Message-ID: <CAEf4Bzbt0kh53xYZL57Nc9AWcYUKga_NQ6uUrTeU4bj8qyTLng@mail.gmail.com>
Date: Fri, 1 Nov 2024 12:06:06 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Juntong Deng <juntong.deng@...look.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com, 
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, memxor@...il.com, snorcht@...il.com, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 1/4] bpf/crib: Introduce task_file open-coded
 iterator kfuncs

On Tue, Oct 29, 2024 at 5:15 PM Juntong Deng <juntong.deng@...look.com> wrote:
>
> This patch adds the open-coded iterator style process file iterator
> kfuncs bpf_iter_task_file_{new,next,destroy} that iterates over all
> files opened by the specified process.
>
> In addition, this patch adds bpf_iter_task_file_get_fd() getter to get
> the file descriptor corresponding to the file in the current iteration.
>
> The reference to struct file acquired by the previous
> bpf_iter_task_file_next() is released in the next
> bpf_iter_task_file_next(), and the last reference is released in the
> last bpf_iter_task_file_next() that returns NULL.
>
> In the bpf_iter_task_file_destroy(), if the iterator does not iterate to
> the end, then the last struct file reference is released at this time.
>
> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
> ---
>  kernel/bpf/Makefile      |   1 +
>  kernel/bpf/crib/Makefile |   3 ++
>  kernel/bpf/crib/crib.c   |  29 +++++++++++
>  kernel/bpf/crib/files.c  | 105 +++++++++++++++++++++++++++++++++++++++
>  4 files changed, 138 insertions(+)
>  create mode 100644 kernel/bpf/crib/Makefile
>  create mode 100644 kernel/bpf/crib/crib.c
>  create mode 100644 kernel/bpf/crib/files.c
>
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 105328f0b9c0..933d36264e5e 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -53,3 +53,4 @@ 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
> +obj-$(CONFIG_BPF_SYSCALL) += crib/
> diff --git a/kernel/bpf/crib/Makefile b/kernel/bpf/crib/Makefile
> new file mode 100644
> index 000000000000..4e1bae1972dd
> --- /dev/null
> +++ b/kernel/bpf/crib/Makefile
> @@ -0,0 +1,3 @@
> +# SPDX-License-Identifier: GPL-2.0
> +
> +obj-$(CONFIG_BPF_SYSCALL) += crib.o files.o
> diff --git a/kernel/bpf/crib/crib.c b/kernel/bpf/crib/crib.c
> new file mode 100644
> index 000000000000..e6536ee9a845
> --- /dev/null
> +++ b/kernel/bpf/crib/crib.c
> @@ -0,0 +1,29 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Checkpoint/Restore In eBPF (CRIB)
> + */
> +
> +#include <linux/bpf.h>
> +#include <linux/btf.h>
> +#include <linux/btf_ids.h>
> +
> +BTF_KFUNCS_START(bpf_crib_kfuncs)
> +
> +BTF_ID_FLAGS(func, bpf_iter_task_file_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_next, KF_ITER_NEXT | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_get_fd)
> +BTF_ID_FLAGS(func, bpf_iter_task_file_destroy, KF_ITER_DESTROY)

This is in no way CRIB-specific, right? So I'd drop the CRIB reference
and move code next to task_file BPF iterator program type
implementation, this is a generic functionality.

Even more so, given Namhyung's recent work on adding kmem_cache
iterator (both program type and open-coded iterator), it seems like it
should be possible to cut down on code duplication by using open-coded
iterator logic inside the BPF iterator program. Now that you are
adding task_file open-coded iterator, can you please check if it can
be reused. See kernel/bpf/task_iter.c (and I think that's where your
code should live as well, btw).

pw-bot: cr

> +
> +BTF_KFUNCS_END(bpf_crib_kfuncs)
> +
> +static const struct btf_kfunc_id_set bpf_crib_kfunc_set = {
> +       .owner = THIS_MODULE,
> +       .set   = &bpf_crib_kfuncs,
> +};
> +
> +static int __init bpf_crib_init(void)
> +{
> +       return register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &bpf_crib_kfunc_set);
> +}
> +
> +late_initcall(bpf_crib_init);
> diff --git a/kernel/bpf/crib/files.c b/kernel/bpf/crib/files.c
> new file mode 100644
> index 000000000000..ececf150303f
> --- /dev/null
> +++ b/kernel/bpf/crib/files.c
> @@ -0,0 +1,105 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/btf.h>
> +#include <linux/file.h>
> +#include <linux/fdtable.h>
> +#include <linux/net.h>
> +
> +struct bpf_iter_task_file {
> +       __u64 __opaque[3];
> +} __aligned(8);
> +
> +struct bpf_iter_task_file_kern {
> +       struct task_struct *task;
> +       struct file *file;
> +       int fd;
> +} __aligned(8);
> +
> +__bpf_kfunc_start_defs();
> +
> +/**
> + * bpf_iter_task_file_new() - Initialize a new task file iterator for a task,
> + * used to iterate over all files opened by a specified task
> + *
> + * @it: the new bpf_iter_task_file to be created
> + * @task: a pointer pointing to a task to be iterated over
> + */
> +__bpf_kfunc int bpf_iter_task_file_new(struct bpf_iter_task_file *it,
> +               struct task_struct *task)
> +{
> +       struct bpf_iter_task_file_kern *kit = (void *)it;
> +
> +       BUILD_BUG_ON(sizeof(struct bpf_iter_task_file_kern) > sizeof(struct bpf_iter_task_file));
> +       BUILD_BUG_ON(__alignof__(struct bpf_iter_task_file_kern) !=
> +                    __alignof__(struct bpf_iter_task_file));
> +
> +       kit->task = task;
> +       kit->fd = -1;
> +       kit->file = NULL;
> +
> +       return 0;
> +}
> +
> +/**
> + * bpf_iter_task_file_next() - Get the next file in bpf_iter_task_file
> + *
> + * bpf_iter_task_file_next acquires a reference to the returned struct file.
> + *
> + * The reference to struct file acquired by the previous
> + * bpf_iter_task_file_next() is released in the next bpf_iter_task_file_next(),
> + * and the last reference is released in the last bpf_iter_task_file_next()
> + * that returns NULL.
> + *
> + * @it: the bpf_iter_task_file to be checked
> + *
> + * @returns a pointer to the struct file of the next file if further files
> + * are available, otherwise returns NULL
> + */
> +__bpf_kfunc struct file *bpf_iter_task_file_next(struct bpf_iter_task_file *it)
> +{
> +       struct bpf_iter_task_file_kern *kit = (void *)it;
> +
> +       if (kit->file)
> +               fput(kit->file);
> +
> +       kit->fd++;
> +
> +       rcu_read_lock();
> +       kit->file = task_lookup_next_fdget_rcu(kit->task, &kit->fd);
> +       rcu_read_unlock();
> +
> +       return kit->file;
> +}
> +
> +/**
> + * bpf_iter_task_file_get_fd() - Get the file descriptor corresponding to
> + * the file in the current iteration
> + *
> + * @it: the bpf_iter_task_file to be checked
> + *
> + * @returns the file descriptor
> + */
> +__bpf_kfunc int bpf_iter_task_file_get_fd(struct bpf_iter_task_file *it__iter)
> +{
> +       struct bpf_iter_task_file_kern *kit = (void *)it__iter;
> +
> +       return kit->fd;
> +}
> +

I don't think we need this. It's probably better to return a pointer
to a small struct representing "item" returned from this iterator.
Something like

struct bpf_iter_task_file_item {
    struct task_struct *task;
    struct file *file;
    int fd;
};

You can then embed this struct into struct bpf_iter_task_file and
return a pointer to it on each next() call (avoiding memory
allocation)


(naming just for illustrative purposes, I spent 0 seconds thinking about it)

> +/**
> + * bpf_iter_task_file_destroy() - Destroy a bpf_iter_task_file
> + *
> + * If the iterator does not iterate to the end, then the last
> + * struct file reference is released at this time.
> + *
> + * @it: the bpf_iter_task_file to be destroyed
> + */
> +__bpf_kfunc void bpf_iter_task_file_destroy(struct bpf_iter_task_file *it)
> +{
> +       struct bpf_iter_task_file_kern *kit = (void *)it;
> +
> +       if (kit->file)
> +               fput(kit->file);
> +}
> +
> +__bpf_kfunc_end_defs();
> --
> 2.39.5
>

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