[v2,2/6] bpf: Add dmabuf iterator

The dmabuf iterator traverses the list of all DMA buffers.

DMA buffers are refcounted through their associated struct file. A
reference is taken on each buffer as the list is iterated to ensure each
buffer persists for the duration of the bpf program execution without
holding the list mutex.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
---
 kernel/bpf/Makefile      |   3 +
 kernel/bpf/dmabuf_iter.c | 134 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 137 insertions(+)
 create mode 100644 kernel/bpf/dmabuf_iter.c

On Mon, May 5, 2025 at 4:17 AM Christian König <christian.koenig@amd.com> wrote:
>
> On 5/5/25 00:41, T.J. Mercier wrote:
> > The dmabuf iterator traverses the list of all DMA buffers.
> >
> > DMA buffers are refcounted through their associated struct file. A
> > reference is taken on each buffer as the list is iterated to ensure each
> > buffer persists for the duration of the bpf program execution without
> > holding the list mutex.
> >
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
> > ---
> >  kernel/bpf/Makefile      |   3 +
> >  kernel/bpf/dmabuf_iter.c | 134 +++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 137 insertions(+)
> >  create mode 100644 kernel/bpf/dmabuf_iter.c
> >
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index 70502f038b92..3a335c50e6e3 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -53,6 +53,9 @@ obj-$(CONFIG_BPF_SYSCALL) += relo_core.o
> >  obj-$(CONFIG_BPF_SYSCALL) += btf_iter.o
> >  obj-$(CONFIG_BPF_SYSCALL) += btf_relocate.o
> >  obj-$(CONFIG_BPF_SYSCALL) += kmem_cache_iter.o
> > +ifeq ($(CONFIG_DMA_SHARED_BUFFER),y)
> > +obj-$(CONFIG_BPF_SYSCALL) += dmabuf_iter.o
> > +endif
> >
> >  CFLAGS_REMOVE_percpu_freelist.o = $(CC_FLAGS_FTRACE)
> >  CFLAGS_REMOVE_bpf_lru_list.o = $(CC_FLAGS_FTRACE)
> > diff --git a/kernel/bpf/dmabuf_iter.c b/kernel/bpf/dmabuf_iter.c
> > new file mode 100644
> > index 000000000000..968762e11f73
> > --- /dev/null
> > +++ b/kernel/bpf/dmabuf_iter.c
> > @@ -0,0 +1,134 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/* Copyright (c) 2025 Google LLC */
> > +#include <linux/bpf.h>
> > +#include <linux/btf_ids.h>
> > +#include <linux/dma-buf.h>
> > +#include <linux/kernel.h>
> > +#include <linux/seq_file.h>
> > +
> > +BTF_ID_LIST_SINGLE(bpf_dmabuf_btf_id, struct, dma_buf)
> > +DEFINE_BPF_ITER_FUNC(dmabuf, struct bpf_iter_meta *meta, struct dma_buf *dmabuf)
> > +
> > +static struct dma_buf *get_next_dmabuf(struct dma_buf *dmabuf)
> > +{
> > +     struct dma_buf *ret = NULL;
> > +
> > +     /*
> > +      * Look for the first/next 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. We cannot call get_dma_buf() since the
> > +      * caller of this program may not already own a reference to the buffer.
> > +      */
> > +     mutex_lock(&dmabuf_list_mutex);
> > +     if (dmabuf) {
>
> That looks like you try to mangle the start and next functionality in just one function.
>
> I would just inline that into the dmabuf_iter_seq_start() and dmabuf_iter_seq_next() functions.

Primarily this is to share between the open coded iterator (next
patch) and this normal iterator since I didn't want to duplicate the
same list traversal code across both of them.
>
>
> > +             dma_buf_put(dmabuf);
> > +             list_for_each_entry_continue(dmabuf, &dmabuf_list, list_node) {
>
> That you can put the DMA-buf and then still uses it in list_for_each_entry_continue() only works because the mutex is locked in the destroy path.

Yup, this was deliberate.
>
>
> I strongly suggest to just put those two functions into drivers/dma-buf/dma-buf.c right next to the __dma_buf_debugfs_list_add() and __dma_buf_debugfs_list_del() functions.

By two functions, you mean a get_first_dmabuf(void) and a
get_next_dmabuf(struct dma_buf*)? To make the dma_buf_put() call a
little less scary since all the mutex ops are right there?
>
>
> Apart from those style suggestions looks good to me from the technical side, but I'm not an expert for the BPF stuff.
>
> Regards,
> Christian.

Thanks for your comments and reviews!

> > +                     if (file_ref_get(&dmabuf->file->f_ref)) {
> > +                             ret = dmabuf;
> > +                             break;
> > +                     }
> > +             }
> > +     } else {
> > +             list_for_each_entry(dmabuf, &dmabuf_list, list_node) {
> > +                     if (file_ref_get(&dmabuf->file->f_ref)) {
> > +                             ret = dmabuf;
> > +                             break;
> > +                     }
> > +             }
> > +     }
> > +     mutex_unlock(&dmabuf_list_mutex);
> > +     return ret;
> > +}
> > +
> > +static void *dmabuf_iter_seq_start(struct seq_file *seq, loff_t *pos)
> > +{
> > +     if (*pos)
> > +             return NULL;
> > +
> > +     return get_next_dmabuf(NULL);
> > +}
> > +
> > +static void *dmabuf_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> > +{
> > +     struct dma_buf *dmabuf = v;
> > +
> > +     ++*pos;
> > +
> > +     return get_next_dmabuf(dmabuf);
> > +}
> > +
> > +struct bpf_iter__dmabuf {
> > +     __bpf_md_ptr(struct bpf_iter_meta *, meta);
> > +     __bpf_md_ptr(struct dma_buf *, dmabuf);
> > +};
> > +
> > +static int __dmabuf_seq_show(struct seq_file *seq, void *v, bool in_stop)
> > +{
> > +     struct bpf_iter_meta meta = {
> > +             .seq = seq,
> > +     };
> > +     struct bpf_iter__dmabuf ctx = {
> > +             .meta = &meta,
> > +             .dmabuf = v,
> > +     };
> > +     struct bpf_prog *prog = bpf_iter_get_info(&meta, in_stop);
> > +
> > +     if (prog)
> > +             return bpf_iter_run_prog(prog, &ctx);
> > +
> > +     return 0;
> > +}
> > +
> > +static int dmabuf_iter_seq_show(struct seq_file *seq, void *v)
> > +{
> > +     return __dmabuf_seq_show(seq, v, false);
> > +}
> > +
> > +static void dmabuf_iter_seq_stop(struct seq_file *seq, void *v)
> > +{
> > +     struct dma_buf *dmabuf = v;
> > +
> > +     if (dmabuf)
> > +             dma_buf_put(dmabuf);
> > +}
> > +
> > +static const struct seq_operations dmabuf_iter_seq_ops = {
> > +     .start  = dmabuf_iter_seq_start,
> > +     .next   = dmabuf_iter_seq_next,
> > +     .stop   = dmabuf_iter_seq_stop,
> > +     .show   = dmabuf_iter_seq_show,
> > +};
> > +
> > +static void bpf_iter_dmabuf_show_fdinfo(const struct bpf_iter_aux_info *aux,
> > +                                     struct seq_file *seq)
> > +{
> > +     seq_puts(seq, "dmabuf iter\n");
> > +}
> > +
> > +static const struct bpf_iter_seq_info dmabuf_iter_seq_info = {
> > +     .seq_ops                = &dmabuf_iter_seq_ops,
> > +     .init_seq_private       = NULL,
> > +     .fini_seq_private       = NULL,
> > +     .seq_priv_size          = 0,
> > +};
> > +
> > +static struct bpf_iter_reg bpf_dmabuf_reg_info = {
> > +     .target                 = "dmabuf",
> > +     .feature                = BPF_ITER_RESCHED,
> > +     .show_fdinfo            = bpf_iter_dmabuf_show_fdinfo,
> > +     .ctx_arg_info_size      = 1,
> > +     .ctx_arg_info           = {
> > +             { offsetof(struct bpf_iter__dmabuf, dmabuf),
> > +               PTR_TO_BTF_ID_OR_NULL },
> > +     },
> > +     .seq_info               = &dmabuf_iter_seq_info,
> > +};
> > +
> > +static int __init dmabuf_iter_init(void)
> > +{
> > +     bpf_dmabuf_reg_info.ctx_arg_info[0].btf_id = bpf_dmabuf_btf_id[0];
> > +     return bpf_iter_reg_target(&bpf_dmabuf_reg_info);
> > +}
> > +
> > +late_initcall(dmabuf_iter_init);
>