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Subject: Re: [RFC PATCH] bpf: Add helpers to read useful task_struct members
To:     "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Alexei Starovoitov <ast@fb.com>, netdev@vger.kernel.org
Cc:     daniel@iogearbox.net, Martin KaFai Lau <kafai@fb.com>,
        linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        Brendan Gregg <brendan.d.gregg@gmail.com>
References: <20171103065833.8076-1-sandipan@linux.vnet.ibm.com>
 <94a4761f-1b51-8b70-fb7f-3cea91c69717@fb.com>
 <1509815348.8zu63uatdo.naveen@linux.ibm.com>
From:   Sandipan Das <sandipan@linux.vnet.ibm.com>
Date:   Mon, 6 Nov 2017 10:46:46 +0530
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Hi Alexei, Naveen,

On 11/04/2017 11:01 PM, Naveen N. Rao wrote:
> 
> I think the offsets described in dwarf were incorrect with CONFIG_GCC_PLUGIN_RANDSTRUCT, but I'll let Sandipan confirm that.
> 

I think that the offsets described in dwarf are probably incorrect when
CONFIG_GCC_PLUGIN_RANDSTRUCT is enabled. To verify this, I used perf
to attach a probe to try_to_wake_up() which is the also the function to
which waker() is attached in the previously mentioned kernel sample. So,
if the run the following:

# perf probe "try_to_wake_up" "p->pid"
# perf record -a -e probe:try_to_wake_up
# perf script

The value of p->pid is reported as 0. Similarly, if I try to read
p->comm, it is reported to be an empty string. The same problem is
seen with systemtap as well.

Also, if I do a printk with offsetof(struct task_struct, pid) and
offsetof(struct task_struct, comm) inside the kernel code and then
compare the values with the offsets reported by pahole, they are
completely different.

- Sandipan


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