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.

2 files changed, 51 insertions, 10 deletions

diff --git a/lib/std/os/linux/test.zig b/lib/std/os/linux/test.zig
index fa6f822329..097c5a388c 100644
--- a/lib/std/os/linux/test.zig
+++ b/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" {
diff --git a/lib/std/os/linux/x86_64.zig b/lib/std/os/linux/x86_64.zig
index 90147876c8..7ef86fdcf8 100644
--- a/lib/std/os/linux/x86_64.zig
+++ b/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"
     );
 }