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//! Signal handler for rustc
//! Primarily used to extract a backtrace from stack overflow
use std::alloc::{alloc, Layout};
use std::{fmt, mem, ptr};
extern "C" {
fn backtrace_symbols_fd(buffer: *const *mut libc::c_void, size: libc::c_int, fd: libc::c_int);
}
fn backtrace_stderr(buffer: &[*mut libc::c_void]) {
let size = buffer.len().try_into().unwrap_or_default();
unsafe { backtrace_symbols_fd(buffer.as_ptr(), size, libc::STDERR_FILENO) };
}
/// Unbuffered, unsynchronized writer to stderr.
///
/// Only acceptable because everything will end soon anyways.
struct RawStderr(());
impl fmt::Write for RawStderr {
fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
let ret = unsafe { libc::write(libc::STDERR_FILENO, s.as_ptr().cast(), s.len()) };
if ret == -1 { Err(fmt::Error) } else { Ok(()) }
}
}
/// We don't really care how many bytes we actually get out. SIGSEGV comes for our head.
/// Splash stderr with letters of our own blood to warn our friends about the monster.
macro raw_errln($tokens:tt) {
let _ = ::core::fmt::Write::write_fmt(&mut RawStderr(()), format_args!($tokens));
let _ = ::core::fmt::Write::write_char(&mut RawStderr(()), '\n');
}
/// Signal handler installed for SIGSEGV
extern "C" fn print_stack_trace(_: libc::c_int) {
const MAX_FRAMES: usize = 256;
// Reserve data segment so we don't have to malloc in a signal handler, which might fail
// in incredibly undesirable and unexpected ways due to e.g. the allocator deadlocking
static mut STACK_TRACE: [*mut libc::c_void; MAX_FRAMES] = [ptr::null_mut(); MAX_FRAMES];
let stack = unsafe {
// Collect return addresses
let depth = libc::backtrace(STACK_TRACE.as_mut_ptr(), MAX_FRAMES as i32);
if depth == 0 {
return;
}
&STACK_TRACE.as_slice()[0..(depth as _)]
};
// Just a stack trace is cryptic. Explain what we're doing.
raw_errln!("error: rustc interrupted by SIGSEGV, printing backtrace\n");
let mut written = 1;
let mut consumed = 0;
// Begin elaborating return addrs into symbols and writing them directly to stderr
// Most backtraces are stack overflow, most stack overflows are from recursion
// Check for cycles before writing 250 lines of the same ~5 symbols
let cycled = |(runner, walker)| runner == walker;
let mut cyclic = false;
if let Some(period) = stack.iter().skip(1).step_by(2).zip(stack).position(cycled) {
let period = period.saturating_add(1); // avoid "what if wrapped?" branches
let Some(offset) = stack.iter().skip(period).zip(stack).position(cycled) else {
// impossible.
return;
};
// Count matching trace slices, else we could miscount "biphasic cycles"
// with the same period + loop entry but a different inner loop
let next_cycle = stack[offset..].chunks_exact(period).skip(1);
let cycles = 1 + next_cycle
.zip(stack[offset..].chunks_exact(period))
.filter(|(next, prev)| next == prev)
.count();
backtrace_stderr(&stack[..offset]);
written += offset;
consumed += offset;
if cycles > 1 {
raw_errln!("\n### cycle encountered after {offset} frames with period {period}");
backtrace_stderr(&stack[consumed..consumed + period]);
raw_errln!("### recursed {cycles} times\n");
written += period + 4;
consumed += period * cycles;
cyclic = true;
};
}
let rem = &stack[consumed..];
backtrace_stderr(rem);
raw_errln!("");
written += rem.len() + 1;
let random_depth = || 8 * 16; // chosen by random diceroll (2d20)
if cyclic || stack.len() > random_depth() {
// technically speculation, but assert it with confidence anyway.
// rustc only arrived in this signal handler because bad things happened
// and this message is for explaining it's not the programmer's fault
raw_errln!("note: rustc unexpectedly overflowed its stack! this is a bug");
written += 1;
}
if stack.len() == MAX_FRAMES {
raw_errln!("note: maximum backtrace depth reached, frames may have been lost");
written += 1;
}
raw_errln!("note: we would appreciate a report at https://github.com/rust-lang/rust");
written += 1;
if written > 24 {
// We probably just scrolled the earlier "we got SIGSEGV" message off the terminal
raw_errln!("note: backtrace dumped due to SIGSEGV! resuming signal");
};
}
/// When SIGSEGV is delivered to the process, print a stack trace and then exit.
pub(super) fn install() {
unsafe {
let alt_stack_size: usize = min_sigstack_size() + 64 * 1024;
let mut alt_stack: libc::stack_t = mem::zeroed();
alt_stack.ss_sp = alloc(Layout::from_size_align(alt_stack_size, 1).unwrap()).cast();
alt_stack.ss_size = alt_stack_size;
libc::sigaltstack(&alt_stack, ptr::null_mut());
let mut sa: libc::sigaction = mem::zeroed();
sa.sa_sigaction = print_stack_trace as libc::sighandler_t;
sa.sa_flags = libc::SA_NODEFER | libc::SA_RESETHAND | libc::SA_ONSTACK;
libc::sigemptyset(&mut sa.sa_mask);
libc::sigaction(libc::SIGSEGV, &sa, ptr::null_mut());
}
}
/// Modern kernels on modern hardware can have dynamic signal stack sizes.
#[cfg(any(target_os = "linux", target_os = "android"))]
fn min_sigstack_size() -> usize {
const AT_MINSIGSTKSZ: core::ffi::c_ulong = 51;
let dynamic_sigstksz = unsafe { libc::getauxval(AT_MINSIGSTKSZ) };
// If getauxval couldn't find the entry, it returns 0,
// so take the higher of the "constant" and auxval.
// This transparently supports older kernels which don't provide AT_MINSIGSTKSZ
libc::MINSIGSTKSZ.max(dynamic_sigstksz as _)
}
/// Not all OS support hardware where this is needed.
#[cfg(not(any(target_os = "linux", target_os = "android")))]
fn min_sigstack_size() -> usize {
libc::MINSIGSTKSZ
}
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