Re: [RFC PATCH] bpf: Add helpers to read useful task_struct members

[Sandipan Das <sandipan@linux.vnet.ibm.com>]

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