Subject: Re: [PATCH v13 2/4] bpf: added new helper bpf_get_ns_current_pid_tgid

From: Carlos Antonio Neira Bustos <[email-masked]>
Date: Wed, 9 Oct 2019 17:30:15 -0300

On Wed, Oct 09, 2019 at 12:50:10PM -0700, Andrii Nakryiko wrote:
> On Wed, Oct 9, 2019 at 10:45 AM Carlos Antonio Neira Bustos
> <[email-masked]> wrote:
> >
> > On Wed, Oct 09, 2019 at 09:14:42AM -0700, Andrii Nakryiko wrote:
> > > On Wed, Oct 9, 2019 at 8:27 AM Carlos Neira <[email-masked]> 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 | 22 +++++++++++++++++++-
> > > >  kernel/bpf/core.c        |  1 +
> > > >  kernel/bpf/helpers.c     | 43 ++++++++++++++++++++++++++++++++++++++++
> > > >  kernel/trace/bpf_trace.c |  2 ++
> > > >  5 files changed, 68 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..6ad3f2abf00d 100644
> > > > --- a/include/uapi/linux/bpf.h
> > > > +++ b/include/uapi/linux/bpf.h
> > > > @@ -2750,6 +2750,19 @@ 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(struct *bpf_pidns_info, u32 size)
> > >
> > > Should be:
> > >
> > > struct bpf_pidns_info *nsdata
> > >
> > > > + *     Return
> > > > + *             0 on success, values for pid and tgid from nsinfo will be as seen
> > > > + *             from the namespace that matches dev and inum from nsinfo.
> > >
> > > I think its cleaner to have a Description section, explaining that it
> > > will return pid/tgid in bpf_pidns_info, and then describe exit codes
> > > in Return section.
> > >
> > > > + *
> > > > + *             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 +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_ns_current_pid_tgid),
> > > >
> > > >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > > >   * function eBPF program intends to call
> > > > @@ -3613,4 +3627,10 @@ struct bpf_sockopt {
> > > >         __s32   retval;
> > > >  };
> > > >
> > > > +struct bpf_pidns_info {
> > > > +       __u64 dev;
> > > > +       __u64 inum;
> > >
> > > seems like conventionally this should be named "ino", this is what
> > > ns_match calls it, so let's stay consistent.
> > >
> > > > +       __u32 pid;
> > > > +       __u32 tgid;
> > > > +};
> > >
> > > So it seems like dev and inum are treated as input parameters, while
> > > pid/tgid is output parameter, right? Wouldn't it be cleaner to have
> > > dev and inum as explicit arguments into bpf_get_ns_current_pid_tgid()?
> > > What's also not great, is that on failure you'll memset this entire
> > > struct to zero, and user will lose its dev/inum. So in practice you'll
> > > be keeping dev/inum somewhere else, then constructing and filling in
> > > this bpf_pidns_info struct every time you need to invoke
> > > bpf_get_ns_current_pid_tgid.
> > >
> > > Maybe it was discussed already, but IMO feels cleaner to have only
> > > pid/tgid in bpf_pidns_info and pass dev/inum as direct arguments.
> > >
> > > >  #endif /* _UAPI__LINUX_BPF_H__ */
> > > > 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..78a1ce7726aa 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 +489,44 @@ const struct bpf_func_proto bpf_strtoul_proto = {
> > > >         .arg4_type      = ARG_PTR_TO_LONG,
> > > >  };
> > > >  #endif
> > > > +
> > > > +BPF_CALL_2(bpf_get_ns_current_pid_tgid, struct bpf_pidns_info *, nsdata, u32,
> > > > +       size)
> > > > +{
> > > > +       struct task_struct *task = current;
> > > > +       struct pid_namespace *pidns;
> > > > +       int err = -EINVAL;
> > > > +
> > > > +       if (unlikely(size != sizeof(struct bpf_pidns_info)))
> > > > +               goto clear;
> > > > +
> > > > +       if ((u64)(dev_t)nsdata->dev != nsdata->dev)
> > >
> > > this seems unlikely() as well :)
> > >
> > > > +               goto clear;
> > > > +
> > > > +       if (unlikely(!task))
> > > > +               goto clear;
> > > > +
> > > > +       pidns = task_active_pid_ns(task);
> > > > +       if (unlikely(!pidns)) {
> > > > +               err = -ENOENT;
> > > > +               goto clear;
> > > > +       }
> > > > +
> > > > +       if (!ns_match(&pidns->ns, (dev_t)nsdata->dev, nsdata->inum))
> > > > +               goto clear;
> > > > +
> > > > +       nsdata->pid = task_pid_nr_ns(task, pidns);
> > > > +       nsdata->tgid = task_tgid_nr_ns(task, pidns);
> > > > +       return 0;
> > > > +clear:
> > > > +       memset((void *)nsdata, 0, (size_t) size);
> > > > +       return err;
> > > > +}
> > > > +
> > > > +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_PTR_TO_UNINIT_MEM,
> > >
> > > So this is a lie, you do expect part of that struct to be initialized.
> > > One more reason to just split off dev/inum(ino?).
> > >
> > >
> > > > +       .arg2_type      = ARG_CONST_SIZE,
> > > > +};
> > > > 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;
> > > >         }
> > > > --
> > > > 2.20.1
> > > >
> > Thanks for reviewing this, I'll make the changes you suggest.
> > I'm not sure about removing dev and ino from struct bpf_pidns_info, I
> > think is useful to know to which ns pid/tgid belong to using dev and ino
> > to figure it out. Maybe in the future we could filter bpf_pidns_info
> > structs by dev/ino, just an idea.
> 
> I'm not following. dev/ino are specified by the caller to this helper,
> right? So caller already know those values. With current set up,
> though, the behavior is weird: this struct's dev/ino is preserved on
> success, but zeroed out on failure, even though helper itself has
> nothing to do with returning dev/ino. It also plays badly with
> ARG_PTR_TO_UNINT_MEM, because that memory is expected to be at least
> partially initialized. I see only downsides, to be honest.
> 
> >
> > Bests
Andrii,

Oh you are right dev/ino are specific to the caller of this helper,
and what you state is correct dev/ino are also not returned by the caller
based on that, yes this should be changed. Now I see the point, I'll
change the helper to take dev/ino and just return pid/tgid.
Thanks again!.

Bests