vmm: fall back if CLONE_CLEAR_SIGHAND unsupported

This will allow the SIGWINCH listener to run on kernels older than
5.5, although on those kernels it will have to make 64 syscalls to
reset all the signal handlers.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

arch/src/aarch64/mod.rs

@@ -21,6 +21,8 @@ use std::fmt::Debug;
 use std::sync::{Arc, Mutex};
 use vm_memory::{Address, GuestAddress, GuestMemory, GuestMemoryAtomic, GuestUsize};
 
+pub const _NSIG: i32 = 65;
+
 /// Errors thrown while configuring aarch64 system.
 #[derive(Debug)]
 pub enum Error {

arch/src/lib.rs

@@ -82,7 +82,7 @@ pub mod aarch64;
 pub use aarch64::{
     arch_memory_regions, configure_system, configure_vcpu, fdt::DeviceInfoForFdt,
     get_host_cpu_phys_bits, initramfs_load_addr, layout, layout::CMDLINE_MAX_SIZE,
-    layout::IRQ_BASE, uefi, EntryPoint,
+    layout::IRQ_BASE, uefi, EntryPoint, _NSIG,
 };
 
 #[cfg(target_arch = "x86_64")]
@@ -92,7 +92,7 @@ pub mod x86_64;
 pub use x86_64::{
     arch_memory_regions, configure_system, configure_vcpu, generate_common_cpuid,
     get_host_cpu_phys_bits, initramfs_load_addr, layout, layout::CMDLINE_MAX_SIZE,
-    layout::CMDLINE_START, regs, CpuidFeatureEntry, EntryPoint,
+    layout::CMDLINE_START, regs, CpuidFeatureEntry, EntryPoint, _NSIG,
 };
 
 /// Safe wrapper for `sysconf(_SC_PAGESIZE)`.

arch/src/x86_64/mod.rs

@@ -53,6 +53,8 @@ const KVM_FEATURE_ASYNC_PF_VMEXIT_BIT: u8 = 10;
 #[cfg(feature = "tdx")]
 const KVM_FEATURE_STEAL_TIME_BIT: u8 = 5;
 
+pub const _NSIG: i32 = 65;
+
 #[derive(Debug, Copy, Clone)]
 /// Specifies the entry point address where the guest must start
 /// executing code, as well as which of the supported boot protocols

vmm/src/sigwinch_listener.rs

@@ -2,10 +2,11 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use crate::clone3::{clone3, clone_args, CLONE_CLEAR_SIGHAND};
+use arch::_NSIG;
 use libc::{
-    c_int, c_void, close, getpgrp, ioctl, pipe2, poll, pollfd, setsid, sigemptyset, siginfo_t,
-    sigprocmask, syscall, tcsetpgrp, SYS_close_range, ENOSYS, O_CLOEXEC, POLLERR, SIGWINCH,
-    SIG_SETMASK, STDERR_FILENO, TIOCSCTTY,
+    c_int, c_void, close, fork, getpgrp, ioctl, pipe2, poll, pollfd, setsid, sigemptyset,
+    siginfo_t, signal, sigprocmask, syscall, tcsetpgrp, SYS_close_range, EINVAL, ENOSYS, O_CLOEXEC,
+    POLLERR, SIGWINCH, SIG_DFL, SIG_SETMASK, STDERR_FILENO, TIOCSCTTY,
 };
 use seccompiler::{apply_filter, BpfProgram};
 use std::cell::RefCell;
@@ -176,6 +177,35 @@ fn sigwinch_listener_main(seccomp_filter: BpfProgram, tx: File, pty: File) -> !
     exit(0);
 }
 
+/// # Safety
+///
+/// Same as [`fork`].
+unsafe fn clone_clear_sighand() -> io::Result<u64> {
+    let mut args = clone_args::default();
+    args.flags |= CLONE_CLEAR_SIGHAND;
+    let r = clone3(&mut args, size_of::<clone_args>());
+    if r != -1 {
+        return Ok(r.try_into().unwrap());
+    }
+    let e = io::Error::last_os_error();
+    if e.raw_os_error() != Some(ENOSYS) && e.raw_os_error() != Some(EINVAL) {
+        return Err(e);
+    }
+
+    // If CLONE_CLEAR_SIGHAND isn't available, fall back to resetting
+    // all the signal handlers one by one.
+    let r = fork();
+    if r == -1 {
+        return Err(io::Error::last_os_error());
+    }
+    if r == 0 {
+        for signum in 1.._NSIG {
+            let _ = signal(signum, SIG_DFL);
+        }
+    }
+    Ok(r.try_into().unwrap())
+}
+
 pub fn start_sigwinch_listener(seccomp_filter: BpfProgram, tty_sub: File) -> io::Result<File> {
     let mut pipe = [-1; 2];
     // SAFETY: FFI call with valid arguments
@@ -188,16 +218,9 @@ pub fn start_sigwinch_listener(seccomp_filter: BpfProgram, tty_sub: File) -> io:
     // SAFETY: pipe[1] is valid
     let tx = unsafe { File::from_raw_fd(pipe[1]) };
 
-    let mut args = clone_args::default();
-    args.flags |= CLONE_CLEAR_SIGHAND;
-
     // SAFETY: FFI call
-    match unsafe { clone3(&mut args, size_of::<clone_args>()) } {
-        -1 => return Err(io::Error::last_os_error()),
-        0 => {
-            sigwinch_listener_main(seccomp_filter, tx, tty_sub);
-        }
-        _ => (),
+    if unsafe { clone_clear_sighand() }? == 0 {
+        sigwinch_listener_main(seccomp_filter, tx, tty_sub);
     }
 
     drop(tx);