Add `__rust_probestack` intrinsic

Will be required for rust-lang/rust#42816

src/lib.rs

@@ -53,3 +53,5 @@ pub mod arm;
 
 #[cfg(target_arch = "x86_64")]
 pub mod x86_64;
+
+pub mod probestack;

src/probestack.rs

@@ -0,0 +1,112 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! This module defines the `__rust_probestack` intrinsic which is used in the
+//! implementation of "stack probes" on certain platforms.
+//!
+//! The purpose of a stack probe is to provide a static guarantee that if a
+//! thread has a guard page then a stack overflow is guaranteed to hit that
+//! guard page. If a function did not have a stack probe then there's a risk of
+//! having a stack frame *larger* than the guard page, so a function call could
+//! skip over the guard page entirely and then later hit maybe the heap or
+//! another thread, possibly leading to security vulnerabilities such as [The
+//! Stack Clash], for example.
+//!
+//! [The Stack Clash]: https://blog.qualys.com/securitylabs/2017/06/19/the-stack-clash
+//!
+//! The `__rust_probestack` is called in the prologue of functions whose stack
+//! size is larger than the guard page, for example larger than 4096 bytes on
+//! x86. This function is then responsible for "touching" all pages relevant to
+//! the stack to ensure that that if any of them are the guard page we'll hit
+//! them guaranteed.
+//!
+//! The precise ABI for how this function operates is defined by LLVM. There's
+//! no real documentation as to what this is, so you'd basically need to read
+//! the LLVM source code for reference. Often though the test cases can be
+//! illuminating as to the ABI that's generated, or just looking at the output
+//! of `llc`.
+//!
+//! Note that `#[naked]` is typically used here for the stack probe because the
+//! ABI corresponds to no actual ABI.
+//!
+//! Finally it's worth noting that at the time of this writing LLVM only has
+//! support for stack probes on x86 and x86_64. There's no support for stack
+//! probes on any other architecture like ARM or PowerPC64. LLVM I'm sure would
+//! be more than welcome to accept such a change!
+
+#![cfg(not(windows))] // Windows already has builtins to do this
+
+#[naked]
+#[no_mangle]
+#[cfg(target_arch = "x86_64")]
+pub unsafe extern fn __rust_probestack() {
+    // Our goal here is to touch each page between %rsp+8 and %rsp+8-%rax,
+    // ensuring that if any pages are unmapped we'll make a page fault.
+    //
+    // The ABI here is that the stack frame size is located in `%eax`. Upon
+    // return we're not supposed to modify `%esp` or `%eax`.
+    asm!("
+        lea    8(%rsp),%r11     // rsp before calling this routine -> r11
+
+        // Main loop, taken in one page increments. We're decrementing r11 by
+        // a page each time until there's less than a page remaining. We're
+        // guaranteed that this function isn't called unless there's more than a
+        // page needed
+    2:
+        sub    $$0x1000,%r11
+        test   %r11,(%r11)
+        sub    $$0x1000,%rax
+        cmp    $$0x1000,%rax
+        ja     2b
+
+        // Finish up the last remaining stack space requested, getting the last
+        // bits out of rax
+        sub    %rax,%r11
+        test   %r11,(%r11)
+
+        // We now know that %r11 is (%rsp + 8 - %rax) so to recover rax
+        // we calculate (%rsp + 8) - %r11 which will give us %rax
+        lea    8(%rsp),%rax
+        sub    %r11,%rax
+
+        ret
+    ");
+    ::core::intrinsics::unreachable();
+}
+
+#[naked]
+#[no_mangle]
+#[cfg(target_arch = "x86")]
+pub unsafe extern fn __rust_probestack() {
+    // This is the same as x86_64 above, only translated for 32-bit sizes. Note
+    // that on Unix we're expected to restore everything as it was, this
+    // function basically can't tamper with anything.
+    //
+    // The ABI here is the same as x86_64, except everything is 32-bits large.
+    asm!("
+        push   %ecx
+        lea    8(%esp),%ecx
+    2:
+        sub    $$0x1000,%ecx
+        test   %ecx,(%ecx)
+        sub    $$0x1000,%eax
+        cmp    $$0x1000,%eax
+        ja     2b
+
+        sub    %eax,%ecx
+        test   %ecx,(%ecx)
+
+        lea    8(%esp),%eax
+        sub    %ecx,%eax
+        pop    %ecx
+        ret
+    ");
+    ::core::intrinsics::unreachable();
+}