On Tue, 2010-05-25 at 09:36 -0400, Chad Mynhier wrote:
> On Tue, May 25, 2010 at 5:13 AM, Mark Phalan <mark.pha...@sun.com> wrote:
> > On Mon, 2010-05-24 at 15:16 -0400, Chad Mynhier wrote:
> >>
> >> Even though "string" is a separate type in DTrace, a string is still
> >> just stored as a null-terminated sequence of characters. stringof()
> >> isn't doing anything to null-terminate what you give it, it's just
> >> assuming that you're giving it something that's null-terminated. In
> >> this case, it appears that there's enough garbage after the copied-in
> >> string to ...
> >
> > I thought that stringof() was being clever. I see this example:
> >
> > "To print only as much of the string as the caller intended, use the
> > copyin() subroutine, which takes a size as its second argument:
> >
> >
> > syscall::write:entry
> > {
> > printf("%s", stringof(copyin(arg1, arg2)));
> > }"
> >
> > from: http://docs.sun.com/app/docs/doc/817-6223/chp-user?a=view
> >
> > which seems to imply that stringof() should NULL-terminate.
> >
> > Is the above example incorrect?
> >
>
> Nope, the above example is mostly working by sheer luck. In this
> case, the destination of the copyin() just happens to be zeroed, so
> the string it copies in will necessarily be null-terminated.
Ok. I guess its a doc bug then.
>
> Note that stringof() doesn't actually _do_ anything. stringof() isn't
> a subroutine that walks what you give it to verify that it's
> null-terminated. (And it _can't_ do that, as it doesn't have a length
> argument and thus has no way of determining the proper length of that
> string.) stringof() isn't much more than a type cast.
Ok.
>
> OTOH, copyinstr() _does_ take a second argument that specifies a max
> length, so the workaround you're looking for is to use that:
>
> data_string = strjoin("@",
> copyinstr((uintptr_t)(*((uint32_t *)
> copyin((uintptr_t)&P->data, sizeof (uint32_t)))), *((uint32_t *)
> copyin((uintptr_t)&P->length, sizeof (uint32_t)))));
>
> copyinstr() will null-terminate the resulting string. You can see
> this here:
> http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/dtrace/dtrace.c#3174.
>
copyinstr() will work when the source is NULL terminated. It mostly
won't be in the real-world case I'll need this for. I think my
workaround with substr() (see other mail) should work though.
Thanks for the help,
-M
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