[PATCH 2/4] bpf: Add dmabuf iterator

[T.J. Mercier]

On Mon, Apr 21, 2025 at 10:58 AM Song Liu <song at kernel.org> wrote:
>
> On Mon, Apr 14, 2025 at 3:53 PM T.J. Mercier <tjmercier at 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 at 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.
>
> [...]