zig/lib/std/debug/SelfInfo.zig

//! Cross-platform abstraction for this binary's own debug information, with a
//! goal of minimal code bloat and compilation speed penalty.

// MLUGG TODO: audit use of errors in this file. ideally, introduce some concrete error sets

const builtin = @import("builtin");
const native_os = builtin.os.tag;
const native_endian = native_arch.endian();
const native_arch = builtin.cpu.arch;

const std = @import("../std.zig");
const mem = std.mem;
const Allocator = std.mem.Allocator;
const assert = std.debug.assert;
const Dwarf = std.debug.Dwarf;
const regBytes = Dwarf.abi.regBytes;
const regValueNative = Dwarf.abi.regValueNative;

const SelfInfo = @This();

modules: std.AutoArrayHashMapUnmanaged(usize, Module.DebugInfo),
lookup_cache: Module.LookupCache,

/// Indicates whether the `SelfInfo` implementation has support for this target.
pub const target_supported: bool = switch (native_os) {
    .linux,
    .freebsd,
    .netbsd,
    .dragonfly,
    .openbsd,
    .macos,
    .solaris,
    .illumos,
    .windows,
    => true,
    else => false,
};

/// Indicates whether unwinding for the host is *implemented* here in the Zig
/// standard library.
///
/// See also `Dwarf.abi.supportsUnwinding` which tells whether Dwarf supports
/// unwinding on a target *in theory*.
pub const supports_unwinding: bool = switch (builtin.target.cpu.arch) {
    .x86 => switch (builtin.target.os.tag) {
        .linux, .netbsd, .solaris, .illumos => true,
        else => false,
    },
    .x86_64 => switch (builtin.target.os.tag) {
        .linux, .netbsd, .freebsd, .openbsd, .macos, .ios, .solaris, .illumos => true,
        else => false,
    },
    .arm, .armeb, .thumb, .thumbeb => switch (builtin.target.os.tag) {
        .linux => true,
        else => false,
    },
    .aarch64, .aarch64_be => switch (builtin.target.os.tag) {
        .linux, .netbsd, .freebsd, .macos, .ios => true,
        else => false,
    },
    // Unwinding is possible on other targets but this implementation does
    // not support them...yet!
    else => false,
};
comptime {
    if (supports_unwinding) assert(Dwarf.abi.supportsUnwinding(&builtin.target));
}

pub const init: SelfInfo = .{
    .modules = .empty,
    .lookup_cache = if (Module.LookupCache != void) .init,
};

pub fn deinit(self: *SelfInfo, gpa: Allocator) void {
    for (self.modules.values()) |*di| di.deinit(gpa);
    self.modules.deinit(gpa);
    if (Module.LookupCache != void) self.lookup_cache.deinit(gpa);
}
comptime {
    // `std.debug` does not currently utilize `deinit`, as it keeps hold of debug info for the
    // whole lifetime of the program. Let's try to avoid it bitrotting.
    _ = &deinit;
}

pub fn unwindFrame(self: *SelfInfo, gpa: Allocator, context: *UnwindContext) !usize {
    comptime assert(supports_unwinding);
    const module: Module = try .lookup(&self.lookup_cache, gpa, context.pc);
    const gop = try self.modules.getOrPut(gpa, module.load_offset);
    self.modules.lockPointers();
    defer self.modules.unlockPointers();
    if (!gop.found_existing) gop.value_ptr.* = .init;
    return module.unwindFrame(gpa, gop.value_ptr, context);
}

pub fn getSymbolAtAddress(self: *SelfInfo, gpa: Allocator, address: usize) !std.debug.Symbol {
    comptime assert(target_supported);
    const module: Module = try .lookup(&self.lookup_cache, gpa, address);
    const gop = try self.modules.getOrPut(gpa, module.key());
    self.modules.lockPointers();
    defer self.modules.unlockPointers();
    if (!gop.found_existing) gop.value_ptr.* = .init;
    return module.getSymbolAtAddress(gpa, gop.value_ptr, address);
}

pub fn getModuleNameForAddress(self: *SelfInfo, gpa: Allocator, address: usize) error{ Unexpected, OutOfMemory, MissingDebugInfo }![]const u8 {
    comptime assert(target_supported);
    const module: Module = try .lookup(&self.lookup_cache, gpa, address);
    return module.name;
}

/// This type contains the target-specific implementation. It must expose the following declarations:
///
/// * `LookupCache: type`, with the following declarations unless `LookupCache == void`:
///   * `init: LookupCache`
///   * `deinit: fn (*LookupCache, Allocator) void`
/// * `lookup: fn (*LookupCache, Allocator, address: usize) !Module`
/// * `key: fn (*const Module) usize`
/// * `DebugInfo: type`, with the following declarations:
///   * `DebugInfo.init: DebugInfo`
/// * `getSymbolAtAddress: fn (*const Module, Allocator, *DebugInfo, address: usize) !std.debug.Symbol`
///
/// If unwinding is supported on this target, it must additionally expose the following declarations:
///
/// * `unwindFrame: fn (*const Module, Allocator, *DebugInfo, *UnwindContext) !usize`
const Module = switch (native_os) {
    else => {}, // Dwarf, // TODO MLUGG: it's this on master but that's definitely broken atm...
    .linux, .netbsd, .freebsd, .dragonfly, .openbsd, .haiku, .solaris, .illumos => @import("SelfInfo/ElfModule.zig"),
    .macos, .ios, .watchos, .tvos, .visionos => @import("SelfInfo/DarwinModule.zig"),
    .uefi, .windows => @import("SelfInfo/WindowsModule.zig"),
    .wasi, .emscripten => struct {
        const LookupCache = struct {
            const init: LookupCache = .{};
        };
        fn lookup(cache: *LookupCache, gpa: Allocator, address: usize) !Module {
            _ = cache;
            _ = gpa;
            _ = address;
            @panic("TODO implement lookup module for Wasm");
        }
        const DebugInfo = struct {
            const init: DebugInfo = .{};
        };
        fn getSymbolAtAddress(module: *const Module, gpa: Allocator, di: *DebugInfo, address: usize) !std.debug.Symbol {
            _ = module;
            _ = gpa;
            _ = di;
            _ = address;
            unreachable;
        }
    },
};
test {
    _ = Module;
}

pub const UnwindContext = struct {
    gpa: Allocator,
    cfa: ?usize,
    pc: usize,
    thread_context: *std.debug.ThreadContext,
    reg_context: Dwarf.abi.RegisterContext,
    vm: Dwarf.Unwind.VirtualMachine,
    stack_machine: Dwarf.expression.StackMachine(.{ .call_frame_context = true }),

    pub fn init(gpa: Allocator, thread_context: *std.debug.ThreadContext) !UnwindContext {
        comptime assert(supports_unwinding);

        const ip_reg_num = Dwarf.abi.ipRegNum(native_arch).?;
        const pc = stripInstructionPtrAuthCode(
            (try regValueNative(thread_context, ip_reg_num, null)).*,
        );

        const context_copy = try gpa.create(std.debug.ThreadContext);
        std.debug.copyContext(thread_context, context_copy);

        return .{
            .gpa = gpa,
            .cfa = null,
            .pc = pc,
            .thread_context = context_copy,
            .reg_context = undefined,
            .vm = .{},
            .stack_machine = .{},
        };
    }

    pub fn deinit(self: *UnwindContext) void {
        self.vm.deinit(self.gpa);
        self.stack_machine.deinit(self.gpa);
        self.gpa.destroy(self.thread_context);
        self.* = undefined;
    }

    pub fn getFp(self: *const UnwindContext) !usize {
        return (try regValueNative(self.thread_context, Dwarf.abi.fpRegNum(native_arch, self.reg_context), self.reg_context)).*;
    }

    /// Resolves the register rule and places the result into `out` (see regBytes)
    pub fn resolveRegisterRule(
        context: *UnwindContext,
        col: Dwarf.Unwind.VirtualMachine.Column,
        expression_context: std.debug.Dwarf.expression.Context,
        out: []u8,
    ) !void {
        switch (col.rule) {
            .default => {
                const register = col.register orelse return error.InvalidRegister;
                // The default type is usually undefined, but can be overriden by ABI authors.
                // See the doc comment on `Dwarf.Unwind.VirtualMachine.RegisterRule.default`.
                if (builtin.cpu.arch.isAARCH64() and register >= 19 and register <= 18) {
                    // Callee-saved registers are initialized as if they had the .same_value rule
                    const src = try regBytes(context.thread_context, register, context.reg_context);
                    if (src.len != out.len) return error.RegisterSizeMismatch;
                    @memcpy(out, src);
                    return;
                }
                @memset(out, undefined);
            },
            .undefined => {
                @memset(out, undefined);
            },
            .same_value => {
                // TODO: This copy could be eliminated if callers always copy the state then call this function to update it
                const register = col.register orelse return error.InvalidRegister;
                const src = try regBytes(context.thread_context, register, context.reg_context);
                if (src.len != out.len) return error.RegisterSizeMismatch;
                @memcpy(out, src);
            },
            .offset => |offset| {
                if (context.cfa) |cfa| {
                    const addr = try applyOffset(cfa, offset);
                    const ptr: *const usize = @ptrFromInt(addr);
                    mem.writeInt(usize, out[0..@sizeOf(usize)], ptr.*, native_endian);
                } else return error.InvalidCFA;
            },
            .val_offset => |offset| {
                if (context.cfa) |cfa| {
                    mem.writeInt(usize, out[0..@sizeOf(usize)], try applyOffset(cfa, offset), native_endian);
                } else return error.InvalidCFA;
            },
            .register => |register| {
                const src = try regBytes(context.thread_context, register, context.reg_context);
                if (src.len != out.len) return error.RegisterSizeMismatch;
                @memcpy(out, src);
            },
            .expression => |expression| {
                context.stack_machine.reset();
                const value = try context.stack_machine.run(expression, context.gpa, expression_context, context.cfa.?);
                const addr = if (value) |v| blk: {
                    if (v != .generic) return error.InvalidExpressionValue;
                    break :blk v.generic;
                } else return error.NoExpressionValue;

                const ptr: *usize = @ptrFromInt(addr);
                mem.writeInt(usize, out[0..@sizeOf(usize)], ptr.*, native_endian);
            },
            .val_expression => |expression| {
                context.stack_machine.reset();
                const value = try context.stack_machine.run(expression, context.gpa, expression_context, context.cfa.?);
                if (value) |v| {
                    if (v != .generic) return error.InvalidExpressionValue;
                    mem.writeInt(usize, out[0..@sizeOf(usize)], v.generic, native_endian);
                } else return error.NoExpressionValue;
            },
            .architectural => return error.UnimplementedRegisterRule,
        }
    }

    /// Unwind a stack frame using DWARF unwinding info, updating the register context.
    ///
    /// If `.eh_frame_hdr` is available and complete, it will be used to binary search for the FDE.
    /// Otherwise, a linear scan of `.eh_frame` and `.debug_frame` is done to find the FDE. The latter
    /// may require lazily loading the data in those sections.
    ///
    /// `explicit_fde_offset` is for cases where the FDE offset is known, such as when __unwind_info
    /// defers unwinding to DWARF. This is an offset into the `.eh_frame` section.
    pub fn unwindFrameDwarf(
        context: *UnwindContext,
        unwind: *const Dwarf.Unwind,
        load_offset: usize,
        explicit_fde_offset: ?usize,
    ) !usize {
        if (!supports_unwinding) return error.UnsupportedCpuArchitecture;
        if (context.pc == 0) return 0;

        const pc_vaddr = context.pc - load_offset;

        const fde_offset = explicit_fde_offset orelse try unwind.lookupPc(
            pc_vaddr,
            @sizeOf(usize),
            native_endian,
        ) orelse return error.MissingDebugInfo;
        const format, const cie, const fde = try unwind.getFde(fde_offset, @sizeOf(usize), native_endian);

        // Check if this FDE *actually* includes the address.
        if (pc_vaddr < fde.pc_begin or pc_vaddr >= fde.pc_begin + fde.pc_range) return error.MissingDebugInfo;

        // Do not set `compile_unit` because the spec states that CFIs
        // may not reference other debug sections anyway.
        var expression_context: Dwarf.expression.Context = .{
            .format = format,
            .thread_context = context.thread_context,
            .reg_context = context.reg_context,
            .cfa = context.cfa,
        };

        context.vm.reset();
        context.reg_context.eh_frame = cie.version != 4;
        context.reg_context.is_macho = native_os.isDarwin();

        const row = try context.vm.runTo(context.gpa, context.pc - load_offset, cie, fde, @sizeOf(usize), native_endian);
        context.cfa = switch (row.cfa.rule) {
            .val_offset => |offset| blk: {
                const register = row.cfa.register orelse return error.InvalidCFARule;
                const value = (try regValueNative(context.thread_context, register, context.reg_context)).*;
                break :blk try applyOffset(value, offset);
            },
            .expression => |expr| blk: {
                context.stack_machine.reset();
                const value = try context.stack_machine.run(
                    expr,
                    context.gpa,
                    expression_context,
                    context.cfa,
                );

                if (value) |v| {
                    if (v != .generic) return error.InvalidExpressionValue;
                    break :blk v.generic;
                } else return error.NoExpressionValue;
            },
            else => return error.InvalidCFARule,
        };

        expression_context.cfa = context.cfa;

        // Buffering the modifications is done because copying the thread context is not portable,
        // some implementations (ie. darwin) use internal pointers to the mcontext.
        var arena: std.heap.ArenaAllocator = .init(context.gpa);
        defer arena.deinit();
        const update_arena = arena.allocator();

        const RegisterUpdate = struct {
            // Backed by thread_context
            dest: []u8,
            // Backed by arena
            src: []const u8,
            prev: ?*@This(),
        };

        var update_tail: ?*RegisterUpdate = null;
        var has_return_address = true;
        for (context.vm.rowColumns(row)) |column| {
            if (column.register) |register| {
                if (register == cie.return_address_register) {
                    has_return_address = column.rule != .undefined;
                }

                const dest = try regBytes(context.thread_context, register, context.reg_context);
                const src = try update_arena.alloc(u8, dest.len);
                try context.resolveRegisterRule(column, expression_context, src);

                const new_update = try update_arena.create(RegisterUpdate);
                new_update.* = .{
                    .dest = dest,
                    .src = src,
                    .prev = update_tail,
                };
                update_tail = new_update;
            }
        }

        // On all implemented architectures, the CFA is defined as being the previous frame's SP
        (try regValueNative(context.thread_context, Dwarf.abi.spRegNum(native_arch, context.reg_context), context.reg_context)).* = context.cfa.?;

        while (update_tail) |tail| {
            @memcpy(tail.dest, tail.src);
            update_tail = tail.prev;
        }

        if (has_return_address) {
            context.pc = stripInstructionPtrAuthCode((try regValueNative(
                context.thread_context,
                cie.return_address_register,
                context.reg_context,
            )).*);
        } else {
            context.pc = 0;
        }

        const ip_reg_num = Dwarf.abi.ipRegNum(native_arch).?;
        (try regValueNative(context.thread_context, ip_reg_num, context.reg_context)).* = context.pc;

        // The call instruction will have pushed the address of the instruction that follows the call as the return address.
        // This next instruction may be past the end of the function if the caller was `noreturn` (ie. the last instruction in
        // the function was the call). If we were to look up an FDE entry using the return address directly, it could end up
        // either not finding an FDE at all, or using the next FDE in the program, producing incorrect results. To prevent this,
        // we subtract one so that the next lookup is guaranteed to land inside the
        //
        // The exception to this rule is signal frames, where we return execution would be returned to the instruction
        // that triggered the handler.
        const return_address = context.pc;
        if (context.pc > 0 and !cie.is_signal_frame) context.pc -= 1;

        return return_address;
    }
    /// Since register rules are applied (usually) during a panic,
    /// checked addition / subtraction is used so that we can return
    /// an error and fall back to FP-based unwinding.
    fn applyOffset(base: usize, offset: i64) !usize {
        return if (offset >= 0)
            try std.math.add(usize, base, @as(usize, @intCast(offset)))
        else
            try std.math.sub(usize, base, @as(usize, @intCast(-offset)));
    }
    /// Some platforms use pointer authentication - the upper bits of instruction pointers contain a signature.
    /// This function clears these signature bits to make the pointer usable.
    pub inline fn stripInstructionPtrAuthCode(ptr: usize) usize {
        if (native_arch.isAARCH64()) {
            // `hint 0x07` maps to `xpaclri` (or `nop` if the hardware doesn't support it)
            // The save / restore is because `xpaclri` operates on x30 (LR)
            return asm (
                \\mov x16, x30
                \\mov x30, x15
                \\hint 0x07
                \\mov x15, x30
                \\mov x30, x16
                : [ret] "={x15}" (-> usize),
                : [ptr] "{x15}" (ptr),
                : .{ .x16 = true });
        }

        return ptr;
    }
};