linux-kernel - Re: [PATCH 2/4] bpf: Add dmabuf iterator

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CABdmKX0bgxZFYuvQvQPK0AnAHEE3FebY_eA1+Vo=ScH1MbfzMg@mail.gmail.com>
Date: Wed, 16 Apr 2025 15:51:38 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Song Liu <song@...nel.org>
Cc: sumit.semwal@...aro.org, christian.koenig@....com, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	skhan@...uxfoundation.org, linux-kernel@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, android-mm@...gle.com, 
	simona@...ll.ch, corbet@....net, eddyz87@...il.com, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	jolsa@...nel.org, mykolal@...com
Subject: Re: [PATCH 2/4] bpf: Add dmabuf iterator

On Wed, Apr 16, 2025 at 3:02 PM Song Liu <song@...nel.org> wrote:
>
> On Mon, Apr 14, 2025 at 3:53 PM T.J. Mercier <tjmercier@...gle.com> wrote:
> [...]
> > +
> > +BTF_ID_LIST_GLOBAL_SINGLE(bpf_dmabuf_btf_id, struct, dma_buf)
> > +DEFINE_BPF_ITER_FUNC(dmabuf, struct bpf_iter_meta *meta, struct dma_buf *dmabuf)
> > +
> > +static void *dmabuf_iter_seq_start(struct seq_file *seq, loff_t *pos)
> > +{
> > +       struct dma_buf *dmabuf, *ret = NULL;
> > +
> > +       if (*pos) {
> > +               *pos = 0;
> > +               return NULL;
> > +       }
> > +       /* Look for the first buffer we can obtain a reference to.
> > +        * The list mutex does not protect a dmabuf's refcount, so it can be
> > +        * zeroed while we are iterating. Therefore we cannot call get_dma_buf()
> > +        * since the caller of this program may not already own a reference to
> > +        * the buffer.
> > +        */
> > +       mutex_lock(&dmabuf_debugfs_list_mutex);
> > +       list_for_each_entry(dmabuf, &dmabuf_debugfs_list, list_node) {
> > +               if (file_ref_get(&dmabuf->file->f_ref)) {
> > +                       ret = dmabuf;
> > +                       break;
> > +               }
> > +       }
> > +       mutex_unlock(&dmabuf_debugfs_list_mutex);
>
> IIUC, the iterator simply traverses elements in a linked list. I feel it is
> an overkill to implement a new BPF iterator for it.

Like other BPF iterators such as kmem_cache_iter or task_iter.
Cgroup_iter iterates trees instead of lists. This is iterating over
kernel objects just like the docs say, "A BPF iterator is a type of
BPF program that allows users to iterate over specific types of kernel
objects". More complicated iteration should not be a requirement here.

> Maybe we simply
> use debugging tools like crash or drgn for this? The access with
> these tools will not be protected by the mutex. But from my personal
> experience, this is not a big issue for user space debugging tools.

drgn is *way* too slow, and even if it weren't the dependencies for
running it aren't available. crash needs debug symbols which also
aren't available on user builds. This is not just for manual
debugging, it's for reporting memory use in production. Or anything
else someone might care to extract like attachment info or refcounts.

> Thanks,
> Song
>
>
> > +
> > +       return ret;
> > +}