std: rework/remove ucontext_t

Our usage of `ucontext_t` in the standard library was kind of
problematic. We unnecessarily mimiced libc-specific structures, and our
`getcontext` implementation was overkill for our use case of stack
tracing.

This commit introduces a new namespace, `std.debug.cpu_context`, which
contains "context" types for various architectures (currently x86,
x86_64, ARM, and AARCH64) containing the general-purpose CPU registers;
the ones needed in practice for stack unwinding. Each implementation has
a function `current` which populates the structure using inline
assembly. The structure is user-overrideable, though that should only be
necessary if the standard library does not have an implementation for
the *architecture*: that is to say, none of this is OS-dependent.

Of course, in POSIX signal handlers, we get a `ucontext_t` from the
kernel. The function `std.debug.cpu_context.fromPosixSignalContext`
converts this to a `std.debug.cpu_context.Native` with a big ol' target
switch.

This functionality is not exposed from `std.c` or `std.posix`, and
neither are `ucontext_t`, `mcontext_t`, or `getcontext`. The rationale
is that these types and functions do not conform to a specific ABI, and
in fact tend to get updated over time based on CPU features and
extensions; in addition, different libcs use different structures which
are "partially compatible" with the kernel structure. Overall, it's a
mess, but all we need is the kernel context, so we can just define a
kernel-compatible structure as long as we don't claim C compatibility by
putting it in `std.c` or `std.posix`.

This change resulted in a few nice `std.debug` simplifications, but
nothing too noteworthy. However, the main benefit of this change is that
DWARF unwinding---sometimes necessary for collecting stack traces
reliably---now requires far less target-specific integration.

Also fix a bug I noticed in `PageAllocator` (I found this due to a bug
in my distro's QEMU distribution; thanks, broken QEMU patch!) and I
think a couple of minor bugs in `std.debug`.

Resolves: #23801
Resolves: #23802

1 files changed, 37 insertions, 3 deletions

diff --git a/lib/std/debug/SelfInfo/WindowsModule.zig b/lib/std/debug/SelfInfo/WindowsModule.zig
index 4bbc220c5b..8c88bd8b2f 100644
--- a/lib/std/debug/SelfInfo/WindowsModule.zig
+++ b/lib/std/debug/SelfInfo/WindowsModule.zig
@@ -295,11 +295,45 @@ pub const UnwindContext = struct {
     pc: usize,
     cur: windows.CONTEXT,
     history_table: windows.UNWIND_HISTORY_TABLE,
-    pub fn init(ctx: *const windows.CONTEXT, gpa: Allocator) Allocator.Error!UnwindContext {
-        _ = gpa;
+    pub fn init(ctx: *const std.debug.cpu_context.Native) UnwindContext {
         return .{
             .pc = @returnAddress(),
-            .cur = ctx.*,
+            .cur = switch (builtin.cpu.arch) {
+                .x86_64 => std.mem.zeroInit(windows.CONTEXT, .{
+                    .Rax = ctx.gprs.get(.rax),
+                    .Rcx = ctx.gprs.get(.rcx),
+                    .Rdx = ctx.gprs.get(.rdx),
+                    .Rbx = ctx.gprs.get(.rbx),
+                    .Rsp = ctx.gprs.get(.rsp),
+                    .Rbp = ctx.gprs.get(.rbp),
+                    .Rsi = ctx.gprs.get(.rsi),
+                    .Rdi = ctx.gprs.get(.rdi),
+                    .R8 = ctx.gprs.get(.r8),
+                    .R9 = ctx.gprs.get(.r9),
+                    .R10 = ctx.gprs.get(.r10),
+                    .R11 = ctx.gprs.get(.r11),
+                    .R12 = ctx.gprs.get(.r12),
+                    .R13 = ctx.gprs.get(.r13),
+                    .R14 = ctx.gprs.get(.r14),
+                    .R15 = ctx.gprs.get(.r15),
+                    .Rip = ctx.gprs.get(.rip),
+                }),
+                .aarch64, .aarch64_be => .{
+                    .ContextFlags = 0,
+                    .Cpsr = 0,
+                    .DUMMYUNIONNAME = .{ .X = ctx.x },
+                    .Sp = ctx.sp,
+                    .Pc = ctx.pc,
+                    .V = @splat(.{ .B = @splat(0) }),
+                    .Fpcr = 0,
+                    .Fpsr = 0,
+                    .Bcr = @splat(0),
+                    .Bvr = @splat(0),
+                    .Wcr = @splat(0),
+                    .Wvr = @splat(0),
+                },
+                else => comptime unreachable,
+            },
             .history_table = std.mem.zeroes(windows.UNWIND_HISTORY_TABLE),
         };
     }