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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Fri, 27 Mar 2020 08:39:23 +0800
Message-ID: <CALOAHbB3mByKY4jfkHLBMu+GOt5XiXURXYwVhExXc4DdZThhVQ@mail.gmail.com>
Subject: Re: [PATCH] kernel/taskstats: fix wrong nla type for
 {cgroup,task}stats policy
To:     David Ahern <dsahern@gmail.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        bsingharora@gmail.com, LKML <linux-kernel@vger.kernel.org>,
        stable@vger.kernel.org, "David S.Miller" <davem@davemloft.net>
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On Fri, Mar 27, 2020 at 5:11 AM David Ahern <dsahern@gmail.com> wrote:
>
> On 3/26/20 2:28 PM, Johannes Berg wrote:
> >
> > And looking at this ... well, that code is completely wrong?
> >
> > E.g.
> >
> >                 rc = send_cmd(nl_sd, id, mypid, TASKSTATS_CMD_GET,
> >                               cmd_type, &tid, sizeof(__u32));
> >
> > (cmd_type is one of TASKSTATS_CMD_ATTR_TGID, TASKSTATS_CMD_ATTR_PID)
> >
> > or it might do
> >
> >                 rc = send_cmd(nl_sd, id, mypid, CGROUPSTATS_CMD_GET,
> >                               CGROUPSTATS_CMD_ATTR_FD, &cfd, sizeof(__u32));
> >
> > so clearly it wants to produce a u32 attribute.
> >
> > But then
> >
> > static int send_cmd(int sd, __u16 nlmsg_type, __u32 nlmsg_pid,
> >              __u8 genl_cmd, __u16 nla_type,
> >              void *nla_data, int nla_len)
> > {
> > ...
> >
> >         na = (struct nlattr *) GENLMSG_DATA(&msg);
> >
> > // this is still fine
> >
> >         na->nla_type = nla_type;
> >
> > // this is also fine
> >
> >         na->nla_len = nla_len + 1 + NLA_HDRLEN;
> >
> > // but this??? the nla_len of a netlink attribute should just be
> > // the len ... what's NLA_HDRLEN doing here? this isn't nested
> > // here we end up just reserving 1+NLA_HDRLEN too much space
> >
> >         memcpy(NLA_DATA(na), nla_data, nla_len);
> >
> > // but then it anyway only fills the first nla_len bytes, which
> > // is just like a regular attribute.
> >
> >         msg.n.nlmsg_len += NLMSG_ALIGN(na->nla_len);
> > // note that this is also wrong - it should be
> > // += NLA_ALIGN(NLA_HDRLEN + nla_len)
> >
> >
> >
> > So really I think what happened here is precisely what we wanted -
> > David's kernel patch caught the broken userspace tool.
>
> agreed. The tool needs to be fixed, not the kernel policy.
>
> I do not get the error message with this change as Johannes points out
> above:
>
> diff --git a/tools/accounting/getdelays.c b/tools/accounting/getdelays.c
> index 8cb504d30384..e90fd133df0e 100644
> --- a/tools/accounting/getdelays.c
> +++ b/tools/accounting/getdelays.c
> @@ -136,7 +136,7 @@ static int send_cmd(int sd, __u16 nlmsg_type, __u32
> nlmsg_pid,
>         msg.g.version = 0x1;
>         na = (struct nlattr *) GENLMSG_DATA(&msg);
>         na->nla_type = nla_type;
> -       na->nla_len = nla_len + 1 + NLA_HDRLEN;
> +       na->nla_len = nla_len + NLA_HDRLEN;
>         memcpy(NLA_DATA(na), nla_data, nla_len);
>         msg.n.nlmsg_len += NLMSG_ALIGN(na->nla_len);
>

Right. This is the right thing to do.
I missed that the nla_len() will minus the NLA_HDRLEN.

Would you pls. submit a patch ?

Feel free to add:
Tested-by: Yafang Shao <laoar.shao@gmail.com>

Thanks
Yafang