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Write page zero as first segment for Mach-O exes

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According to the Mach-O file format reference, the first
load command should be a `__PAGEZERO` segment command. The
segment is located at virtual memory location 0, has no protection
rights, and causes acccesses to NULL to immediately crash.

Signed-off-by: Jakub Konka <kubkon@jakubkonka.com>
This commit is contained in:
Jakub Konka 2020-08-20 08:30:54 +02:00
parent 91de5c212f
commit 9164daaa39
1 changed files with 56 additions and 4 deletions
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60
src-self-hosted/link/MachO.zig
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@ -6,6 +6,8 @@ const assert = std.debug.assert;
const fs = std.fs;
const log = std.log.scoped(.link);
const macho = std.macho;
const math = std.math;
const mem = std.mem;
const Module = @import("../Module.zig");
const link = @import("../link.zig");
@ -15,6 +17,14 @@ pub const base_tag: Tag = File.Tag.macho;
base: File,
/// Stored in native-endian format, depending on target endianness needs to be bswapped on read/write.
/// Same order as in the file.
segment_cmds: std.ArrayListUnmanaged(macho.segment_command_64) = std.ArrayListUnmanaged(macho.segment_command_64){},
/// Stored in native-endian format, depending on target endianness needs to be bswapped on read/write.
/// Same order as in the file.
sections: std.ArrayListUnmanaged(macho.@"section_64") = std.ArrayListUnmanaged(macho.@"section_64"){},
entry_addr: ?u64 = null,
error_flags: File.ErrorFlags = File.ErrorFlags{},
@ -86,9 +96,34 @@ fn createFile(allocator: *Allocator, file: fs.File, options: link.Options) !Mach
};
errdefer self.deinit();
if (options.output_mode == .Exe) {
// The first segment command for executables is always a __PAGEZERO segment.
try self.segment_cmds.append(allocator, .{
.cmd = macho.LC_SEGMENT_64,
.cmdsize = @sizeOf(macho.segment_command_64),
.segname = self.makeString("__PAGEZERO"),
.vmaddr = 0,
.vmsize = 0,
.fileoff = 0,
.filesize = 0,
.maxprot = 0,
.initprot = 0,
.nsects = 0,
.flags = 0,
});
}
return self;
}
fn makeString(self: *MachO, comptime bytes: []const u8) [16]u8 {
if (bytes.len > 16) @compileError("MachO segment/section name too long");
var buf: [16]u8 = undefined;
mem.copy(u8, buf[0..], bytes);
return buf;
}
fn writeMachOHeader(self: *MachO) !void {
var hdr: macho.mach_header_64 = undefined;
hdr.magic = macho.MH_MAGIC_64;
@ -122,9 +157,12 @@ fn writeMachOHeader(self: *MachO) !void {
};
hdr.filetype = filetype;
// TODO the rest of the header
hdr.ncmds = 0;
hdr.sizeofcmds = 0;
// TODO consider other commands
const ncmds = try math.cast(u32, self.segment_cmds.items.len);
hdr.ncmds = ncmds;
hdr.sizeofcmds = ncmds * @sizeOf(macho.segment_command_64);
// TODO should these be set to something else?
hdr.flags = 0;
hdr.reserved = 0;
@ -133,6 +171,17 @@ fn writeMachOHeader(self: *MachO) !void {
pub fn flush(self: *MachO, module: *Module) !void {
// TODO implement flush
{
const buf = try self.base.allocator.alloc(macho.segment_command_64, self.segment_cmds.items.len);
defer self.base.allocator.free(buf);
for (buf) |*seg, i| {
seg.* = self.segment_cmds.items[i];
}
try self.base.file.?.pwriteAll(mem.sliceAsBytes(buf), @sizeOf(macho.mach_header_64));
}
if (self.entry_addr == null and self.base.options.output_mode == .Exe) {
log.debug("flushing. no_entry_point_found = true\n", .{});
self.error_flags.no_entry_point_found = true;
@ -143,7 +192,10 @@ pub fn flush(self: *MachO, module: *Module) !void {
}
}
pub fn deinit(self: *MachO) void {}
pub fn deinit(self: *MachO) void {
self.segment_cmds.deinit(self.base.allocator);
self.@"sections".deinit(self.base.allocator);
}
pub fn allocateDeclIndexes(self: *MachO, decl: *Module.Decl) !void {}