[Bug 194933] New: DTrace: Stack() command does not print function name if it is from a kernel module for i386.

bugzilla-noreply at freebsd.org bugzilla-noreply at freebsd.org
Mon Nov 10 19:19:53 UTC 2014


            Bug ID: 194933
           Summary: DTrace: Stack() command does not print function name
                    if it is from a kernel module for i386.
           Product: Base System
           Version: 10.0-STABLE
          Hardware: i386
                OS: Any
            Status: Needs Triage
          Severity: Affects Many People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: shrikanth07 at gmail.com

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"

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

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

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

            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.

Filing a bug report after discussion on freebsd-hackers, please refer the
following thread

Reference: commit revision 210425. And also the related thread

Note that this is not an issue on amd64.

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