#systemtap on 2016-08-02 — irc logs at freenode.irclog.whitequark.org

2015-11-12 23:18 fche changed the topic of #systemtap to: http://sourceware.org/systemtap; email systemtap@sourceware.org if answers here not timely, conversations may be logged

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15:44 <p53ud0nym> How can i use systemtap with debugging symbols on a custom kernel?

15:44 <p53ud0nym> I am compiling with CONFIG_DEBUG_INFO=y and DWARF4 enabled but systemtap doesn't find a modules debug symbols

15:45 <p53ud0nym> does it look in /usrr/lib/debug/`uname -r`/?

15:45 <p53ud0nym> I basically have a custom kernel modul ethat I want to place probes in. I am currently running with `stap -g -d MODNAME script.stp -v`.

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15:54 <fche> hey p53ud0nym

15:54 <fche> so to probe out-of-tree modules, stap has been getting better, e.g., supporting module("/full/path/to/foo.ko") probe points

15:55 <fche> but the easiest way is to install your debuginfo-laden .ko under /lib/modules/`uname -r`/SUBDIRECTORY; stap will find it there easily

16:22 <p53ud0nym> fche: Thanks but I'm still receiving `no line information` on a statement probe when i pass the direct path of the kernel module. NB: stap doesn't complain about the function probes. The module is compiled with `-g -DDEBUG -ggdb`

16:23 <p53ud0nym> I've check objdump and DWARF symbols are there.

16:23 <p53ud0nym> I'm about to try and compile with the `-O0` flag.

16:24 <fche> hm this is sounding familiar; was this a CONFIG_DEBUG_INFO_SPLIT=y configuration perhaps?

16:26 <p53ud0nym> CONFIG_DEBUG_INFO_SPLIT is not set

16:26 <p53ud0nym> I can force it to N and re-compile?

16:26 <fche> worth a shot

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16:31 <fche> as is

16:31 <fche> % stap -l 'module("foo").statement("*@*:*")'

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16:51 <p53ud0nym> fche: Your last suggestion printed out every line. Thanks.

16:51 <fche> so that same probe point should work as in probe module("foo").statement("bar...") { }

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17:16 <p53ud0nym> fche: ...It worked

17:16 <fche> hoorray

17:16 <p53ud0nym> So when i ran `stap -l .....` is only printed certain lines.

17:17 <fche> yes, when stap can reasonably-unambiguously pick a source level line that corresponds to a PC address

17:17 <p53ud0nym> I was using `module("foo").statement("*@*:510")` but that didn;t exist in the ouptut from `stap -l`. However lines 509 and 511 did.

17:18 <p53ud0nym> Thanks a lot!

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20:38 <p53ud0nym> Surely this is a bug: https://paste.debian.net/786674/

20:53 <fche> sometimes the list mode is optimistic; this is a bug, yeah

20:55 <p53ud0nym> As far as I'm concerned, this is also another bug: https://paste.debian.net/786675/

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20:56 <fche> probably, but it's a bug in gcc and/or linux kconfig

20:58 <fche> http://www.spinics.net/lists/kernel/msg1974263.html

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20:59 <fche> fedora carries that as a kernel patch; unfortunately lkml has not seen eye-to-eye on this point

20:59 <fche> they basically degrade debuginfo deliberately

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21:14 <p53ud0nym> Maybe I should have just used fedora with a stock kernel. I'm recompiling the trying it again.

21:14 <fche> if you build your module with V=1, we can see the CFLAGS better

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21:40 <fche> p53ud0nym, any luck?

21:42 <p53ud0nym> No. V=1 shows the module and entire kernel is built with -fvar-tracking-assignments but I still can't compile due to lack of references.

21:43 <p53ud0nym> *can't compile the stp script

21:44 <p53ud0nym> Same error as before, with an output that exists in `stap -l module()(*@*:*)` cannot be found.

21:44 <p53ud0nym> I'm about to give up tbh.

21:47 <p53ud0nym> I have a deadline and I'm running of time. Probably manually edit driver code for now on a few drivers. If I get time I'll use fedora with their stock kernel without my custom kernel module. If I do, you will see me back here In a month.

21:47 <p53ud0nym> Thanks for all your help.

21:48 <fche> ok

21:48 <fche> sorry it didn't work out

21:48 <fche> one other thing worth trying btw is to add in some kernel tracepoints

21:48 <fche> those one are reliably placed & parameters reliably passed, without debuginfo issues

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22:02 <p53ud0nym> Thanks, i'll look into it.

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