Prog, like
> >> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
> >>
> >> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
> >> parameters defining iteration order and starting css. Here we also reuse
> >> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
> >> BPF_CGROUP_ITER_ANCESTORS_UP enums.
> >>
> >> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> >> ---
> >> kernel/bpf/cgroup_iter.c | 57 +++++++++++++++++++
> >> kernel/bpf/helpers.c | 3 +
> >> .../testing/selftests/bpf/bpf_experimental.h | 6 ++
> >> 3 files changed, 66 insertions(+)
> >>
> >> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> >> index 810378f04fbc..ebc3d9471f52 100644
> >> --- a/kernel/bpf/cgroup_iter.c
> >> +++ b/kernel/bpf/cgroup_iter.c
> >> @@ -294,3 +294,60 @@ static int __init bpf_cgroup_iter_init(void)
> >> }
> >>
> >> late_initcall(bpf_cgroup_iter_init);
> >> +
> >> +struct bpf_iter_css {
> >> + __u64 __opaque[2];
> >> + __u32 __opaque_int[1];
> >> +} __attribute__((aligned(8)));
> >> +
> >
> > same as before, __opaque[3] only
> >
> >
> >> +struct bpf_iter_css_kern {
> >> + struct cgroup_subsys_state *start;
> >> + struct cgroup_subsys_state *pos;
> >> + int order;
> >> +} __attribute__((aligned(8)));
> >> +
> >> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
> >> + struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order)
> >
> > Similarly, I wonder if we should go for a more generic "flags" argument?
> >
> >> +{
> >> + struct bpf_iter_css_kern *kit = (void *)it;
> >
> > empty line
> >
> >> + kit->start = NULL;
> >> + BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
> >> + BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
> >
> > please move this up before kit->start assignment, and separate by empty lines
> >
> >> + switch (order) {
> >> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
> >> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
> >> + case BPF_CGROUP_ITER_ANCESTORS_UP:
> >> + break;
> >> + default:
> >> + return -EINVAL;
> >> + }
> >> +
> >> + kit->start = start;
> >> + kit->pos = NULL;
> >> + kit->order = order;
> >> + return 0;
> >> +}
> >> +
> >> +__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
> >> +{
> >> + struct bpf_iter_css_kern *kit = (void *)it;
> >
> > empty line
> >
> >> + if (!kit->start)
> >> + return NULL;
> >> +
> >> + switch (kit->order) {
> >> + case BPF_CGROUP_ITER_DESCENDANTS_PRE:
> >> + kit->pos = css_next_descendant_pre(kit->pos, kit->start);
> >> + break;
> >> + case BPF_CGROUP_ITER_DESCENDANTS_POST:
> >> + kit->pos = css_next_descendant_post(kit->pos, kit->start);
> >> + break;
> >> + default:
> >
> > we know it's BPF_CGROUP_ITER_ANCESTORS_UP, so why not have that here explicitly?
> >
> >> + kit->pos = kit->pos ? kit->pos->parent : kit->start;
> >> + }
> >> +
> >> + return kit->pos;
> >
> > wouldn't this implementation never return the "start" css? is that intentional?
> >
>
> Thanks for the review.
>
> This implementation actually would return the "start" css.
>
> 1. BPF_CGROUP_ITER_DESCENDANTS_PRE:
> 1.1 when we first call next(), css_next_descendant_pre(NULL, kit->start)
> will return kit->start.
> 1.2 second call next(), css_next_descendant_pre(kit->start, kit->start)
> would return a first valid child under kit->start with pre-order
> 1.3 third call next, css_next_descendant_pre(last_valid_child,
> kit->start) would return the next valid child
> ...
> util css_next_descendant_pre return a NULL pointer, which means we have
> visited all valid child including "start" css itself.
>
> The above logic is equal to macro 'css_for_each_descendant_pre' in kernel.
>
> Same, BPF_CGROUP_ITER_DESCENDANTS_POST is equal to macro
> 'css_for_each_descendant_post' which would return 'start' css when we
> have visited all valid child.
>
> 2. BPF_CGROUP_ITER_ANCESTORS_UP
> 2.1 when we fisrt call next(), kit->pos is NULL, and we would return
> kit->start.
>
>
> The selftest in patch7 whould check:
> 1. when we use BPF_CGROUP_ITER_DESCENDANTS_PRE to iterate a cgroup tree,
> the first cgroup we visted should be root('start') cgroup.
> 2. when we use BPF_CGROUP_ITER_DESCENDANTS_POST to iterate a cgroup
> tree, the last cgroup we visited should be root('start') cgroup.
>
>
> Am I miss something important?
>
No, again, my bad, I didn't trace the logic completely before asking.
All makes sense with kit->pos being initialized to NULL. Thanks for
elaborating!
>
> Thanks.
>
>
>
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