std.os.linux: export kernel-sized sigset_t and operations

The kernel ABI sigset_t is smaller than the glibc one.  Define the
right-sized sigset_t and fixup the sigaction() wrapper to leverage it.
The Sigaction wrapper here is not an ABI, so relax it (drop the "extern"
and the "restorer" fields), the existing `k_sigaction` is the ABI
sigaction struct.

Linux defines `sigset_t` with a c_ulong, so it can be 32-bit or 64-bit,
depending on the platform.  This can make a difference on big-endian
systems.

Patch up `ucontext_t` so that this change doesn't impact its layout.
AFAICT, its currently the glibc layout.

lib/std/os/linux.zig

@@ -1745,8 +1745,9 @@ pub fn sigprocmask(flags: u32, noalias set: ?*const sigset_t, noalias oldset: ?*
     return syscall4(.rt_sigprocmask, flags, @intFromPtr(set), @intFromPtr(oldset), NSIG / 8);
 }
 
-pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) usize {
-    assert(sig >= 1);
+pub fn sigaction(sig: u8, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) usize {
+    assert(sig > 0);
+    assert(sig < NSIG);
     assert(sig != SIG.KILL);
     assert(sig != SIG.STOP);
 
@@ -1755,14 +1756,15 @@ pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigact
     const mask_size = @sizeOf(@TypeOf(ksa.mask));
 
     if (act) |new| {
+        // Zig needs to install our arch restorer function with any signal handler, so
+        // must copy the Sigaction struct
         const restorer_fn = if ((new.flags & SA.SIGINFO) != 0) &restore_rt else &restore;
         ksa = k_sigaction{
             .handler = new.handler.handler,
             .flags = new.flags | SA.RESTORER,
-            .mask = undefined,
+            .mask = new.mask,
             .restorer = @ptrCast(restorer_fn),
         };
-        @memcpy(@as([*]u8, @ptrCast(&ksa.mask))[0..mask_size], @as([*]const u8, @ptrCast(&new.mask)));
     }
 
     const ksa_arg = if (act != null) @intFromPtr(&ksa) else 0;
@@ -1777,8 +1779,8 @@ pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigact
 
     if (oact) |old| {
         old.handler.handler = oldksa.handler;
-        old.flags = @as(c_uint, @truncate(oldksa.flags));
-        @memcpy(@as([*]u8, @ptrCast(&old.mask))[0..mask_size], @as([*]const u8, @ptrCast(&oldksa.mask)));
+        old.flags = oldksa.flags;
+        old.mask = oldksa.mask;
     }
 
     return 0;
@@ -1786,25 +1788,31 @@ pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigact
 
 const usize_bits = @typeInfo(usize).int.bits;
 
-pub const sigset_t = [1024 / 32]u32;
+/// Defined as one greater than the largest defined signal number.
+pub const NSIG = if (is_mips) 128 else 65;
+
+/// Linux kernel's sigset_t.  This is logically 64-bit on most
+/// architectures, but 128-bit on MIPS.  Contrast with the 1024-bit
+/// sigset_t exported by the glibc and musl library ABIs.
+pub const sigset_t = [(NSIG - 1 + 7) / @bitSizeOf(SigsetElement)]SigsetElement;
+
+const SigsetElement = c_ulong;
 
 const sigset_len = @typeInfo(sigset_t).array.len;
 
-/// Empty set to initialize sigset_t instances from.
-pub const empty_sigset: sigset_t = [_]u32{0} ** sigset_len;
+/// Empty set to initialize sigset_t instances from.  No need for `sigemptyset`.
+pub const empty_sigset: sigset_t = [_]SigsetElement{0} ** sigset_len;
 
-pub const filled_sigset: sigset_t = [_]u32{0x7fff_ffff} ++ [_]u32{0} ** (sigset_len - 1);
+/// Filled set to initialize sigset_t instances from.  No need for `sigfillset`.
+pub const filled_sigset: sigset_t = [_]SigsetElement{~@as(SigsetElement, 0)} ** sigset_len;
 
-pub const all_mask: sigset_t = [_]u32{0xffff_ffff} ** sigset_len;
-
-fn sigset_bit_index(sig: usize) struct { word: usize, mask: u32 } {
+fn sigset_bit_index(sig: usize) struct { word: usize, mask: SigsetElement } {
     assert(sig > 0);
     assert(sig < NSIG);
     const bit = sig - 1;
-    const shift = @as(u5, @truncate(bit % 32));
     return .{
-        .word = bit / 32,
-        .mask = @as(u32, 1) << shift,
+        .word = bit / @bitSizeOf(SigsetElement),
+        .mask = @as(SigsetElement, 1) << @truncate(bit % @bitSizeOf(SigsetElement)),
     };
 }
 
@@ -5479,38 +5487,33 @@ pub const TFD = switch (native_arch) {
     },
 };
 
-/// NSIG is the total number of signals defined.
-/// As signal numbers are sequential, NSIG is one greater than the largest defined signal number.
-pub const NSIG = if (is_mips) 128 else 65;
-
 const k_sigaction_funcs = struct {
     const handler = ?*align(1) const fn (i32) callconv(.c) void;
     const restorer = *const fn () callconv(.c) void;
 };
 
+/// Kernel sigaction struct, as expected by the `rt_sigaction` syscall.  Includes restorer.
 pub const k_sigaction = switch (native_arch) {
-    .mips, .mipsel => extern struct {
+    .mips, .mipsel, .mips64, .mips64el => extern struct {
         flags: c_uint,
         handler: k_sigaction_funcs.handler,
-        mask: [4]c_ulong,
-        restorer: k_sigaction_funcs.restorer,
-    },
-    .mips64, .mips64el => extern struct {
-        flags: c_uint,
-        handler: k_sigaction_funcs.handler,
-        mask: [2]c_ulong,
+        mask: sigset_t,
         restorer: k_sigaction_funcs.restorer,
     },
     else => extern struct {
         handler: k_sigaction_funcs.handler,
         flags: c_ulong,
         restorer: k_sigaction_funcs.restorer,
-        mask: [2]c_uint,
+        mask: sigset_t,
     },
 };
 
+/// Kernel Sigaction wrapper for the actual ABI `k_sigaction`.  The Zig
+/// linux.zig wrapper library still does some pre-processing on
+/// sigaction() calls (to add the `restorer` field).
+///
 /// Renamed from `sigaction` to `Sigaction` to avoid conflict with the syscall.
-pub const Sigaction = extern struct {
+pub const Sigaction = struct {
     pub const handler_fn = *align(1) const fn (i32) callconv(.c) void;
     pub const sigaction_fn = *const fn (i32, *const siginfo_t, ?*anyopaque) callconv(.c) void;
 
@@ -5519,8 +5522,10 @@ pub const Sigaction = extern struct {
         sigaction: ?sigaction_fn,
     },
     mask: sigset_t,
-    flags: c_uint,
-    restorer: ?*const fn () callconv(.c) void = null,
+    flags: switch (native_arch) {
+        .mips, .mipsel, .mips64, .mips64el => c_uint,
+        else => c_ulong,
+    },
 };
 
 pub const SFD = struct {

lib/std/os/linux/test.zig

@@ -126,6 +126,8 @@ test "fadvise" {
 }
 
 test "sigset_t" {
+    std.debug.assert(@sizeOf(linux.sigset_t) == (linux.NSIG / 8));
+
     var sigset = linux.empty_sigset;
 
     // See that none are set, then set each one, see that they're all set, then
@@ -138,6 +140,7 @@ test "sigset_t" {
     }
     for (1..linux.NSIG) |i| {
         try expectEqual(linux.sigismember(&sigset, @truncate(i)), true);
+        try expectEqual(linux.sigismember(&linux.filled_sigset, @truncate(i)), true);
         try expectEqual(linux.sigismember(&linux.empty_sigset, @truncate(i)), false);
     }
     for (1..linux.NSIG) |i| {
@@ -147,22 +150,52 @@ test "sigset_t" {
         try expectEqual(linux.sigismember(&sigset, @truncate(i)), false);
     }
 
+    // Kernel sigset_t is either 2+ 32-bit values or 1+ 64-bit value(s).
+    const sigset_len = @typeInfo(linux.sigset_t).array.len;
+    const sigset_elemis64 = 64 == @bitSizeOf(@typeInfo(linux.sigset_t).array.child);
+
     linux.sigaddset(&sigset, 1);
     try expectEqual(sigset[0], 1);
-    try expectEqual(sigset[1], 0);
+    if (sigset_len > 1) {
+        try expectEqual(sigset[1], 0);
+    }
 
     linux.sigaddset(&sigset, 31);
     try expectEqual(sigset[0], 0x4000_0001);
-    try expectEqual(sigset[1], 0);
+    if (sigset_len > 1) {
+        try expectEqual(sigset[1], 0);
+    }
 
     linux.sigaddset(&sigset, 36);
-    try expectEqual(sigset[0], 0x4000_0001);
-    try expectEqual(sigset[1], 0x8);
+    if (sigset_elemis64) {
+        try expectEqual(sigset[0], 0x8_4000_0001);
+    } else {
+        try expectEqual(sigset[0], 0x4000_0001);
+        try expectEqual(sigset[1], 0x8);
+    }
 
     linux.sigaddset(&sigset, 64);
-    try expectEqual(sigset[0], 0x4000_0001);
-    try expectEqual(sigset[1], 0x8000_0008);
-    try expectEqual(sigset[2], 0);
+    if (sigset_elemis64) {
+        try expectEqual(sigset[0], 0x8000_0008_4000_0001);
+    } else {
+        try expectEqual(sigset[0], 0x4000_0001);
+        try expectEqual(sigset[1], 0x8000_0008);
+    }
+}
+
+test "filled_sigset" {
+    // unlike the C library, all the signals are set in the kernel-level fillset
+    const sigset = linux.filled_sigset;
+    for (1..linux.NSIG) |i| {
+        try expectEqual(linux.sigismember(&sigset, @truncate(i)), true);
+    }
+}
+
+test "empty_sigset" {
+    const sigset = linux.empty_sigset;
+    for (1..linux.NSIG) |i| {
+        try expectEqual(linux.sigismember(&sigset, @truncate(i)), false);
+    }
 }
 
 test "sysinfo" {

lib/std/os/linux/x86_64.zig

@@ -369,13 +369,21 @@ pub const mcontext_t = extern struct {
     reserved1: [8]usize = undefined,
 };
 
+/// ucontext_t is part of the state pushed on the stack by the kernel for
+/// a signal handler.  And also a subset of the state returned from the
+/// makecontext/getcontext/swapcontext POSIX APIs.
+///
+/// Currently this structure matches the glibc/musl layout.  It contains a
+/// 1024-bit signal mask, and `fpregs_mem`.  This structure should be
+/// split into one for the kernel ABI and c.zig should define a glibc/musl
+/// compatible structure.
 pub const ucontext_t = extern struct {
     flags: usize,
     link: ?*ucontext_t,
     stack: stack_t,
     mcontext: mcontext_t,
-    sigmask: sigset_t,
-    fpregs_mem: [64]usize,
+    sigmask: [1024 / @bitSizeOf(c_ulong)]c_ulong, // Currently a glibc-compatible (1024-bit) sigmask.
+    fpregs_mem: [64]usize, // Not part of kernel ABI, only part of glibc ucontext_t
 };
 
 fn gpRegisterOffset(comptime reg_index: comptime_int) usize {
@@ -455,7 +463,7 @@ fn getContextInternal() callconv(.naked) usize {
           [stack_offset] "i" (@offsetOf(ucontext_t, "stack")),
           [sigprocmask] "i" (@intFromEnum(linux.SYS.rt_sigprocmask)),
           [sigmask_offset] "i" (@offsetOf(ucontext_t, "sigmask")),
-          [sigset_size] "i" (linux.NSIG / 8),
+          [sigset_size] "i" (@sizeOf(sigset_t)),
         : "cc", "memory", "rax", "rcx", "rdx", "rdi", "rsi", "r8", "r10", "r11"
     );
 }