Change SigAction::flags to use from_bits_truncated

[Detegr]

On Linux, if the signal trampoline code is in the C library, sigaction
sets the SA_RESTORER flag (0x04000000) in the sa_flags field of old
sigaction (see sigreturn(2)).

This is not intended for application use and is missing from SaFlags,
therefore from_bits fails and unwrapping panics the user program.

This fix just drops the bits that are not defined in SaFlags.

[asomers]

Do you have an example testcase that demonstrates the failure?

[Detegr]

Sure.

extern crate nix;
use nix::sys::signal;

extern "C" fn handler(_: nix::libc::c_int) {}
fn main() {
    let act = signal::SigAction::new(
        signal::SigHandler::Handler(handler),
        signal::SaFlags::empty(),
        signal::SigSet::empty(),
    );
    let oact = unsafe { signal::sigaction(signal::SIGINT, &act) }.unwrap();
    println!("{:?}", oact.flags()); // Works, the default handler flags do not have SA_RESTORER set
    let oact = unsafe { signal::sigaction(signal::SIGINT, &act) }.unwrap();
    println!("{:?}", oact.flags()); // Panics, as sa_flags contains SA_RESTORER
}

[asomers]

Yeah, that fails for me too, on Linux but not FreeBSD. Could you please add it to tests/sys/test_signal.rs?

[Detegr]

Added.

[asomers]

bors r+

[bors]

Build succeeded

• buildbot/nix-rust/nix amd64_fbsd11
• buildbot/nix-rust/nix i386_fbsd11
• continuous-integration/travis-ci/push

[Susurrus]

This needed a CHANGELOG entry indicating the fix as well. @Detegr Would you be willing to submit one?

[Detegr]

Sure. Is it okay to have the commit in this PR or should I open a new one?

[Susurrus]

Since this was merged, you should open a new one. As all you're changing is the CHANGELOG, put "[skip ci]" at the start of the commit message. Thanks!