[PATCH 2/4] bpf: Add dmabuf iterator

["T.J. Mercier" <tjmercier@google.com>]

The dmabuf iterator traverses the list of all DMA buffers. The list is
maintained only when CONFIG_DEBUG_FS is enabled.

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>
---
 include/linux/btf_ids.h  |   1 +
 kernel/bpf/Makefile      |   3 +
 kernel/bpf/dmabuf_iter.c | 130 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 134 insertions(+)
 create mode 100644 kernel/bpf/dmabuf_iter.c

diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
index 139bdececdcf..899ead57d89d 100644
--- a/include/linux/btf_ids.h
+++ b/include/linux/btf_ids.h
@@ -284,5 +284,6 @@ extern u32 bpf_cgroup_btf_id[];
 extern u32 bpf_local_storage_map_btf_id[];
 extern u32 btf_bpf_map_id[];
 extern u32 bpf_kmem_cache_btf_id[];
+extern u32 bpf_dmabuf_btf_id[];
 
 #endif
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index 70502f038b92..5b30d37ef055 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 ($(CONFIG_DEBUG_FS),y)
+obj-$(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..b4b8be1d6aa4
--- /dev/null
+++ b/kernel/bpf/dmabuf_iter.c
@@ -0,0 +1,130 @@
+// 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_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);
+
+	return ret;
+}
+
+static void *dmabuf_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct dma_buf *dmabuf = v, *ret = NULL;
+
+	++*pos;
+
+	mutex_lock(&dmabuf_debugfs_list_mutex);
+	dma_buf_put(dmabuf);
+	while (!list_is_last(&dmabuf->list_node, &dmabuf_debugfs_list)) {
+		dmabuf = list_next_entry(dmabuf, list_node);
+		if (file_ref_get(&dmabuf->file->f_ref)) {
+			ret = dmabuf;
+			break;
+		}
+	}
+	mutex_unlock(&dmabuf_debugfs_list_mutex);
+
+	return ret;
+}
+
+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",
+	.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);
-- 
2.49.0.604.gff1f9ca942-goog