DTrace: stack() does not print kernel module functions for i386

[Rui Paulo]

On Nov 9, 2014, at 01:36, Konstantin Belousov <kostikbel at gmail.com> wrote:
> 
> On Sat, Nov 08, 2014 at 02:06:39PM -0800, Shrikanth Kamath wrote:
>> I verified this on a FreeBSD 10.0 STABLE, the stack() feature does not
>> appear to print functions from kernel modules. I believe there is a
>> glitch in libdtrace in the function "dt_module_update"
>> (cddl/contrib/opensolaris/lib/libdtrace/common/dt_module.c).
>> 
>> The section header address computation which is currently being done
>> in the function dt_module_update for elf type ET_REL, a similar
>> computation needs to be done for the ET_DYN maybe. I lack background
>> on the elf types but for experiment sakes I changed the line
>> 
>> @@ -948,7 +948,7 @@ dt_module_update(dtrace_hdl_t *dtp, struct kld_fil
>> #if defined(__FreeBSD__)
>>        mapbase = (uintptr_t)k_stat->address;
>>        gelf_getehdr(dmp->dm_elf, &ehdr);
>> -       is_elf_obj = (ehdr.e_type == ET_REL);
>> +      is_elf_obj = (ehdr.e_type == ET_REL) || (ehdr.e_type == ET_DYN);
>>        if (is_elf_obj) {
>>                dmp->dm_sec_offsets =
>>                    malloc(ehdr.e_shnum * sizeof(*dmp->dm_sec_offsets));
>> 
>> So from a previous run where I was running a dtrace one liner
>> %dtrace -n 'fbt:hwpmc:: { stack(); }'
>> The output without the above change shows a sample as
>> 
>> 0  50825 pmc_find_process_descriptor:entry
>>              0xc3c35bf5                                  <-- Address
>> not matched to symbol
>>              kernel`syscall+0x48b
>>              kernel`0xc0fd2121
>> 
>> whereas with the above nit change to include ET_DYN for section offset
>> adjustment I get the complete stack trace as
>> 
>> 0  50825 pmc_find_process_descriptor:entry
>>              hwpmc.ko`pmc_hook_handler+0x6a5   <--address matched to symbol
>>              kernel`syscall+0x48b
>>              kernel`0xc0fd2121
>> 
>> I believe without the correct section offset adjustment the following
>> check in the function dtrace_lookup_by_addr fails to match the address
>> to the correct symbol
>> 
>>            if (addr - dmp->dm_text_va < dmp->dm_text_size ||
>>                    addr - dmp->dm_data_va < dmp->dm_data_size ||
>>                    addr - dmp->dm_bss_va < dmp->dm_bss_size)
>> 
>> because dml->dm_text_va was not setup correctly in dt_module_update.
>> 
>> Can somebody help clarify this?
>> 
>> Reference: commit revision 210425.
> 
> I have no idea about DTrace guts, but can add one note you might find
> useful.
> 
> From the backtace above I can conclude that your platform is i386.
> A difference between i386 and amd64 is that i386 modules are dso's
> (ET_DYN), while on amd64 modules are only linked to object files
> (ET_REL).  The original author of the code probably tested on amd64.
> 
> You may want to special case amd64 by #ifdef, and use ET_DYN on other
> arches.

I agree with your analysis.  I think this patch should go in with the #ifdef __amd64__ for ET_REL.

--
Rui Paulo