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Subject: Re: [PATCH] kernel/taskstats: fix wrong nla type for
 {cgroup,task}stats policy
To:     Johannes Berg <johannes@sipsolutions.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yafang Shao <laoar.shao@gmail.com>
Cc:     bsingharora@gmail.com, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, "David S.Miller" <davem@davemloft.net>
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 <20200326130808.ccbacd6cba99a40326936fea@linux-foundation.org>
 <b879b50324b502cbd3f8439182d63532518d7315.camel@sipsolutions.net>
From:   David Ahern <dsahern@gmail.com>
Message-ID: <ed7c4063-8b45-9fc8-5b92-a903d9da4054@gmail.com>
Date:   Thu, 26 Mar 2020 15:11:21 -0600
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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);