Elfutils module reporting

Elfutils module reporting

[Nurdin Premji]

Roland, I'm trying to refresh dwfl objects in terms of the dwfl_modules. 
To optimize code and not create a new dwfl object every time a task changes.
As a first pass I was trying to remove all old modules and re-report new 
modules just by using dwfl_report_begin and dwfl_report_end. The 
libdwfl.h comments lead me to believe this is all that is necessary to 
remove all old modules and report only new modules. I plan to switch to 
dwfl_report_begin_add later on and only report modules that have changed.

I got an assertion failure at the dwfl_report_end stage
 /home/yyz/npremji/mainworkspace/frysk/frysk-imports/elfutils/libdwfl/dwfl_module.c:249: 
dwfl_report_end: Assertion `i == dwfl->nmodules' failed.

Looking at the dwfl_module.c file I see that dwfl_report_begin calls 
dwfl_report_begin_add, which clears all the modules. I thought that this 
function was supposed to preserver all the old modules?

Also how would I go about removing single modules in an updating 
scenario? i.e. is there a dwfl_report_remove_module function to link 
with dwfl_report_begin_add?

Thank you,
Nurdin.

Re: Elfutils module reporting

[Roland McGrath]

The expected pattern is that you do:

	dwfl_report_begin
	dwfl_report_foo all modules that are in the address space now
	dwfl_report_end

i.e., just like with a fresh Dwfl, but dwfl_report_begin instead of dwfl_begin.
This preserves the existing data structures for modules that haven't
changed since before dwfl_report_begin, and removes any old modules that
you didn't report again.  

dwfl_report_begin_add is intended for something like gdb's add-symbol-file,
where you are always just adding one new thing as opposed to re-synch'ing
to the new set of mappings after they've changed.  It's just a shorthand
and mild optimization for re-reporting all the existing modules after
dwfl_report_begin.

I could add something like a dwfl_report_remove to de-report a reported
module in a reporting loop (which after dwfl_report_begin_add would be
equivalent to removing the preexisting module).  But I'd rather exercise
the interface as it stands a bit more before deciding to add a
microoptimization.

> I got an assertion failure at the dwfl_report_end stage
>  /home/yyz/npremji/mainworkspace/frysk/frysk-imports/elfutils/libdwfl/dwfl_module.c:249: 
> dwfl_report_end: Assertion `i == dwfl->nmodules' failed.

Please send me a small C test program to reproduce this bug.


Thanks,
Roland

Re: Elfutils module reporting

[Nurdin Premji]

[-- Attachment #1: Type: text/plain, Size: 1441 bytes --]

Roland McGrath wrote:
> The expected pattern is that you do:
>
> 	dwfl_report_begin
> 	dwfl_report_foo all modules that are in the address space now
> 	dwfl_report_end
>
> i.e., just like with a fresh Dwfl, but dwfl_report_begin instead of dwfl_begin.
> This preserves the existing data structures for modules that haven't
> changed since before dwfl_report_begin, and removes any old modules that
> you didn't report again.  
>
> dwfl_report_begin_add is intended for something like gdb's add-symbol-file,
> where you are always just adding one new thing as opposed to re-synch'ing
> to the new set of mappings after they've changed.  It's just a shorthand
> and mild optimization for re-reporting all the existing modules after
> dwfl_report_begin.
>
> I could add something like a dwfl_report_remove to de-report a reported
> module in a reporting loop (which after dwfl_report_begin_add would be
> equivalent to removing the preexisting module).  But I'd rather exercise
> the interface as it stands a bit more before deciding to add a
> microoptimization.
>
>   
>> I got an assertion failure at the dwfl_report_end stage
>>  /home/yyz/npremji/mainworkspace/frysk/frysk-imports/elfutils/libdwfl/dwfl_module.c:249: 
>> dwfl_report_end: Assertion `i == dwfl->nmodules' failed.
>>     
>
> Please send me a small C test program to reproduce this bug.
>
>
> Thanks,
> Roland
>   
Try this. It was tested against elfutils 0.128 in fedora 7.

[-- Attachment #2: dwfltest.c --]
[-- Type: text/x-csrc, Size: 620 bytes --]

#include <libdwfl.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

int main()
{
static char *flags = "-:.debug:/usr/lib/debug";
Dwfl_Callbacks *cbs = (Dwfl_Callbacks *) malloc(sizeof (Dwfl_Callbacks));
cbs->find_elf = dwfl_linux_proc_find_elf;
cbs->find_debuginfo = dwfl_standard_find_debuginfo;
cbs->debuginfo_path = &flags; 

Dwfl* dwfl = dwfl_begin(cbs);
dwfl_report_begin(dwfl);
dwfl_report_module(dwfl, "module1", 0, 10);
dwfl_report_end(dwfl, NULL, NULL);
dwfl_report_begin(dwfl);
dwfl_report_module(dwfl, "module1", 0, 10);
dwfl_report_end(dwfl, NULL, NULL);
}

Re: Elfutils module reporting

[Roland McGrath]

I knew there was a reason I wanted a user! ;-)
The reporting interface has never really been used with updates before.
It's a one-line fix.  

--- libdwfl/dwfl_module.c	16e7ba3c4f28363c68c66975b4a123da1383e52a
+++ libdwfl/dwfl_module.c	dbf8292265056d2e7302f58f09e72dcdac51cf8a
@@ -149,6 +149,7 @@ dwfl_report_module (Dwfl *dwfl, const ch
 	  m->next = *tailp;
 	  m->gc = false;
 	  *tailp = m;
+	  ++dwfl->nmodules;
 	  return m;
 	}