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Message-ID: <b3655d46-5c42-407e-adc1-b17865432e45@linux.dev>
Date: Fri, 18 Oct 2024 11:22:00 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Namhyung Kim <namhyung@...nel.org>
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
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?
> + 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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