Make SIGSEGV handler emit nicer backtraces

compiler/rustc_driver_impl/src/lib.rs

@@ -78,6 +78,15 @@ pub mod pretty;
 #[macro_use]
 mod print;
 mod session_diagnostics;
+#[cfg(all(unix, any(target_env = "gnu", target_os = "macos")))]
+mod signal_handler;
+
+#[cfg(not(all(unix, any(target_env = "gnu", target_os = "macos"))))]
+mod signal_handler {
+    /// On platforms which don't support our signal handler's requirements,
+    /// simply use the default signal handler provided by std.
+    pub(super) fn install() {}
+}
 
 use crate::session_diagnostics::{
     RLinkEmptyVersionNumber, RLinkEncodingVersionMismatch, RLinkRustcVersionMismatch,
@@ -1427,72 +1436,6 @@ pub fn init_env_logger(handler: &EarlyErrorHandler, env: &str) {
     }
 }
 
-#[cfg(all(unix, any(target_env = "gnu", target_os = "macos")))]
-mod signal_handler {
-    extern "C" {
-        fn backtrace_symbols_fd(
-            buffer: *const *mut libc::c_void,
-            size: libc::c_int,
-            fd: libc::c_int,
-        );
-    }
-
-    extern "C" fn print_stack_trace(_: libc::c_int) {
-        const MAX_FRAMES: usize = 256;
-        static mut STACK_TRACE: [*mut libc::c_void; MAX_FRAMES] =
-            [std::ptr::null_mut(); MAX_FRAMES];
-        unsafe {
-            let depth = libc::backtrace(STACK_TRACE.as_mut_ptr(), MAX_FRAMES as i32);
-            if depth == 0 {
-                return;
-            }
-            backtrace_symbols_fd(STACK_TRACE.as_ptr(), depth, 2);
-        }
-    }
-
-    /// When an error signal (such as SIGABRT or SIGSEGV) is delivered to the
-    /// process, print a stack trace and then exit.
-    pub(super) fn install() {
-        use std::alloc::{alloc, Layout};
-
-        unsafe {
-            let alt_stack_size: usize = min_sigstack_size() + 64 * 1024;
-            let mut alt_stack: libc::stack_t = std::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, std::ptr::null_mut());
-
-            let mut sa: libc::sigaction = std::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, std::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
-    }
-}
-
-#[cfg(not(all(unix, any(target_env = "gnu", target_os = "macos"))))]
-mod signal_handler {
-    pub(super) fn install() {}
-}
-
 pub fn main() -> ! {
     let start_time = Instant::now();
     let start_rss = get_resident_set_size();

compiler/rustc_driver_impl/src/signal_handler.rs

@@ -0,0 +1,142 @@
+//! 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
+}