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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Aug 27, 2025 at 01:50:18PM -0700, Shakeel Butt wrote: > On Wed, Aug 27, 2025 at 04:34:48PM +0100, Lorenzo Stoakes wrote: > > > +__bpf_kfunc_start_defs(); > > > + > > > +/** > > > + * bpf_mm_get_mem_cgroup - Get the memory cgroup associated with a mm_struct. > > > + * @mm: The mm_struct to query > > > + * > > > + * The obtained mem_cgroup must be released by calling bpf_put_mem_cgroup(). > > > + * > > > + * Return: The associated mem_cgroup on success, or NULL on failure. Note that > > > + * this function depends on CONFIG_MEMCG being enabled - it will always return > > > + * NULL if CONFIG_MEMCG is not configured. > > > > What kind of locking is assumed here? > > > > Are we protected against mmdrop() clearing out the mm? > > No locking is needed. Just the valid mm object or NULL. Usually the > underlying function (get_mem_cgroup_from_mm) is called in page fault > context where the current is holding mm. Here the only requirement is > that mm is valid either through explicit reference or the context. I mean this may be down to me being not so familiar with BPF, but my concern is that we're handing _any_ mm here. So presumably this could also be a remote mm? If not then why are we accepting an mm parameter at all, when we could just grab current->mm? If it's a remote mm, then we need to be absolutely sure that we won't UAF. I also feel we should talk about this in the kdoc, unless BPF always somehow asserts these things to be the case + verifies them smoehow. > > > > > > + */ > > > +__bpf_kfunc struct mem_cgroup *bpf_mm_get_mem_cgroup(struct mm_struct *mm) > > > +{ > > > + return get_mem_cgroup_from_mm(mm); > > > +} > > > + > > > +/** > > > + * bpf_put_mem_cgroup - Release a memory cgroup obtained from bpf_mm_get_mem_cgroup() > > > + * @memcg: The memory cgroup to release > > > + */ > > > +__bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg) > > > +{ > > > +#ifdef CONFIG_MEMCG > > > + if (!memcg) > > > + return; > > > + css_put(&memcg->css); > > > > Feels weird to have an ifdef here but not elsewhere, maybe the whole thing > > should be ifdef...? > > > > Is there not a put equivalent for get_mem_cgroup_from_mm()? That is a bit weird. > > > > Also do we now refrence the memcg global? That's pretty gross, could we not > > actually implement such a helper? > > > > Is it valid to do this also? Maybe cgroup people can chime in. > > There is mem_cgroup_put() which should handle !CONFIG_MEMCG configs. > OK Yafang - let's use this instead then?