On Mon, May 5, 2025 at 4:17 AM Christian König <christian.koe...@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 <tjmerc...@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);
>
