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Message-ID: <ZxflTe2O2iktiv8G@google.com>
Date: Tue, 22 Oct 2024 10:47:57 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
LKML <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Vlastimil Babka <vbabka@...e.cz>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Kees Cook <kees@...nel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: Add open coded version of kmem_cache
iterator
Hello,
On Fri, Oct 18, 2024 at 11:22:00AM -0700, Martin KaFai Lau wrote:
> On 10/17/24 1:06 AM, Namhyung Kim wrote:
> > Add a new open coded iterator for kmem_cache which can be called from a
> > BPF program like below. It doesn't take any argument and traverses all
> > kmem_cache entries.
> >
> > struct kmem_cache *pos;
> >
> > bpf_for_each(kmem_cache, pos) {
> > ...
> > }
> >
> > As it needs to grab slab_mutex, it should be called from sleepable BPF
> > programs only.
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > kernel/bpf/helpers.c | 3 ++
> > kernel/bpf/kmem_cache_iter.c | 87 ++++++++++++++++++++++++++++++++++++
> > 2 files changed, 90 insertions(+)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 073e6f04f4d765ff..d1dfa4f335577914 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -3111,6 +3111,9 @@ BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
> > BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
> > BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
> > BTF_ID_FLAGS(func, bpf_get_kmem_cache)
> > +BTF_ID_FLAGS(func, bpf_iter_kmem_cache_new, KF_ITER_NEW | KF_SLEEPABLE)
> > +BTF_ID_FLAGS(func, bpf_iter_kmem_cache_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPABLE)
> > +BTF_ID_FLAGS(func, bpf_iter_kmem_cache_destroy, KF_ITER_DESTROY | KF_SLEEPABLE)
> > BTF_KFUNCS_END(common_btf_ids)
> > static const struct btf_kfunc_id_set common_kfunc_set = {
> > diff --git a/kernel/bpf/kmem_cache_iter.c b/kernel/bpf/kmem_cache_iter.c
> > index ebc101d7da51b57c..31ddaf452b20a458 100644
> > --- a/kernel/bpf/kmem_cache_iter.c
> > +++ b/kernel/bpf/kmem_cache_iter.c
> > @@ -145,6 +145,93 @@ static const struct bpf_iter_seq_info kmem_cache_iter_seq_info = {
> > .seq_ops = &kmem_cache_iter_seq_ops,
> > };
> > +/* open-coded version */
> > +struct bpf_iter_kmem_cache {
> > + __u64 __opaque[1];
> > +} __attribute__((aligned(8)));
> > +
> > +struct bpf_iter_kmem_cache_kern {
> > + struct kmem_cache *pos;
> > +} __attribute__((aligned(8)));
> > +
> > +__bpf_kfunc_start_defs();
> > +
> > +__bpf_kfunc int bpf_iter_kmem_cache_new(struct bpf_iter_kmem_cache *it)
> > +{
> > + struct bpf_iter_kmem_cache_kern *kit = (void *)it;
> > +
> > + BUILD_BUG_ON(sizeof(*kit) > sizeof(*it));
> > + BUILD_BUG_ON(__alignof__(*kit) != __alignof__(*it));
> > +
> > + kit->pos = NULL;
> > + return 0;
> > +}
> > +
> > +__bpf_kfunc struct kmem_cache *bpf_iter_kmem_cache_next(struct bpf_iter_kmem_cache *it)
> > +{
> > + struct bpf_iter_kmem_cache_kern *kit = (void *)it;
> > + struct kmem_cache *prev = kit->pos;
> > + struct kmem_cache *next;
> > + bool destroy = false;
> > +
> > + mutex_lock(&slab_mutex);
>
> I think taking mutex_lock here should be fine since sleepable tracing prog
> should be limited to the error injection whitelist. Those functions should
> not have held the mutex afaict.
>
> > +
> > + if (list_empty(&slab_caches)) {
> > + mutex_unlock(&slab_mutex);
> > + return NULL;
> > + }
> > +
> > + if (prev == NULL)
> > + next = list_first_entry(&slab_caches, struct kmem_cache, list);
> > + else if (list_last_entry(&slab_caches, struct kmem_cache, list) == prev)
> > + next = NULL;
>
> At the last entry, next is NULL.
>
> > + else
> > + next = list_next_entry(prev, list);
> > +
> > + /* boot_caches have negative refcount, don't touch them */
> > + if (next && next->refcount > 0)
> > + next->refcount++;
> > +
> > + /* Skip kmem_cache_destroy() for active entries */
> > + if (prev && prev->refcount > 1)
> > + prev->refcount--;
> > + else if (prev && prev->refcount == 1)
> > + destroy = true;
> > +
> > + mutex_unlock(&slab_mutex);
> > +
> > + if (destroy)
> > + kmem_cache_destroy(prev);
> > +
> > + kit->pos = next;
>
> so kit->pos will be NULL also. Does it mean the bpf prog will be able to
> call bpf_iter_kmem_cache_next() again and re-loop from the beginning of the
> slab_caches list?
Right, I'll mark the start pos differently to prevent that.
Thanks,
Namhyung
>
> > + return next;
> > +}
> > +
> > +__bpf_kfunc void bpf_iter_kmem_cache_destroy(struct bpf_iter_kmem_cache *it)
> > +{
> > + struct bpf_iter_kmem_cache_kern *kit = (void *)it;
> > + struct kmem_cache *s = kit->pos;
> > + bool destroy = false;
> > +
> > + if (s == NULL)
> > + return;
> > +
> > + mutex_lock(&slab_mutex);
> > +
> > + /* Skip kmem_cache_destroy() for active entries */
> > + if (s->refcount > 1)
> > + s->refcount--;
> > + else if (s->refcount == 1)
> > + destroy = true;
> > +
> > + mutex_unlock(&slab_mutex);
> > +
> > + if (destroy)
> > + kmem_cache_destroy(s);
> > +}
> > +
> > +__bpf_kfunc_end_defs();
> > +
> > static void bpf_iter_kmem_cache_show_fdinfo(const struct bpf_iter_aux_info *aux,
> > struct seq_file *seq)
> > {
>
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