Subject: Re: [PATCH V12 2/4] bpf: added new helper bpf_get_ns_current_pid_tgid

From: Carlos Antonio Neira Bustos <[email-masked]>
Date: Thu, 3 Oct 2019 11:52:11 -0300

On Wed, Oct 02, 2019 at 12:52:29PM +0200, Daniel Borkmann wrote:
> On 10/1/19 11:41 PM, Carlos Neira wrote:
> > New bpf helper bpf_get_ns_current_pid_tgid,
> > This helper will return pid and tgid from current task
> > which namespace matches dev_t and inode number provided,
> > this will allows us to instrument a process inside a container.
> > 
> > Signed-off-by: Carlos Neira <[email-masked]>
> > ---
> >   include/linux/bpf.h      |  1 +
> >   include/uapi/linux/bpf.h | 18 +++++++++++++++++-
> >   kernel/bpf/core.c        |  1 +
> >   kernel/bpf/helpers.c     | 36 ++++++++++++++++++++++++++++++++++++
> >   kernel/trace/bpf_trace.c |  2 ++
> >   5 files changed, 57 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 5b9d22338606..231001475504 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1055,6 +1055,7 @@ 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_ns_current_pid_tgid_proto;
> >   /* Shared helpers among cBPF and eBPF. */
> >   void bpf_user_rnd_init_once(void);
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 77c6be96d676..ea8145d7f897 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2750,6 +2750,21 @@ union bpf_attr {
> >    *		**-EOPNOTSUPP** kernel configuration does not enable SYN cookies
> >    *
> >    *		**-EPROTONOSUPPORT** IP packet version is not 4 or 6
> > + *
> > + * u64 bpf_get_ns_current_pid_tgid(u64 dev, u64 inum)
> > + *	Return
> > + *		A 64-bit integer containing the current tgid and pid from current task
> > + *              which namespace inode and dev_t matches , and is create as such:
> > + *		*current_task*\ **->tgid << 32 \|**
> > + *		*current_task*\ **->pid**.
> > + *
> > + *		On failure, the returned value is one of the following:
> > + *
> > + *		**-EINVAL** if dev and inum supplied don't match dev_t and inode number
> > + *              with nsfs of current task, or if dev conversion to dev_t lost high bits.
> > + *
> > + *		**-ENOENT** if /proc/self/ns does not exists.
> > + *
> >    */
> >   #define __BPF_FUNC_MAPPER(FN)		\
> >   	FN(unspec),			\
> > @@ -2862,7 +2877,8 @@ union bpf_attr {
> >   	FN(sk_storage_get),		\
> >   	FN(sk_storage_delete),		\
> >   	FN(send_signal),		\
> > -	FN(tcp_gen_syncookie),
> > +	FN(tcp_gen_syncookie),          \
> > +	FN(get_ns_current_pid_tgid),
> >   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> >    * function eBPF program intends to call
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 66088a9e9b9e..b2fd5358f472 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -2042,6 +2042,7 @@ 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_ns_current_pid_tgid_proto __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..8777181d1717 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -11,6 +11,8 @@
> >   #include <linux/uidgid.h>
> >   #include <linux/filter.h>
> >   #include <linux/ctype.h>
> > +#include <linux/pid_namespace.h>
> > +#include <linux/proc_ns.h>
> >   #include "../../lib/kstrtox.h"
> > @@ -487,3 +489,37 @@ const struct bpf_func_proto bpf_strtoul_proto = {
> >   	.arg4_type	= ARG_PTR_TO_LONG,
> >   };
> >   #endif
> > +
> > +BPF_CALL_2(bpf_get_ns_current_pid_tgid, u64, dev, u64, inum)
> > +{
> > +	struct task_struct *task = current;
> > +	struct pid_namespace *pidns;
> > +	pid_t pid, tgid;
> > +
> > +	if ((u64)(dev_t)dev != dev)
> > +		return -EINVAL;
> > +
> > +	if (unlikely(!task))
> > +		return -EINVAL;
> > +
> > +	pidns = task_active_pid_ns(task);
> > +	if (unlikely(!pidns))
> > +		return -ENOENT;
> > +
> > +
> > +	if (!ns_match(&pidns->ns, (dev_t)dev, inum))
> > +		return -EINVAL;
> > +
> > +	pid = task_pid_nr_ns(task, pidns);
> > +	tgid = task_tgid_nr_ns(task, pidns);
> > +
> > +	return (u64) tgid << 32 | pid;
> 
> Basically here you are overlapping the 64-bit return value for the valid
> outcome with the error codes above for the invalid case. If you look at
> bpf_perf_event_read() we already had such broken occasion that bit us in
> the past, and needed to introduce bpf_perf_event_read_value() instead.
> Lets not go there again and design it similarly to the latter.
> 
> > +}
> > +
> > +const struct bpf_func_proto bpf_get_ns_current_pid_tgid_proto = {
> > +	.func		= bpf_get_ns_current_pid_tgid,
> > +	.gpl_only	= false,
> > +	.ret_type	= RET_INTEGER,
> > +	.arg1_type	= ARG_ANYTHING,
> > +	.arg2_type	= ARG_ANYTHING,
> > +};
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 44bd08f2443b..32331a1dcb6d 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -735,6 +735,8 @@ 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_ns_current_pid_tgid:
> > +		return &bpf_get_ns_current_pid_tgid_proto;
> >   	default:
> >   		return NULL;
> >   	}
> > 
> 
Daniel,
If I understand correctly, to avoid problems I need to change the helper's function signature to something like the following:

struct bpf_ns_current_pid_tgid_storage {
	__u64 dev;
	__u64 inum;
	__u64 pidtgid;
};

BPF_CALL_2(bpf_get_ns_current_pid_tgid,
	   struct bpf_ns_current_pid_tgid_storage *, buf, u32, size);  

then use dev and inum provided by the user and return the requested
value into pidtgid of the struct. Would that work?

Thanks for your help.