Date: Fri, 06 Jun 2014 15:40:36 +0400 From: Dmitry Yu Okunev <dyokunev@ut.mephi.ru> To: Mark Johnston <markj@freebsd.org> Cc: freebsd-dtrace@freebsd.org Subject: Re: failed to resolve cwd: Unknown variable name Message-ID: <5391A8B4.4030906@ut.mephi.ru> In-Reply-To: <CAMw1wOw4dkUeOrrdbHqmO-A_qB09dMaCr_aDTJSfSvZ8Ku3EUg@mail.gmail.com> References: <5388A227.7050805@ut.mephi.ru> <CAMw1wOwMU--0tDEjnK%2BcWW00d=d%2B%2B8BVmSCQ7XbR85bMQ0fhWw@mail.gmail.com> <538EAAF1.1030005@ut.mephi.ru> <CAMw1wOw4dkUeOrrdbHqmO-A_qB09dMaCr_aDTJSfSvZ8Ku3EUg@mail.gmail.com>
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On 06/04/2014 05:25 PM, Mark Johnston wrote:
> On Wed, Jun 4, 2014 at 1:13 AM, Dmitry Yu Okunev <dyokunev@ut.mephi.ru>=
wrote:
>> On 06/04/2014 06:28 AM, Mark Johnston wrote:
>>> On Fri, May 30, 2014 at 11:22 AM, Dmitry Yu Okunev <dyokunev@ut.mephi=
=2Eru> wrote:
>>>> But when I try to use the variable I get a failure:
>>>>> dtrace: invalid probe specifier syscall:::entry { printf("%s", cwd)=
;
>>>> }: in action list: failed to resolve cwd: Unknown variable name
>>>>
>>>> You can get the same error by running a dtrace-script from official
>>>> FreeBSD distribution:
>>>>> /usr/share/dtrace/toolkit/opensnoop -c
>>> So one can get a bit of the way there with something ugly like
>>>
>>> inline string cwd =3D
>>> stringof(curthread->td_proc->p_fd->fd_cdir->v_cache_dst.tqh_first->nc=
_name);
>>>
>>> to get the last component of a process' cwd (it needs a check for a
>>> missing cache entry), but I don't see any easy way to get at the full=
>>> cwd. Calling vn_fullpath() in probe context would be a pretty bad ide=
a
>>> since it may invoke VFS operations
>>
>> Hm. If it's performance problem only, then personally I can endure tha=
t.
>>
>> Can I use vn_fullpath*() in dtrace probes on current FreeBSD?
>=20
> It's a safety thing. DTrace probes execute with interrupts disabled,
> so they're fairly limited in what they're allowed to do. Name
> resolution potentially requires the underlying filesystem code to read
> from disk or invoke an RPC, which cannot be done in probe context.
> Hence my suggestion of a cache-only lookup function that could be
> callable from within a probe.
Well. Here're two problems:
1. The solution returns only the last component of cwd. How to get the
rest? I can handle it myself (if it's possible), but if somebody already
have working solution that would be great :)
2. Is this solution steady to FreeBSD version updates? I think that this
headers can be changed with every major version.
--=20
Best regards, Dmitry,
head of UNIX-tech department NRNU MEPhI,
tel. 8 (495) 788-56-99, ext. 8255
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