dwarf information missing. how to rescue?

dwarf information missing. how to rescue?

[Shiyao Ma]

Hi,

sudo stap -L 'kernel.function("tcp_cong_control")'
gives this mere output:
kernel.function("tcp_cong_control@./net/ipv4/tcp_input.c:3336")

No params, locals are included.

Should I recompile the kernel?  if so, with what flags?

I'd like all this functions be probe-able.


Best.

Re: dwarf information missing. how to rescue?

[Frank Ch. Eigler]

Shiyao Ma <i@introo.me> writes:

> [...]
> sudo stap -L 'kernel.function("tcp_cong_control")'
> gives this mere output:
> kernel.function("tcp_cong_control@./net/ipv4/tcp_input.c:3336")
>
> No params, locals are included.
> Should I recompile the kernel?  if so, with what flags?

Over here on fedora 25, that function is inlined by the compiler, so a
compiler optimizer has to work hard to keep debuginfo correct &
available.  I wonder if perhaps you're affected by a years-old kernel
error of judgement, wherein gcc's debuginfo generation is deliberately
hobbled.  The HHGTG ^W systemtap release notes have had this to say:

  - An upstream kernel commit #2062afb4f804a put
  "-fno-var-tracking-assignments" into KCFLAGS, reducing debuginfo
  quality which can cause debuginfo failures.  A proposed workaround to
  this issue exists in: https://lkml.org/lkml/2014/11/21/505 . Fedora
  kernels are not affected by this issue.

If this may be affecting you, just nuke this line of the top level
kernel Makefile and rebuild it.  Distros everywhere should do the same.

Makefile:
KBUILD_CFLAGS   += $(call cc-option, -fno-var-tracking-assignments)

- FChE

Re: dwarf information missing. how to rescue?

[Shiyao Ma]

Hi,


I am on Debian sid with kernel 4.9.

How did you find out the function `tcp_cong_control' is compiled
inline?  Grepping /proc/kallsyms and didn't find it?

Thanks.

Best.




On Sun, Apr 2, 2017 at 5:43 AM, Frank Ch. Eigler <fche@redhat.com> wrote:
> Shiyao Ma <i@introo.me> writes:
>
>> [...]
>> sudo stap -L 'kernel.function("tcp_cong_control")'
>> gives this mere output:
>> kernel.function("tcp_cong_control@./net/ipv4/tcp_input.c:3336")
>>
>> No params, locals are included.
>> Should I recompile the kernel?  if so, with what flags?
>
> Over here on fedora 25, that function is inlined by the compiler, so a
> compiler optimizer has to work hard to keep debuginfo correct &
> available.  I wonder if perhaps you're affected by a years-old kernel
> error of judgement, wherein gcc's debuginfo generation is deliberately
> hobbled.  The HHGTG ^W systemtap release notes have had this to say:
>
>   - An upstream kernel commit #2062afb4f804a put
>   "-fno-var-tracking-assignments" into KCFLAGS, reducing debuginfo
>   quality which can cause debuginfo failures.  A proposed workaround to
>   this issue exists in: https://lkml.org/lkml/2014/11/21/505 . Fedora
>   kernels are not affected by this issue.
>
> If this may be affecting you, just nuke this line of the top level
> kernel Makefile and rebuild it.  Distros everywhere should do the same.
>
> Makefile:
> KBUILD_CFLAGS   += $(call cc-option, -fno-var-tracking-assignments)
>
> - FChE

Re: dwarf information missing. how to rescue?

[Frank Ch. Eigler]

Hi -

> How did you find out the function `tcp_cong_control' is compiled
> inline?  Grepping /proc/kallsyms and didn't find it?

I ran

% stap -L 'kernel.function("tcp_cong_control").*'

and it showed an .inline variant only, no .call/.return.

- FChE

Re: dwarf information missing. how to rescue?

[Arkady]

In Ubuntu kernel 4.8.0-45-generic there is inline function only

On Sun, Apr 2, 2017 at 2:58 PM, Frank Ch. Eigler <fche@redhat.com> wrote:
> Hi -
>
>> How did you find out the function `tcp_cong_control' is compiled
>> inline?  Grepping /proc/kallsyms and didn't find it?
>
> I ran
>
> % stap -L 'kernel.function("tcp_cong_control").*'
>
> and it showed an .inline variant only, no .call/.return.
>
> - FChE
>