Re: [PATCH v2 2/6] bpf: Add dmabuf iterator

["Christian König" <christian.koenig@amd.com>]

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.

> +		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.

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.

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.

> +			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);