Re: dwarf information missing. how to rescue?

[Shiyao Ma <i at introo dot me>]

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