Re: [PATCH bpf-next 1/4] bpf: introduce task_vma bpf_iter

[Yonghong Song <yhs@fb.com>]

On 12/11/20 6:48 PM, Song Liu wrote:
> Introduce task_vma bpf_iter to print memory information of a process. It
> can be used to print customized information similar to /proc/<pid>/maps.
> 
> task_vma iterator releases mmap_lock before calling the BPF program.
> Therefore, we cannot pass vm_area_struct directly to the BPF program. A
> new __vm_area_struct is introduced to keep key information of a vma. On
> each iteration, task_vma gathers information in __vm_area_struct and
> passes it to the BPF program.
> 
> If the vma maps to a file, task_vma also holds a reference to the file
> while calling the BPF program.
> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>   include/linux/bpf.h      |   2 +-
>   include/uapi/linux/bpf.h |   7 ++
>   kernel/bpf/task_iter.c   | 193 ++++++++++++++++++++++++++++++++++++++-
>   3 files changed, 200 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 07cb5d15e7439..49dd1e29c8118 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1325,7 +1325,7 @@ enum bpf_iter_feature {
>   	BPF_ITER_RESCHED	= BIT(0),
>   };
>   
> -#define BPF_ITER_CTX_ARG_MAX 2
> +#define BPF_ITER_CTX_ARG_MAX 3
>   struct bpf_iter_reg {
>   	const char *target;
>   	bpf_iter_attach_target_t attach_target;
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 30b477a264827..c2db8a1d0cbd2 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5151,4 +5151,11 @@ enum {
>   	BTF_F_ZERO	=	(1ULL << 3),
>   };
>   
> +struct __vm_area_struct {
> +	__u64 start;
> +	__u64 end;
> +	__u64 flags;
> +	__u64 pgoff;
> +};

Probably we should not expose the above structure as uapi.
All other bpf_iter ctx argument layouts are btf based
and consider unstable. btf_iter itself is considered
as an unstable interface to dump kernel internal
data structures.

> +
>   #endif /* _UAPI__LINUX_BPF_H__ */
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 0458a40edf10a..30e5475d0831e 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -304,9 +304,171 @@ static const struct seq_operations task_file_seq_ops = {
>   	.show	= task_file_seq_show,
>   };
>   
> +struct bpf_iter_seq_task_vma_info {
> +	/* The first field must be struct bpf_iter_seq_task_common.
> +	 * this is assumed by {init, fini}_seq_pidns() callback functions.
> +	 */
> +	struct bpf_iter_seq_task_common common;
> +	struct task_struct *task;
> +	struct __vm_area_struct vma;
> +	struct file *file;
> +	u32 tid;
> +};
> +
[...]