1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
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
}