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+---
+layout: post
+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
+[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!
+
+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)?
+
+Of course it is! Using an unmodified compiler, I can build a simple
+"Hello, world!" application that dies due to memory corruption:
+
+<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?
+
+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
+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)
+that is being picked up by `LD_PRELOAD`:
+
+```rust
+use std::ffi::c_void;
+use std::ptr::null_mut;
+
+// Start off with an empty pointer
+static mut ALLOC: *mut c_void = null_mut();
+
+#[no_mangle]
+pub extern "C" fn malloc(size: usize) -> *mut c_void {
+    unsafe { 
+        // If we've never allocated anything, ask the operating system
+        // for some memory...
+        if ALLOC == null_mut() {
+            ALLOC = libc::malloc(size)
+        }
+        // ...and then give that same section of memory to everyone,
+        // 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?