Re: [PATCH v2 08/23] mm: introduce BPF kfuncs to deal with memcg pointers

[bot+bpf-ci]

```
commit ec8e2e1769704b4f69261f36e6aaf035d47c5f0f
Author: Roman Gushchin <roman.gushchin@xxxxxxxxx>

mm: introduce BPF kfuncs to deal with memcg pointers

This commit introduces two new BPF kfuncs for converting css pointers
to memcg pointers with proper reference counting: bpf_get_mem_cgroup()
and bpf_put_mem_cgroup().

> diff --git a/mm/Makefile b/mm/Makefile
> index 051e88c69..2d8f9beb3 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -107,6 +107,7 @@ obj-$(CONFIG_MEMCG) += swap_cgroup.o
>  endif
>  ifdef CONFIG_BPF_SYSCALL
>  obj-y += bpf_oom.o
> +obj-$(CONFIG_MEMCG) += bpf_memcontrol.o
>  endif
>  obj-$(CONFIG_CGROUP_HUGETLB) += hugetlb_cgroup.o
>  obj-$(CONFIG_GUP_TEST) += gup_test.o
> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> new file mode 100644
> index 000000000..1e4609774
> --- /dev/null
> +++ b/mm/bpf_memcontrol.c
> @@ -0,0 +1,88 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Memory Controller-related BPF kfuncs and auxiliary code
> + *
> + * Author: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> + */
> +
> +#include <linux/memcontrol.h>
> +#include <linux/bpf.h>
> +
> +__bpf_kfunc_start_defs();
> +
> +/**
> + * bpf_get_mem_cgroup - Get a reference to a memory cgroup
> + * @css: pointer to the css structure
> + *
> + * Returns a pointer to a mem_cgroup structure after bumping
> + * the corresponding css's reference counter.
> + *
> + * It's fine to pass a css which belongs to any cgroup controller,
> + * e.g. unified hierarchy's main css.
> + *
> + * Implements KF_ACQUIRE semantics.
> + */
> +__bpf_kfunc struct mem_cgroup *
> +bpf_get_mem_cgroup(struct cgroup_subsys_state *css)
> +{
> +	struct mem_cgroup *memcg = NULL;
> +	bool rcu_unlock = false;
> +
> +	if (!root_mem_cgroup)
> +		return NULL;
> +
> +	if (root_mem_cgroup->css.ss != css->ss) {
                                            ^^^^^^^

Can this dereference a NULL css pointer?  The function checks css for
NULL at line 42 with "if (css && css_tryget(css))", which suggests css
can be NULL.  If a BPF program calls this function with a NULL css,
wouldn't it crash here before reaching that check?

> +		struct cgroup *cgroup = css->cgroup;
> +		int ssid = root_mem_cgroup->css.ss->id;
> +
> +		rcu_read_lock();
> +		rcu_unlock = true;
> +		css = rcu_dereference_raw(cgroup->subsys[ssid]);
> +	}
> +
> +	if (css && css_tryget(css))
> +		memcg = container_of(css, struct mem_cgroup, css);
> +
> +	if (rcu_unlock)
> +		rcu_read_unlock();
> +
> +	return memcg;
> +}

[ ... ]

> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(bpf_memcontrol_kfuncs)
> +BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_ACQUIRE | KF_RET_NULL | KF_RCU)
> +BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)
> +
> +BTF_KFUNCS_END(bpf_memcontrol_kfuncs)

[ ... ]


```

---
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In-Reply-To-Subject: `mm: introduce BPF kfuncs to deal with memcg pointers`
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/18859027430