Your approach requires extra checks, though, which are easy to forget. Also, NULL is not guaranteed to be the stored as zeros, plus padding is going to make your life annoying.
Well, dangling pointers are also easy to forget... Yes, it requires some discipline. Good code requires discipline, doesn't it?
The trick of checking that buffers are zeroed is purely a debugging tool, so it's okay if it doesn't work on some platforms. And if you allocate with calloc(), the padding will be zeroed for you. It's actually very rare that you will have to call memset() with this technique.
This is like the most clichéd way of saying “my code has security vulnerabilities” that there is. I have yet to see code that has remained secure solely on the “discipline” of programmers remembering to check things.
> The trick of checking that buffers are zeroed is purely a debugging tool, so it's okay if it doesn't work on some platforms.
Fair.
> And if you allocate with calloc(), the padding will be zeroed for you.
It might get unzeroed if you work with the memory.
All code is full of vulnerabilites. If you say your code isn't, then I'm sure it is. I just do the best I can to keep the error rate as low as possible. But it's a rate, and it's never zero.
Also, it's not just about vulns in security-critical code. It's also about ordinary bugs. Why not be a little more careful? It won't hurt.
> It might get unzeroed if you work with the memory.
Maybe, but it isn't very common. I'm not sure when the C standard allows changing padding bytes, but in practice the compilers I've used don't seem to do it. And again, it's just a debugging aid, if it causes too much trouble on some platform, just turn it off.
It’s better to have automatic checks than rely on programmers being careful enough to remember to add them. For padding: this probably happens more on architectures that don’t do unaligned accesses very well.
Help me out here, because I'm really trying to understand. Are you saying that dangling pointers that blow up if you double-free them is an "automatic check"? If not, what kind of automatic check are you talking about?
If the extra code is really that bothersome, just use a macro or wrapper function.
It's a much better situation than NULLing them out, because that hides bugs and makes tools like Address Sanitizer useless. A dangling pointer, when freed, will often throw an assert in your allocator; here's an example of how this looks like on my computer:
$ clang -x c -
#include <stdlib.h>
int main(int argc, char **argv) {
char *foo = malloc(10);
free(foo);
free(foo);
}
$ ./a.out
a.out(14391,0x1024dfd40) malloc: *** error for object 0x11fe06a30: pointer being freed was not allocated
a.out(14391,0x1024dfd40) malloc: *** set a breakpoint in malloc_error_break to debug
Abort trap
As you turn up your (automatic) checking this will be caught more and more often. Setting the pointer to NULL will silently hide the error as free(NULL) is a no-op and nothing will catch it. Thus, the suggestion here was
1. advocating adding additional code, which has historically been hard to actually do in practice, and
2. providing a suggestion that is generally worse.
I can see an argument for wrapping it in a macro so you can turn off nulling in debug builds (ASan might even have hooks so you can automate this, I know Valgrind does). But use-after-free is worse than just double-frees, and if you read a dangling pointer in production there's no real way to catch it AFAIK. Last I heard (admittedly been a few years since I checked), you're not supposed to deploy ASan builds because they actually increase the attack surface.
So, your program's memory is full of these dangling pointers, and at some point you will have a bug you didn't catch and use one. And you can't even write an assertion to check that it's valid. What do you propose?
And again to clarify, I'm not trying to advocate for hiding bugs. I want to catch them early (e.g. with assertions), but I also want to avoid reading garbage at runtime at all costs, because that's how programs get pwn'd.
> But use-after-free is worse than just double-frees
From an exploitability point of view they are largely equivalent.
As for the rest of your comment: my point of view is largely "you should catch these with Address Sanitizer in debug", so I don't usually write code like "I should assert if I marked this as freed by NULLing it out". If I actually need to check this for program logic, then of course I'll add something like this.
The macro you suggest would alleviate my concerns, I suppose, and it wouldn't really be fair for me to shoot that solution down solely because I personally don't like these kinds of assertions in production. So it's not a bad option by any means, other than my top-level comment of this requiring extra code. I know some libraries like to take a pointer-to-a-pointer so they can NULL it out for you, so that is an option for your wrapper. And a double-free that doesn't crash can sometimes open up exploitable bugs too since it messes with program invariants that you didn't expect. But these are much rarer than the typical "attacker controlled uninitialized memory ended up where it shouldn't" so it's not a big deal.
I’m not sure what it could have said after saying that programmers should have disciple after I mentioned that their thing required extra checks to work.
The “discipline” in this case (see the whole thread) is “have programmers remember to insert checks”, which has historically been a good way to have security holes crop up. So I’m not sure what was dishonest about it?
They argued that discipline is necessary, not sufficient, to produce good code. You represented the argument as: "discipline is sufficient for secure (good) code"
You took the original argument, changed it to be fallacious, and used it as a strawman. That's what was dishonest about it.
I don't think that's fair in this case because nulling out pointers isn't the first line of defense. If you forget to do it once, it's not going to cause a bug in and of itself. You can easily grep the code periodically to find any cases you missed.
I think that's the misunderstanding, then, because to me it seemed to be a defensive coding practice (I think it was certainly presented as such in the top comment). My "you need extra checks" claim was mostly aimed at the additional things you add on to your code assuming that you are now zeroing out freed pointers, which I think can lead to dangerous situations where you may come to rely on this being done consistently when it's a manual process that is easy to forget.
Left unsaid due to the fact I was out doing groceries this morning when I posted that was that I don't think this is even a very good practice in general, as I explained in more detail in other comments here.
Indeed, it shouldn't be a first line of defense (nulling + an assert seems reasonable, fwiw), and accessing a nulled out pointer is just as UB as any other UB. It's probably more likely to crash immediately in practice, but it's also easier for an optimizer to "see through", so you may get surprising optimizations if you get it wrong.
Honestly, unless you really cannot afford it time-budget wise, I would just ship everything with ASAN, UBSAN, etc. and deal with the crash reports.
Shipping code with Address Sanitizer enabled is generally not advisable; it has fairly high overhead. You should absolutely use it during testing, though!
>> NULL is not guaranteed to be the stored as zeros
> Is that a real issue, though?
Of course, it's not, but that's one of those factoids that everyone learns at some point and feels like needing to rub it into everyone else's face assuming that these poor schmucks are as oblivious to it as they once were. A circle of life and all that.
Forgive me for encouraging the adoption of portable, compliant code to those who may not otherwise be aware of it. If you want to assume all the world’s an x86 that’s great but you should at least know what part of your code is going to be wrong elsewhere.
