[Linaro-mm-sig] Re: [PATCH v2 2/6] bpf: Add dmabuf iterator

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);