Implement thread signal support

[jstarks]

Add a first layer of support for Linux signal handling. This includes support for:

• Signal delivery
• Signal handler configuration (rt_sigaction)
• Signal masks (rt_sigprocmask)
• Alternate signal stacks (sigaltstack)
• Returning from a signal handler (rt_sigreturn)
• Sending signals to self (tkill, tgkill)
• Delivering signals on hardware exceptions

Known to be missing:

• Cross-thread signaling
• Process-wide signals
• FPU/xsave state in signal contexts
• SIGALRM support (alarm, setitimer)
• Misc. signal syscalls (e.g., rt_sigtimedwait, rt_sigpending)
• SIGSTOP support

[jstarks]

@CvvT, how important is the alarm/setitimer functionality that this removes? Do we need to add that back before this can be merged?

[CvvT]

@CvvT, how important is the alarm/setitimer functionality that this removes? Do we need to add that back before this can be merged?

Not necessary. They are only needed for running UnixBench, which I haven’t upstreamed yet. It’s fine to remove them.

[wdcui]

Thank you for this substantial change, John. I left some comments. I also requested @CvvT to review this PR.

[CvvT]

LGTM! Thanks!

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/enum_missing.ron

Failed in:
  enum litebox_common_linux::Signal, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:870
  enum litebox_common_linux::SigmaskHow, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:959

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
        ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
  variant SyscallRequest::Ioctl 18 -> 22 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1844
  variant SyscallRequest::Pread64 19 -> 23 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1848
  variant SyscallRequest::Pwrite64 20 -> 24 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1854
  variant SyscallRequest::Readv 21 -> 25 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1860
  variant SyscallRequest::Writev 22 -> 26 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1865
  variant SyscallRequest::Access 23 -> 27 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1870
  variant SyscallRequest::Madvise 24 -> 28 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1874
  variant SyscallRequest::Dup 25 -> 29 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1879
  variant SyscallRequest::Socket 26 -> 30 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1884
  variant SyscallRequest::Connect 27 -> 31 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1894
  variant SyscallRequest::Accept 28 -> 32 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1899
  variant SyscallRequest::Sendto 29 -> 33 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1905
  variant SyscallRequest::Sendmsg 30 -> 34 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1913
  variant SyscallRequest::Recvfrom 31 -> 35 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1918
  variant SyscallRequest::Bind 32 -> 36 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1926
  variant SyscallRequest::Listen 33 -> 37 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1931
  variant SyscallRequest::Setsockopt 34 -> 38 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1935
  variant SyscallRequest::Getsockopt 35 -> 39 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1941
  variant SyscallRequest::Getsockname 36 -> 40 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1947
  variant SyscallRequest::Getpeername 37 -> 41 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1952
  variant SyscallRequest::Uname 38 -> 42 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1957
  variant SyscallRequest::Fcntl 39 -> 43 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1960
  variant SyscallRequest::Getcwd 40 -> 44 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1964
  variant SyscallRequest::EpollCtl 41 -> 45 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1968
  variant SyscallRequest::EpollPwait 42 -> 46 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1974
  variant SyscallRequest::EpollCreate 43 -> 47 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1982
  variant SyscallRequest::Ppoll 44 -> 48 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1985
  variant SyscallRequest::Pselect 45 -> 49 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:1992
  variant SyscallRequest::ArchPrctl 46 -> 50 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2000
  variant SyscallRequest::Readlink 47 -> 51 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2003
  variant SyscallRequest::Readlinkat 48 -> 52 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2008
  variant SyscallRequest::Openat 49 -> 53 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2014
  variant SyscallRequest::Ftruncate 50 -> 54 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2020
  variant SyscallRequest::Unlinkat 51 -> 55 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2024
  variant SyscallRequest::Newfstatat 52 -> 56 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2030
  variant SyscallRequest::Eventfd2 53 -> 57 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2043
  variant SyscallRequest::Pipe2 54 -> 58 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2047
  variant SyscallRequest::Clone 55 -> 59 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2051
  variant SyscallRequest::Clone3 56 -> 60 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2054
  variant SyscallRequest::SetThreadArea 57 -> 61 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2059
  variant SyscallRequest::ClockGettime 58 -> 62 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2062
  variant SyscallRequest::ClockGetres 59 -> 63 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2066
  variant SyscallRequest::ClockNanosleep 60 -> 64 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2070
  variant SyscallRequest::Gettimeofday 61 -> 65 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2076
  variant SyscallRequest::Time 62 -> 66 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2080
  variant SyscallRequest::Getrlimit 63 -> 67 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2083
  variant SyscallRequest::Setrlimit 64 -> 68 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2087
  variant SyscallRequest::Prlimit 65 -> 69 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2091
  variant SyscallRequest::SetTidAddress 66 -> 70 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2102
  variant SyscallRequest::Gettid 67 -> 71 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2105
  variant SyscallRequest::SetRobustList 68 -> 72 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2106
  variant SyscallRequest::GetRobustList 69 -> 73 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2109
  variant SyscallRequest::GetRandom 70 -> 74 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2114
  variant SyscallRequest::Getpid 71 -> 75 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2119
  variant SyscallRequest::Getppid 72 -> 76 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2120
  variant SyscallRequest::Getuid 73 -> 77 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2121
  variant SyscallRequest::Geteuid 74 -> 78 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2122
  variant SyscallRequest::Getgid 75 -> 79 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2123
  variant SyscallRequest::Getegid 76 -> 80 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2124
  variant SyscallRequest::Sysinfo 77 -> 81 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2125
  variant SyscallRequest::CapGet 78 -> 82 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2128
  variant SyscallRequest::GetDirent64 79 -> 83 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2132
  variant SyscallRequest::SchedGetAffinity 80 -> 84 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2137
  variant SyscallRequest::SchedYield 81 -> 85 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2142
  variant SyscallRequest::Futex 82 -> 86 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2143
  variant SyscallRequest::Execve 83 -> 87 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2146
  variant SyscallRequest::Umask 84 -> 88 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2151
  variant SyscallRequest::Prctl 85 -> 89 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2154
  variant SyscallRequest::Alarm 86 -> 90 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2157
  variant SyscallRequest::SetITimer 88 -> 91 in /home/runner/work/litebox/litebox/litebox_common_linux/src/lib.rs:2160

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/enum_variant_missing.ron

Failed in:
  variant SyscallRequest::ThreadKill, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2236
  variant PunchthroughSyscall::RtSigprocmask, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2925
  variant PunchthroughSyscall::RtSigaction, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2943
  variant PunchthroughSyscall::RtSigreturn, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2952
  variant PunchthroughSyscall::Alarm, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2959
  variant PunchthroughSyscall::ThreadKill, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2961
  variant PunchthroughSyscall::SetITimer, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:2966

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/struct_missing.ron

Failed in:
  struct litebox_common_linux::SigSet, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:914
  struct litebox_common_linux::SaFlags, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:934
  struct litebox_common_linux::SigAction, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_common_linux/src/lib.rs:950

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#major-any-change-to-trait-item-signatures
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/trait_method_missing.ron

Failed in:
  method rt_sigprocmask of trait HostInterface, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_platform_linux_kernel/src/lib.rs:368

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#major-any-change-to-trait-item-signatures
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.45.0/src/lints/trait_method_missing.ron

Failed in:
  method rt_sigprocmask of trait HostInterface, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/6ae9741a9c8ee766ae681e5369a29bb06040e3f5/litebox_platform_lvbs/src/lib.rs:614