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

[Atish Patra]

On 11/07/2017 03:14 PM, Y Song wrote:
> On Tue, Nov 7, 2017 at 12:37 AM, Naveen N. Rao
> <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> > Alexei Starovoitov wrote:
> > > On 11/7/17 12:55 AM, Naveen N. Rao wrote:
> > > > > I thought such struct shouldn't change layout.
> > > > > If it is we need to fix include/linux/compiler-clang.h to do that
> > > > > anon struct as well.
> > > >
> > > >
> > > > We considered that, but it looked to be very dependent on the version of
> > > > gcc used to build the kernel. But, this may be a simpler approach for
> > > > the shorter term.
> > >
> > > why it would depend on version of gcc?
> >
> >
> > From what I can see, randomized_struct_fields_start is defined only for gcc
> > > = 4.6. For older versions, it does not get mapped to an anonymous
> > structure. We may not care for older gcc versions, but..
> >
> > The other issue was that __randomize_layout maps to __designated_init when
> > randstruct plugin is not enabled, which is in turn an attribute on gcc >=
> > v5.1, but not otherwise.
> >
> > > We just need this, no?
> > >
> > > diff --git a/include/linux/compiler-clang.h
> > > b/include/linux/compiler-clang.h
> > > index de179993e039..4e29ab6187cb 100644
> > > --- a/include/linux/compiler-clang.h
> > > +++ b/include/linux/compiler-clang.h
> > > @@ -15,3 +15,6 @@
> > >    * with any version that can compile the kernel
> > >    */
> > >   #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix),
> > > __COUNTER__)
> > > +
> > > +#define randomized_struct_fields_start struct {
> > > +#define randomized_struct_fields_end   };
> > >
> > > since offsets are mandated by C standard.
> >
> >
> > Yes, this is what we're testing with and is probably sufficient for our
> > purposes.
>
> Just tested this with bcc. bcc actually complains. the rewriter
> is not able to rewrite prev->pid where prev is "struct task_struct *prev".
> I will change bcc rewriter to see whether the field value is correct or not.
>
> Not sure my understanding is correct or not, but I am afraid that
> the above approach for clang compiler change may not work.
> If clang calculates the field offset based on header file, the offset
> may not be the same as kernel one....
>
> I verified that the drawf info with randomized structure config does not
> match randomized structure member offset. Specifically, I tried
> linux/proc_ns.h struct proc_ns_operations,
> dwarf says:
>   field name: offset 0
>   field real_ns_name: offset 8
> But if you print out the real offset at runtime, you get 40 and 16 respectively.
I am also trying to get it work with bcc as any python scripts that 
access task_struct gives wrong output (task_switch.py, runqlen.py). I 
recompiled my kernel (4.14-rc7 & bcc) with the patch.

I am seeing following error.
# ./task_switch.py
/virtual/main.c:17:18: warning: implicit declaration of function 
'bpf_get_task_pid_tgid' is invalid in C99
      [-Wimplicit-function-declaration]
  key.prev_pid = bpf_get_task_pid_tgid(prev);
                 ^
1 warning generated.
LLVM ERROR: Program used external function 'bpf_get_task_pid_tgid' which 
could not be resolved!

Are you also seeing the same issue or something else ?

Regards,
Atish
> > - Naveen