On Mon, Apr 21, 2025 at 10:58 AM Song Liu song@kernel.org wrote:

On Mon, Apr 14, 2025 at 3:53 PM T.J. Mercier tjmercier@google.com wrote:

...

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[];

This is not necessary. See below.

...

#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

Maybe we should add this file to drivers/dma-buf. I would like to hear other folks thoughts on this.

This is fine with me, and would save us the extra CONFIG_DMA_SHARED_BUFFER check that's currently needed in kernel/bpf/Makefile but would require checking CONFIG_BPF instead. Sumit / Christian any objections to moving the dmabuf bpf iterator implementation into drivers/dma-buf?

...

@@ -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)

Use BTF_ID_LIST_SINGLE(), then we don't need this in btf_ids.h

...

+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.
  

•    */
  

We should use kernel comment style for new code. IOW, /*

• Look for ...

*/

Thanks, Song

Thanks, I have incorporated all of your comments and retested. I will give some time for more feedback before sending a v2.

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