[rrd-users] Valgrind error rrd_update.c:982

Mon Jun 16 08:25:07 CEST 2014

Hi,

> The setlocale() thread-safeness problems are well known, but there is also a
> rather significant lack of a decent widely adopted workaround.

I know, but what I want to say is that according to this page [1] the
rrd_update_r function is said to be thread-save but it isn't. Hence,
if I had not stumbled across the first point (aka the memory bug) I
would not have noticed this one. I  would never have assumed that
rrd_update_r uses setlocale. (Perhaps sometimes later in the future
after I would have observed strange behaviour.) That's why I asked for
a big warning at [1].

I do not know yet, if setlocale is the only problem with rrd_update_r,
but at least I see a bug fix for this one. There is no need to use
setlocale at all, if there was an interface like

rrd_update_r( cont char* rrdFile, time_t ts, size_t argc, const char*
desc[], const T* val[] )

for T being a numeric type. This way one could avoid the conversion to
and from a string at all. The calling application is responsible of
providing the correct datatype. That should even simplify rrd_update_r
because there is no need to
parse an argument of the pattern "<ts>:<val 1>:...:<val n>".

Regards, Matthias

[1] http://oss.oetiker.ch/rrdtool/prog/rrdthreads.en.html

2014-06-16 1:47 GMT+02:00 Donovan Baarda <abo at minkirri.apana.org.au>:
> The setlocale() thread-safeness problems are well known, but there is also a
> rather significant lack of a decent widely adopted workaround. This is the
> problem with many of the standard C libraries... they pre-date threading,
> and there was never consensus/standardizing on any thread safe alternative.
>
> For any thread-safe alternative to work, everything needs to use it, but
> without a standard, you can pretty much guarantee that something you link
> will not use it, and some platforms won't even have it.
>
> So instead, your best option is often to just declare your code
> non-thread-safe and recommend subprocesses instead... but then the
> inter-process communication libs are not as well developed as the threading
> libs.
>
> Either that or completely embrace something like glib which replaces nearly
> everything in the standard C libs and only use other libs that also use
> glib. When you do this you also have to accept that your code doesn't look
> like normal C any more, and you might have some platform restrictions.
>
>
>
> On 16 June 2014 08:39, Matthias Nagel <matthias.h.nagel at gmail.com> wrote:
>>
>> Hello,
>> thanks. In the meantime I patched my Debian packages locally. But I ran
>> into another race condition. rrd_update_r() isn't thread-save, because the C
>> locale is an application wide variable. Assume one has rrdlib (A) and some
>> other library (B) and the execution order is as follows:
>>
>> (A1) old_locale = setlocale(...)
>> (B1) old_locale = setlocale(...)
>> (A2) // do some locale-dependent stuff
>> (A3) setlocale( old_locale )
>> (B2) // do some locale-dependent stuff
>> (B3) setlocale( old_locale )
>>
>> Here, library B can be any library that also sets the global C locale
>> within a different thread context. In the best case some strings are
>> misinterpreted, in the worst case the memory gets corrupted :-( At the
>> moment, I wrote a work-around by using an application wide mutex that must
>> be locked by any thread that wants to call any library that might change the
>> global C locale. But of course this isn't very nice.
>>
>> Are there any chances that the rrd_update_r function (and relatives) will
>> be rewritten? For example, C++ locales are bounded to a specific stream and
>> are not global. At least there should be big, BIG warning at
>> http://oss.oetiker.ch/rrdtool/prog/rrdthreads.en.html that the C locale is
>> subject to a race condition.
>>
>> Regards, Matthias
>>
>> Am Sonntag, 15. Juni 2014, 22:37:01 schrieb Tobias Oetiker:
>> > Hi Matthias,
>> >
>> > yes you are right ... we fixed this in master, but not in the 1.4
>> > branch ... it is now ...
>> >
>> > cheers
>> > tobi
>> >
>> > Today Matthias Nagel wrote:
>> >
>> > > Hello,
>> > >
>> > > I am writing a multi-threaded C++ application that uses rrdlib
>> > > natively by calling rrd_update_r().  If I compile without optimazations and
>> > > enable -ggdb everything seems to work fine. As soon as I switch to -O2 and
>> > > disable -ggdb my apllication crashes at runtime.
>> > >
>> > > If it crashes the output is either
>> > >
>> > > *** glibc detected *** rrdtool: <something>
>> > >
>> > > or
>> > >
>> > > expected timestamp not found in data source from <input>
>> > >
>> > > but <input> is not the string that was given to rrd_update_r but
>> > > unreadable garbage. Obviously, it is a memory corruption problem. Therefore,
>> > > I ran the application under valgrind and I noticed that the problems comes
>> > > from inside of the rrdlib. The message is
>> > >
>> > >
>> > > ==11724== Invalid read of size 1
>> > > ==11724==    at 0x4C2A051: __GI_strcmp (mc_replace_strmem.c:712)
>> > > ==11724==    by 0x5A4FF7F: setlocale (setlocale.c:210)
>> > > ==11724==    by 0x505D06B: _rrd_update (rrd_update.c:982)
>> > > ==11724==  Address 0x9deb0d0 is 0 bytes inside a block of size 12
>> > > free'd
>> > > ==11724==    at 0x4C27D4E: free (vg_replace_malloc.c:427)
>> > > ==11724==    by 0x5A4FCBD: setname (setlocale.c:173)
>> > > ==11724==    by 0x5A500B0: setlocale (setlocale.c:417)
>> > > ==11724==    by 0x505D02D: _rrd_update (rrd_update.c:974)
>> > >
>> > > Let's have a look at it:
>> > >
>> > > rrd_update.c:973: old_locale = setlocale(LC_NUMERIC, NULL);
>> > > rrd_update.c:974: setlocale(LC_NUMERIC, "C");
>> > > rrd_update.c:982: setlocale(LC_NUMERIC, old_locale);
>> > >
>> > > The problem is obvious. The variable "old_locale" that is used at the
>> > > 3rd line was assigned at the 1st line. But the 2nd call to "setlocale" freed
>> > > the return value of the first call. According to the man pages the return
>> > > value is a pointer to static memory and freed/allocated on every call.
>> > > Actually the 2nd line (974) should be ommited and it should be
>> > >
>> > > rrd_update.c:973: old_locale = setlocale(LC_NUMERIC, "C" );
>> > > rrd_update.c:974: // deleted
>> > > rrd_update.c:982: setlocale(LC_NUMERIC, old_locale);
>> > >
>> > > Why this double call to "setlocale" anyway?
>> > >
>> > > Best regards, Matthias
>> > >
>> > >
>> >
>> >
>>
>> --
>> Matthias Nagel
>> Parkstraße 27
>> 76131 Karlsruhe
>>
>> Festnetz: +49-721-96869289
>> Mobil: +49-151-15998774
>> e-Mail: matthias.h.nagel at gmail.com
>> ICQ: 499797758
>> Skype: nagmat84
>>
>> _______________________________________________
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>>
>
>
>
> --
> Donovan Baarda <abo at minkirri.apana.org.au>

-- 
Matthias Nagel
Parkstraße 27
76131 Karlsruhe

Festnetz: +49-721-96869289
Mobil: +49-151-15998774
e-Mail: matthias.h.nagel at gmail.com
ICQ: 499797758
Skype: nagmat84