Date: Thu, 13 Nov 2014 13:36:12 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com>, avg@freebsd.org, Shrikanth Kamath <shrikanth07@gmail.com>, freebsd-dtrace@freebsd.org, Rui Paulo <rpaulo@me.com> Subject: Re: DTrace: stack() does not print kernel module functions for i386 Message-ID: <201411131336.12334.jhb@freebsd.org> In-Reply-To: <9011F920-3092-4E61-9CDC-68FD9092BB7D@me.com> References: <CAEOAkMXnwqC42gZKc0f80cppff077pYGjs5PUPht0DBcyEi8Jw@mail.gmail.com> <20141109093632.GV53947@kib.kiev.ua> <9011F920-3092-4E61-9CDC-68FD9092BB7D@me.com>
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On Sunday, November 09, 2014 1:57:19 pm Rui Paulo wrote:
> On Nov 9, 2014, at 01:36, Konstantin Belousov <kostikbel@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.
>
Why have the #ifdef? In theory other platforms besides amd64 could use
sys/kern/link_elf_obj.c. It doesn't hurt to just let the code always accept
both ET_DYN and ET_REL does it?
--
John Baldwin
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