Re: [PATCH bpf-next V9 1/3] bpf: new helper to obtain namespace data from current task

[Yonghong Song <yhs@fb.com>]

On 8/13/19 11:47 AM, Carlos Neira wrote:
> From: Carlos <cneirabustos@gmail.com>
> 
> New bpf helper bpf_get_current_pidns_info.
> This helper obtains the active namespace from current and returns
> pid, tgid, device and namespace id as seen from that namespace,
> allowing to instrument a process inside a container.
> 
> Signed-off-by: Carlos Neira <cneirabustos@gmail.com>
> ---
>   fs/internal.h            |  2 --
>   fs/namei.c               |  1 -
>   include/linux/bpf.h      |  1 +
>   include/linux/namei.h    |  4 +++
>   include/uapi/linux/bpf.h | 31 ++++++++++++++++++++++-
>   kernel/bpf/core.c        |  1 +
>   kernel/bpf/helpers.c     | 64 ++++++++++++++++++++++++++++++++++++++++++++++++
>   kernel/trace/bpf_trace.c |  2 ++
>   8 files changed, 102 insertions(+), 4 deletions(-)

I prefer to break this into two patches to reduce
the potential merging conflicts:
   patch 1: fs/internal.h, fs/namei.c, include/linux/namei.h
   patch 2: rest of changes
patch 1 is simply a preparing patches to make filename_lookup
available later.

> 
> diff --git a/fs/internal.h b/fs/internal.h
> index 315fcd8d237c..6647e15dd419 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -59,8 +59,6 @@ extern int finish_clean_context(struct fs_context *fc);
>   /*
>    * namei.c
>    */
> -extern int filename_lookup(int dfd, struct filename *name, unsigned flags,
> -			   struct path *path, struct path *root);
>   extern int user_path_mountpoint_at(int, const char __user *, unsigned int, struct path *);
>   extern int vfs_path_lookup(struct dentry *, struct vfsmount *,
>   			   const char *, unsigned int, struct path *);
> diff --git a/fs/namei.c b/fs/namei.c
> index 209c51a5226c..a89fc72a4a10 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -19,7 +19,6 @@
>   #include <linux/export.h>
>   #include <linux/kernel.h>
>   #include <linux/slab.h>
> -#include <linux/fs.h>
>   #include <linux/namei.h>
>   #include <linux/pagemap.h>
>   #include <linux/fsnotify.h>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f9a506147c8a..e4adf5e05afd 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1050,6 +1050,7 @@ extern const struct bpf_func_proto bpf_get_local_storage_proto;
>   extern const struct bpf_func_proto bpf_strtol_proto;
>   extern const struct bpf_func_proto bpf_strtoul_proto;
>   extern const struct bpf_func_proto bpf_tcp_sock_proto;
> +extern const struct bpf_func_proto bpf_get_current_pidns_info_proto;
>   
>   /* Shared helpers among cBPF and eBPF. */
>   void bpf_user_rnd_init_once(void);
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 9138b4471dbf..b45c8b6f7cb4 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -6,6 +6,7 @@
>   #include <linux/path.h>
>   #include <linux/fcntl.h>
>   #include <linux/errno.h>
> +#include <linux/fs.h>
>   
>   enum { MAX_NESTED_LINKS = 8 };
>   
> @@ -97,6 +98,9 @@ extern void unlock_rename(struct dentry *, struct dentry *);
>   
>   extern void nd_jump_link(struct path *path);
>   
> +extern int filename_lookup(int dfd, struct filename *name, unsigned flags,
> +			   struct path *path, struct path *root);
> +
>   static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
>   {
>   	((char *) name)[min(len, maxlen)] = '\0';
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 4393bd4b2419..db241857ec15 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2741,6 +2741,28 @@ union bpf_attr {
>    *		**-EOPNOTSUPP** kernel configuration does not enable SYN cookies
>    *
>    *		**-EPROTONOSUPPORT** IP packet version is not 4 or 6
> + *
> + * int bpf_get_current_pidns_info(struct bpf_pidns_info *pidns, u32 size_of_pidns)

size_of_pidns => size.

> + *	Description
> + *		Copies into *pidns* pid, namespace id and tgid as seen by the
Copies => Copy.
Maybe something like below:
Get tgid, pid and namespace id as seen by the current namespace, and 
device major/minor numbers from device /proc/self/ns/pid. Such
information is stored in *pidns* of size *size*.

> + *		current namespace and also device from /proc/self/ns/pid.
> + *		*size_of_pidns* must be the size of *pidns*
> + *
> + *		This helper is used when pid filtering is needed inside a
> + *		container as bpf_get_current_tgid() helper returns always the

returns always => always returns.

> + *		pid id as seen by the root namespace.
> + *	Return
> + *		0 on success
> + *
> + *		**-EINVAL** if *size_of_pidns* is not valid or unable to get ns, pid
> + *		or tgid of the current task.
> + *
> + *		**-ECHILD** if /proc/self/ns/pid does not exists.
> + *
> + *		**-ENOTDIR** if /proc/self/ns does not exists.

Let us remove ECHILD and ENOTDIR and replace it with ENOENT as I
described below.

Please *do verify* what happens when namespaces or pid_ns are not
configured.

> + *
> + *		**-ENOMEM**  if allocation fails.

helper internal allocation fails.

> + *
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -2853,7 +2875,8 @@ union bpf_attr {
>   	FN(sk_storage_get),		\
>   	FN(sk_storage_delete),		\
>   	FN(send_signal),		\
> -	FN(tcp_gen_syncookie),
> +	FN(tcp_gen_syncookie),		\
> +	FN(get_current_pidns_info),
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>    * function eBPF program intends to call
> @@ -3604,4 +3627,10 @@ struct bpf_sockopt {
>   	__s32	retval;
>   };
>   
> +struct bpf_pidns_info {
> +	__u32 dev;

Please add a comment for dev for how device major and minor number are 
derived. User space gets device major and minor number, they need to
compare to the corresponding major/minor numbers returned by this helper.

> +	__u32 nsid;
> +	__u32 tgid;
> +	__u32 pid;
> +};
>   #endif /* _UAPI__LINUX_BPF_H__ */
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 8191a7db2777..3159f2a0188c 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2038,6 +2038,7 @@ const struct bpf_func_proto bpf_get_current_uid_gid_proto __weak;
>   const struct bpf_func_proto bpf_get_current_comm_proto __weak;
>   const struct bpf_func_proto bpf_get_current_cgroup_id_proto __weak;
>   const struct bpf_func_proto bpf_get_local_storage_proto __weak;
> +const struct bpf_func_proto bpf_get_current_pidns_info __weak;
>   
>   const struct bpf_func_proto * __weak bpf_get_trace_printk_proto(void)
>   {
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 5e28718928ca..41fbf1f28a48 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -11,6 +11,12 @@
>   #include <linux/uidgid.h>
>   #include <linux/filter.h>
>   #include <linux/ctype.h>
> +#include <linux/pid_namespace.h>
> +#include <linux/major.h>
> +#include <linux/stat.h>
> +#include <linux/namei.h>
> +#include <linux/version.h>
> +
>   
>   #include "../../lib/kstrtox.h"
>   
> @@ -312,6 +318,64 @@ void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
>   	preempt_enable();
>   }
>   
> +BPF_CALL_2(bpf_get_current_pidns_info, struct bpf_pidns_info *, pidns_info, u32,
> +	 size)
> +{
> +	const char *pidns_path = "/proc/self/ns/pid";
> +	struct pid_namespace *pidns = NULL;
> +	struct filename *tmp = NULL;

tmp => fname

> +	struct inode *inode;
> +	struct path kp;
> +	pid_t tgid = 0;
> +	pid_t pid = 0;
> +	int ret;
> +	int len;
> +
> +	if (unlikely(size != sizeof(struct bpf_pidns_info)))
> +		return -EINVAL;

Please put an empty line. As a general rule for readability,
put an empty line if control flow is interrupted, e.g., by
"return", "break" or "continue". At least this is what
I saw most in bpf mailing list.

> +	pidns = task_active_pid_ns(current);
> +	if (unlikely(!pidns))
> +		goto clear;

An empty line. Also, there is nothing to clear.
I prefer an error code -ENOENT.

You can set
	ret = -EINVAL;
here

> +	pidns_info->nsid =  pidns->ns.inum;
> +	pid = task_pid_nr_ns(current, pidns);
> +	if (unlikely(!pid))
> +		goto clear;

An empty line.

> +	tgid = task_tgid_nr_ns(current, pidns);
> +	if (unlikely(!tgid))
> +		goto clear;

An empty line.

> +	pidns_info->tgid = (u32) tgid;
> +	pidns_info->pid = (u32) pid;

Different functionality, an empty line.

> +	tmp = kmem_cache_alloc(names_cachep, GFP_ATOMIC);
> +	if (unlikely(!tmp)) {
> +		memset((void *)pidns_info, 0, (size_t) size);
> +		return -ENOMEM;

ret = -ENOMEM;
goto clear;

> +	}

An empty line.

> +	len = strlen(pidns_path) + 1;
> +	memcpy((char *)tmp->name, pidns_path, len);
> +	tmp->uptr = NULL;
> +	tmp->aname = NULL;
> +	tmp->refcnt = 1;
> +	ret = filename_lookup(AT_FDCWD, tmp, 0, &kp, NULL);
Adding below to free kmem cache memory
	kmem_cache_free(names_cachep, fname);

In the above, we checked task_active_pid_ns().
If not returning NULL, we have a valid pid ns. So the above
filename_lookup should not go wrong. We can still keep
the error checking though.

> +	if (ret) {
> +		memset((void *)pidns_info, 0, (size_t) size);
> +		return ret;

goto clear;

> +	}

An empty line.

> +	inode = d_backing_inode(kp.dentry);
> +	pidns_info->dev = inode->i_sb->s_dev;
> +	return 0;

An empty line.

> +clear > +	memset((void *)pidns_info, 0, (size_t) size);
> +	return -EINVAL;
> +}
> +
> +const struct bpf_func_proto bpf_get_current_pidns_info_proto = {
> +	.func		= bpf_get_current_pidns_info,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_UNINIT_MEM,
> +	.arg2_type	= ARG_CONST_SIZE,
> +};
> +
>   #ifdef CONFIG_CGROUPS
>   BPF_CALL_0(bpf_get_current_cgroup_id)
>   {
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index ca1255d14576..5e1dc22765a5 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -709,6 +709,8 @@ tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   #endif
>   	case BPF_FUNC_send_signal:
>   		return &bpf_send_signal_proto;
> +	case BPF_FUNC_get_current_pidns_info:
> +		return &bpf_get_current_pidns_info_proto;
>   	default:
>   		return NULL;
>   	}
>