Mail Archives: djgpp/2019/06/17/02:03:54
On Mon, 17 Jun 2019 01:28:32 -0400
"Frank Sapone (emoaddict15 AT gmail DOT com) [via djgpp AT delorie DOT com]"
<djgpp AT delorie DOT com> wrote:
> On 6/17/2019 1:05 AM, Rod Pemberton wrote:
> > On Mon, 17 Jun 2019 02:27:18 +0200
> > "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]"
> > <djgpp AT delorie DOT com> wrote:
> >> I ran into this issue with malloc(). It seems that, given enough
> >> allocations, malloc() will eventually return a pointer into already
> >> allocated memory.
> >>
> >> The attached program is able to reproduce this rather consistently,
> >> but only under cwsdpmi. It also only happens if the memory has
> >> previously been written to (suggesting a paging issue?). However
> >> the code that first led me to investigate this also exhibits the
> >> same problem under hdpmi. As such, I'm still not entirely
> >> convinced that this initial issue wasn't caused by my own code. I
> >> also find it hard to believe that no one else noticed this rather
> >> obvious problem before me. Still, the attached program
> >> demonstrates this clobbering issue, and I think this would warrant
> >> further investigation.
> >>
> >> Any insight is much appreciated.
> >>
> > First problem is trivial. The code doesn't compile with older DJGPP
> > v1.3. The declaration of 'i' within the for() loop errors, but
> > other C99 declarations only warn.
> >
> > Second problem is you don't call memset() prior to using memory, nor
> > free() after you're done using it. Of course, calling memset()
> > would prevent your method of "clobber" detection from working.
> > But, not calling memset() means you don't know if the magic clobber
> > value is: a) from you setting it within your program, or
> > b) from some random garbage values in memory.
> >
> > Third problem is you apparently didn't test the program without the
> > "p[i] = magic;" line. If you had, you would've noticed that your
> > program clobbers even without setting memory to magic values. In
> > other words, memory is filled with random values, since it wasn't
> > cleared by memset(). Also, some of those random values happen to
> > match your program's random magic value used to detect clobbered
> > memory.
> >
> > Fourth problem is that you can't actually confirm if memory is being
> > clobbered from within a C program for two reasons: inability to
> > distinguish a magic value from an identical random value in memory
> > which hasn't been cleared, and the inability in C to allocate,
> > clear, and free memory, prior to the re-use of the exact same
> > memory for a clobber test. To test this issue properly requires a
> > modified version of the memory allocator, i.e., CWSDPMI in this
> > case.
> >
> > Finally, you didn't report which version of DJGPP, or CWSDPMI, and
> > whether or not your code is operating in a Windows 98/SE/ME/XP etc
> > console. When operating in a Windows console, CWSDPMI is not being
> > used. The Windows DPMI host is being used.
> >
> >
>
> Good points. I was thinking some of the same things when I read this
> post earlier, but I am not as much of a C guru as others here. My
> initial thoughts were that memset was not being called and that it
> was a random integer for the magic value that there is a possibility
> of grabbing the same value.
>
One thing he could do is reduce the likelihood, i.e., probability, that
he'll get a false positive, i.e., random value. He could check for
multiple magic values in a row. The more values he checks for, the
lower the probability will be that it's random. I'd probably
check for an entire block. However, even long sequences can be
random or perhaps stale program data. E.g., a roulette wheel could in
theory land one on the color red for millions of spins or more, while
the odds or likelihood that it lands on either black or red hasn't
changed. I.e., there is no absolute certainty that a detected "clobber"
isn't random.
Rod Pemberton
--
Once upon a time, many decades ago in a place far away, humble people
sought their freedom, and lost. "Ideas are bulletproof."
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