Re: kprobe: __blkdev_put probe is missed

[Masami Hiramatsu]

Hi Ming,

On Sat, 20 Jun 2020 07:28:20 +0800
Ming Lei <ming.lei@xxxxxxxxxx> wrote:

> > 
> > Ah, after all it is as expected. With your kconfig, the kernel is
> > very agressively optimized.
> > 
> > $ objdump -dS vmlinux | less
> > ...
> > ffffffff81256dc3 <__blkdev_put>:
> > {
> > ffffffff81256dc3:       e8 98 85 df ff          callq  ffffffff8104f360 <__fentry__>
> > ffffffff81256dc8:       41 57                   push   %r15
> > ffffffff81256dca:       41 56                   push   %r14
> > ffffffff81256dcc:       41 55                   push   %r13
> > ...
> > ffffffff81256f05:       75 02                   jne    ffffffff81256f09 <__blkdev_put+0x146>
> >         struct block_device *victim = NULL;
> > ffffffff81256f07:       31 db                   xor    %ebx,%ebx
> >                 bdev->bd_contains = NULL;
> > ffffffff81256f09:       48 c7 45 60 00 00 00    movq   $0x0,0x60(%rbp)
> > ffffffff81256f10:       00 
> >                 put_disk_and_module(disk);
> > ffffffff81256f11:       4c 89 f7                mov    %r14,%rdi
> > ffffffff81256f14:       e8 c6 3d 11 00          callq  ffffffff8136acdf <put_disk_and_module>
> >         mutex_unlock(&bdev->bd_mutex);
> > ffffffff81256f19:       4c 89 ff                mov    %r15,%rdi
> >                 __blkdev_put(victim, mode, 1);
> > ffffffff81256f1c:       41 bc 01 00 00 00       mov    $0x1,%r12d
> >         mutex_unlock(&bdev->bd_mutex);
> > ffffffff81256f22:       e8 8d d7 48 00          callq  ffffffff816e46b4 <mutex_unlock>
> >         bdput(bdev);
> > ffffffff81256f27:       48 89 ef                mov    %rbp,%rdi
> > ffffffff81256f2a:       e8 f0 e9 ff ff          callq  ffffffff8125591f <bdput>
> >         if (victim)
> > ffffffff81256f2f:       48 85 db                test   %rbx,%rbx
> > ffffffff81256f32:       74 08                   je     ffffffff81256f3c <__blkdev_put+0x179>
> > ffffffff81256f34:       48 89 dd                mov    %rbx,%rbp
> > ffffffff81256f37:       e9 b4 fe ff ff          jmpq   ffffffff81256df0 <__blkdev_put+0x2d> <<-----THIS!!
> > }
> > ffffffff81256f3c:       48 8b 44 24 28          mov    0x28(%rsp),%rax
> > ffffffff81256f41:       65 48 33 04 25 28 00    xor    %gs:0x28,%rax
> > ffffffff81256f48:       00 00 
> > ffffffff81256f4a:       74 05                   je     ffffffff81256f51 <__blkdev_put+0x18e>
> > ffffffff81256f4c:       e8 5a 4e 48 00          callq  ffffffff816dbdab <__stack_chk_fail>
> > ffffffff81256f51:       48 83 c4 30             add    $0x30,%rsp
> > ffffffff81256f55:       5b                      pop    %rbx
> > ffffffff81256f56:       5d                      pop    %rbp
> > ffffffff81256f57:       41 5c                   pop    %r12
> > ffffffff81256f59:       41 5d                   pop    %r13
> > ffffffff81256f5b:       41 5e                   pop    %r14
> > ffffffff81256f5d:       41 5f                   pop    %r15
> > ffffffff81256f5f:       c3                      retq   
> > 
> > 
> > As you can see, the nested __blkdev_put() is coverted to a loop.
> > If you put kprobe on __blkdev_put+0x2d, you'll see the event twice.
> 
> Thanks for your investigation.
> 
> Some trace tools can just trace on function entry, such as bcc, and some
> user script always trace on function entry.
> 
> I guess the issue should belong to kprobe implementation:
> 
> 1) __blkdev_put() is capable of being kprobed, so from user view, the
> probe on entry of __blkdev_put() should be triggered

Yes, it is correctly triggered.

> 
> 2) from implementation view, I understand exception should be trapped
> on the entry of __blkdev_put(), looks it isn't done.

No, it is correctly trapped the function entry address. The problem is
that the gcc optimized the nested function call into jump to the
beginning of function body (skip prologue).

Usually, a function is compiled as below

func()     (1) the entry address (func:)
{          (2) the function prologue (setup stackframe)  
  int a    (3) the beginning of function body 
   ...
  func()   (4) the nested function call

And in this case, the gcc optimized (4) into jump to (3) instead of
actual function call instruction. Thus, for the nested case (1) and
(2) are skipped.
 IOW, the code flow becomes
  (1)->(2)->(3)->(4)->(3)
 instead of 
  (1)->(2)->(3)->(4)->(1)->(2)->(3)

In this case, if we put a probe on (1) or (2), those are disappeared
in the nested call. Thus if you put a probe on (3) ('perf probe __blkdev_put:2')
you'll see the event twice.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>