Date: Sat, 17 Dec 2016 19:48:53 +0100 From: Domagoj Stolfa <domagoj.stolfa@gmail.com> To: Hiroki Sato <hrs@FreeBSD.org> Cc: freebsd-dtrace@freebsd.org Subject: clause-local variable with copyin() Message-ID: <20161217184853.GA11338@freebsd-laptop> In-Reply-To: <20161217.151014.1579687141761225852.hrs@allbsd.org> References: <20161217.151014.1579687141761225852.hrs@allbsd.org>
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Hello,
> I have trouble with clause-local variable. A minimum working example
> is attached. The "sample" program simply displays a string in an
> infinite loop with a USDT named as "dump-str", sample_debug.d does
> copyin() and printf() the whole buffer assuming it is
> nul-terminated:
>
> | sample$target:::dump-str
> | {
> | this->st = copyin(arg0, 1024);
> |
> | printf("(1)st = %s, %p\n", stringof(this->st),
> | (char *)this->st);
> | }
> | sample$target:::dump-str
> | {
> | printf("(2)st = %s, %p\n", stringof(this->st),
> | (char *)this->st);
> | printf("(3)st = %s\n", stringof(copyin(arg0, 1024)));
> | }
>
> The odd part is that it does not work with splitting the probe into
> the two as above but works fine without the split. The result was as
> follows:
>
> | % sudo make test
> | dtrace -C -I/var/home/hrs/sample_str -s sample_debug.d -c /var/home/hrs/sample_str/sample
> | dtrace: script 'sample_debug.d' matched 5 probes
> | CPU ID FUNCTION:NAME
> | 0 61714 main:dump-str (1)st = test-uname, fffffe0001a19118
> |
> | 0 61714 main:dump-str (2)st = , fffffe0001a19118
> | (3)st = test-uname
>
> this->st became empty at the beginning of the second probe.
>
> The symptom varied depending on the address of this->st, so I am
> guessing that this->st was incorrectly freed at the end of the first
> probe. If I use copyinstr(arg0) instead of copyin(), this problem
> does not occur.
That does seem like what's happening based on the described behaviour
and running the examples you've provided.
> Do I misunderstand clause-local variable? I noticed this when I use
> if-then clause which was recently implemented as a syntax sugar to
> split a probe automatically. The following ended up with the same
> result:
>
> | sample$target:::dump-str
> | {
> | this->st = copyin(arg0, 1024);
> |
> | printf("(1)st = %s, %p\n", stringof(this->st),
> | (char *)this->st);
> | if (0) {
> | }
> | printf("(2)st = %s, %p\n", stringof(this->st),
> | (char *)this->st);
> | printf("(3)st = %s\n", stringof(copyin(arg0, 1024)));
> | }
No, the clause-local variables should remain intact eventhough the probe
is split up into two clauses. See [1] for an example. This is
reproducible on FreeBSD 12.0-CURRENT and seems like a bug.
[1] http://dtrace.org/guide/chp-variables.html#chp-variables-4
--
Best regards,
Domagoj Stolfa.
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