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Add notes about async signal safety

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This commit is contained in:
losfair 2018-11-24 22:25:32 +08:00
parent 5de9f0f7b4
commit a316253b94
1 changed files with 11 additions and 3 deletions
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14
src/recovery.rs
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@ -1,4 +1,8 @@
//! When a WebAssembly module triggers any traps, we perform recovery here. //! When a WebAssembly module triggers any traps, we perform recovery here.
//!
//! This module uses TLS (thread-local storage) to track recovery information. Since the four signals we're handling
//! are very special, the async signal unsafety of Rust's TLS implementation generally does not affect the correctness here
//! unless you have memory unsafety elsewhere in your code.
use std::cell::UnsafeCell; use std::cell::UnsafeCell;
@ -13,8 +17,8 @@ thread_local! {
static SETJMP_BUFFER: UnsafeCell<[::nix::libc::c_int; SETJMP_BUFFER_LEN]> = UnsafeCell::new([0; SETJMP_BUFFER_LEN]); static SETJMP_BUFFER: UnsafeCell<[::nix::libc::c_int; SETJMP_BUFFER_LEN]> = UnsafeCell::new([0; SETJMP_BUFFER_LEN]);
} }
/// Calls a function with longjmp receiver installed. The function must be compiled from WebAssembly; /// Calls a WebAssembly function with longjmp receiver installed. If a non-WebAssembly function is passed in,
/// Otherwise, the behavior is undefined. /// the behavior of protected_call is undefined.
pub unsafe fn protected_call<T, R>(f: fn(T) -> R, p: T) -> Result<R, i32> { pub unsafe fn protected_call<T, R>(f: fn(T) -> R, p: T) -> Result<R, i32> {
let jmp_buf = SETJMP_BUFFER.with(|buf| buf.get()); let jmp_buf = SETJMP_BUFFER.with(|buf| buf.get());
let prev_jmp_buf = *jmp_buf; let prev_jmp_buf = *jmp_buf;
@ -24,7 +28,7 @@ pub unsafe fn protected_call<T, R>(f: fn(T) -> R, p: T) -> Result<R, i32> {
*jmp_buf = prev_jmp_buf; *jmp_buf = prev_jmp_buf;
Err(signum) Err(signum)
} else { } else {
let ret = f(p); let ret = f(p); // TODO: Switch stack?
*jmp_buf = prev_jmp_buf; *jmp_buf = prev_jmp_buf;
Ok(ret) Ok(ret)
} }
@ -32,6 +36,10 @@ pub unsafe fn protected_call<T, R>(f: fn(T) -> R, p: T) -> Result<R, i32> {
/// Unwinds to last protected_call. /// Unwinds to last protected_call.
pub unsafe fn do_unwind(signum: i32) -> ! { pub unsafe fn do_unwind(signum: i32) -> ! {
// Since do_unwind is only expected to get called from WebAssembly code which doesn't hold any host resources (locks etc.)
// itself, accessing TLS here is safe. In case any other code calls this, it often indicates a memory safety bug and you should
// temporarily disable the signal handlers to debug it.
let jmp_buf = SETJMP_BUFFER.with(|buf| buf.get()); let jmp_buf = SETJMP_BUFFER.with(|buf| buf.get());
if *jmp_buf == [0; SETJMP_BUFFER_LEN] { if *jmp_buf == [0; SETJMP_BUFFER_LEN] {
::std::process::abort(); ::std::process::abort();