Ending needs work, but most of the content is good to go.

_posts/2019-02-14-insane-allocators.md

@@ -1,44 +1,43 @@
 ---
 layout: post
-title: "Insane Allocators: SEGFAULTs in safe Rust"
+title: "Insane Allocators: segfaults in safe Rust"
 description: "\"Trusting trust\" with allocators."
 category: rust, memory
 tags: []
 ---
 
-Having recently spent a lot of time studying the weird ways that
+Having recently spent a lot of time down rabbit holes looking at how
 [Rust uses memory](/2019/02/understanding-allocations-in-rust.html),
 I like to think I finally understand the rules well enough to
-break them. Specifically - what are the assumptions that underpin
-Rust's memory model? It wasn't a question particularly relevant
-to understanding how Rust allocates memory, but is certainly worth
-discussing as an addendum. Let's finish off this series on Rust and
-memory by breaking the most important rules Rust has!
+break them. See, Rust will go so far as to claim:
 
-Rust's whole shtick is that it's "memory safe." In practice,
-this (should) mean that there's no undefined behavior in safe Rust,
-because the compiler/borrow checker makes sure you can't get yourself
-into a situation where you misuse or corrupt memory. But is it possible
-for Rust programs, *written without using `unsafe`*, to encounter a
-[segfault](https://en.wikipedia.org/wiki/Segmentation_fault)?
+> If all you do is write Safe Rust, you will never have to worry about type-safety or memory-safety.
+> You will never endure a dangling pointer, a use-after-free, or any other kind of Undefined Behavior.
 
-Of course it is! Using an unmodified compiler, I can build a simple
-"Hello, world!" application that dies due to memory corruption:
+-- [The Nomicon](https://doc.rust-lang.org/nomicon/meet-safe-and-unsafe.html)
+
+...and subject to (relatively infrequent)
+[borrow checker bugs](https://github.com/rust-lang/rust/labels/A-borrow-checker),
+it's correct. There's ongoing work to [formalize](https://plv.mpi-sws.org/rustbelt/popl18/)
+the rules and *prove* that Rust is safe, but for our purposes it's a reasonable assumption.
+
+Until it isn't. It's totally possible for "safe" Rust programs
+(under contrived circumstances) to encounter memory corruption.
+It's even possible for these programs to
+["segfault"](https://en.wikipedia.org/wiki/Segmentation_fault)
+when using an unmodified compiler:
 
 <script id="asciicast-ENIpRYpdDazCkppanf3LSCetX" src="https://asciinema.org/a/ENIpRYpdDazCkppanf3LSCetX.js" async></script>
 
 # Wait, wat?
 
-There's obviously something nefarious going on. I mean, why would
-anyone use `sudo` to run the `rustc` compiler?
+[Wat indeed.](https://www.destroyallsoftware.com/talks/wat)
 
-And for that matter, why does Rust 1.31.0 behave differently
-from Rust 1.32.0?
-
-To pull off this chicanery, I'm making use of a special environment
-variable in Linux called [`LD_PRELOAD`](https://blog.fpmurphy.com/2012/09/all-about-ld_preload.html).
-I won't go into detail the way [Matt Godbolt does](https://www.youtube.com/watch?v=dOfucXtyEsU),
-but the important bit is this: I can insert my own code in place of
+There are two tricks used to pull this off. First, I'm making
+use of a special environment variable in Linux called
+[`LD_PRELOAD`](https://blog.fpmurphy.com/2012/09/all-about-ld_preload.html).
+Matt Godbolt goes into [way more detail](https://www.youtube.com/watch?v=dOfucXtyEsU)
+than I can cover, but the important bit is this: I can insert my own code in place of
 functions typically implemented by the [C standard library](https://www.gnu.org/software/libc/).
 
 Second, there's a very special implementation of [`malloc`](https://linux.die.net/man/3/malloc)
@@ -57,15 +56,52 @@ pub extern "C" fn malloc(size: usize) -> *mut c_void {
         // If we've never allocated anything, ask the operating system
         // for some memory...
         if ALLOC == null_mut() {
+            // Use a `libc` binding to avoid recursive malloc calls
             ALLOC = libc::malloc(size)
         }
-        // ...and then give that same section of memory to everyone,
-        // corrupting the location.
+        // ...and then give that same section of memory to everyone
+        // for all subsequent allocations, corrupting the location.
         return ALLOC;
     }
 }
 ```
 
-Now, there are two questions yet to answer:
-1. Why was `sudo` used to compile?
-2. Why did Rust 1.31 work when 1.32 didn't?
+So how is it possible to run the Rust compiler in this environment?
+`LD_PRELOAD` applies to all programs, so running `ls` will also
+lead to memory corruption and crashing! The answer is that `sudo`
+deletes environment variables like `LD_PRELOAD` and
+`LD_LIBRARY_PATH` when running commands; it's possible to
+crash `sudo` in the same way by using our evil `malloc`
+implementation.
+
+Finally, why does Rust 1.31 work, and 1.32 fail? The answer is in the
+release notes:
+[`jemalloc` is removed by default](https://blog.rust-lang.org/2019/01/17/Rust-1.32.0.html#jemalloc-is-removed-by-default).
+In Rust 1.28 through 1.31, programs were statically compiled against
+[jemalloc](http://jemalloc.net/) by default; our evil `malloc` implementation
+never gets invoked because the program goes straight to the operating
+system to request memory. However, it's still possible to trigger segfaults
+in Rust programs from  1.28 - 1.31 by using the
+[`System`](https://doc.rust-lang.org/std/alloc/struct.System.html)
+global allocator. Rust programs prior to 1.28 aren't subject to this
+`LD_PRELOAD` trick.
+
+# So what?
+
+It should be made very clear: the code demonstrated here isn't a
+security issue. "Safe" Rust programs are only crashing in these
+circumstances because the memory allocator is intentionally lying to it.
+Even in mission critical systems, there are a lot of concerns beyond memory allocation; the
+[F-35 Joint Strike Fighter coding standards](http://www.stroustrup.com/JSF-AV-rules.pdf)
+don't even give it a full page. 
+
+But this example does highlight an assumption of Rust's memory model
+that I haven't seen discussed much: **safe Rust is safe if, and only if,
+the allocator it relies on is "correct"**. And because writing a non-trivial allocator is
+[fundamentally unsafe](https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html#unsafety),
+safe Rust will always rely on unsafe Rust somewhere.
+
+That all said, know that "safe" Rust can only claim to be safe because it stands
+on the shoulders of incredible developers working on jemalloc,
+[kmalloc](https://linux-kernel-labs.github.io/master/labs/kernel_api.html#memory-allocation),
+and others.