tiny.tldr.formats.elf.relocation.relax

1const std = @import("std");2const root = @import("../../../root.zig");3const elf = @import("../root.zig");4const checked = @import("value.zig");56const model = root.model;7const Rela = elf.format.Rela;8const copyInto = elf.format.copyInto;9const writeU32 = elf.format.writeU32;1011fn checkedRelocationPatchOffset(12    contribution_size: u64,13    base_file_offset: u64,14    patch_offset: u64,15    patch_size: u64,16) model.Error!usize {17    if (patch_offset > contribution_size or18        patch_size > contribution_size - patch_offset) return error.InvalidRange;19    return @intCast(base_file_offset + patch_offset);20}2122pub fn tlsGdToLocalExec(23    image: []u8,24    contribution_size: u64,25    base_file_offset: u64,26    relocation: Rela,27    value: i128,28) model.Error!void {29    if (relocation.offset < 4) return error.InvalidRange;30    const patch_offset = try checkedRelocationPatchOffset(contribution_size, base_file_offset, relocation.offset - 4, 16);31    const replacement = [_]u8{32        0x64, 0x48, 0x8b, 0x04, 0x25, 0x00, 0x00, 0x00,33        0x00, 0x48, 0x8d, 0x80, 0x00, 0x00, 0x00, 0x00,34    };35    copyInto(image, patch_offset, &replacement);36    writeU32(image, patch_offset + 12, @bitCast(try checked.checkedI32(value)));37}3839pub fn tlsLdToLocalExec(40    image: []u8,41    contribution_size: u64,42    base_file_offset: u64,43    relocation: Rela,44) model.Error!void {45    if (relocation.offset < 3) return error.InvalidRange;46    const probe_offset = try checkedRelocationPatchOffset(contribution_size, base_file_offset, relocation.offset - 3, 9);47    const direct_call_opcode_offset = probe_offset + 7;48    const replacement = [_]u8{49        0x66, 0x66, 0x66, 0x64, 0x48, 0x8b, 0x04, 0x25,50        0x00, 0x00, 0x00, 0x00,51    };52    if (image[direct_call_opcode_offset] == 0xe8) {53        const patch_offset = try checkedRelocationPatchOffset(contribution_size, base_file_offset, relocation.offset - 3, replacement.len);54        copyInto(image, patch_offset, &replacement);55        return;56    }57    if (image[direct_call_opcode_offset] == 0xff and image[direct_call_opcode_offset + 1] == 0x15) {58        const patch_offset = try checkedRelocationPatchOffset(contribution_size, base_file_offset, relocation.offset - 3, replacement.len + 1);59        image[patch_offset] = 0x66;60        copyInto(image, patch_offset + 1, &replacement);61        return;62    }63    return error.UnsupportedRelocation;64}6566pub const Gotpcrelx = enum {67    pc_relative,68    absolute_signed_32,69};7071pub fn gotpcrelxInstruction(image: []u8, place_offset: usize, relocation_type: u32) ?Gotpcrelx {72    if (relaxGotpcrelxMov(image, place_offset, relocation_type)) return .pc_relative;73    if (relaxGotpcrelxIndirectBranch(image, place_offset, relocation_type)) return .pc_relative;74    if (relaxGotpcrelxBinaryImmediate(image, place_offset, relocation_type)) return .absolute_signed_32;75    return null;76}7778fn relaxGotpcrelxMov(image: []u8, place_offset: usize, relocation_type: u32) bool {79    if (place_offset < 2) return false;80    if (relocation_type == @backingInt(std.elf.R_X86_64.REX_GOTPCRELX)) {81        if (place_offset < 3 or !isRexPrefix(image[place_offset - 3])) return false;82    }83    const opcode_offset = place_offset - 2;84    if (image[opcode_offset] != 0x8b) return false;85    if (!isRipRelativeMemoryOperand(image[place_offset - 1])) return false;86    image[opcode_offset] = 0x8d;87    return true;88}8990fn relaxGotpcrelxIndirectBranch(image: []u8, place_offset: usize, relocation_type: u32) bool {91    if (relocation_type == @backingInt(std.elf.R_X86_64.REX_GOTPCRELX)) return false;92    if (place_offset < 2) return false;93    const opcode_offset = place_offset - 2;94    if (image[opcode_offset] != 0xff) return false;95    const modrm = image[place_offset - 1];96    if (!isRipRelativeMemoryOperand(modrm)) return false;97    const reg = (modrm >> 3) & 0x7;98    const opcode: u8 = switch (reg) {99        2 => 0xe8,100        4 => 0xe9,101        else => return false,102    };103    image[opcode_offset] = 0x67;104    image[opcode_offset + 1] = opcode;105    return true;106}107108pub fn gotpcrelWeakUndefinedNullCheck(image: []u8, place_offset: usize) bool {109    if (place_offset < 3 or place_offset + 4 >= image.len) return false;110    const rex_offset = place_offset - 3;111    const rex = image[rex_offset];112    if (!isRexPrefix(rex) or (rex & 0x08) == 0) return false;113    if (image[place_offset - 2] != 0x83) return false;114    const modrm = image[place_offset - 1];115    if (!isRipRelativeMemoryOperand(modrm) or ((modrm >> 3) & 0x7) != 7) return false;116    if (image[place_offset + 4] != 0) return false;117    const replacement = [_]u8{ 0x48, 0x39, 0xe4, 0x0f, 0x1f, 0x44, 0x00, 0x00 };118    @memcpy(image[rex_offset..][0..replacement.len], &replacement);119    return true;120}121122pub fn gottpoffToLocalExec(image: []u8, place_offset: usize, value: i128) model.Error!void {123    if (place_offset < 3 or place_offset > image.len or 4 > image.len - place_offset) return error.InvalidRange;124    const rex_offset = place_offset - 3;125    const rex = image[rex_offset];126    if (!isRexPrefix(rex) or (rex & 0x08) == 0) return error.UnsupportedRelocation;127    if (image[place_offset - 2] != 0x8b) return error.UnsupportedRelocation;128    const modrm = image[place_offset - 1];129    if (!isRipRelativeMemoryOperand(modrm)) return error.UnsupportedRelocation;130    const reg = (modrm >> 3) & 0x7;131    const high_reg = (rex >> 2) & 0x1;132    image[rex_offset] = 0x48 | high_reg;133    image[place_offset - 2] = 0xc7;134    image[place_offset - 1] = 0xc0 | reg;135    writeU32(image, place_offset, @bitCast(try checked.checkedI32(value)));136}137138fn relaxGotpcrelxBinaryImmediate(image: []u8, place_offset: usize, relocation_type: u32) bool {139    if (relocation_type != @backingInt(std.elf.R_X86_64.REX_GOTPCRELX)) return false;140    if (place_offset < 3) return false;141    const rex_offset = place_offset - 3;142    const rex = image[rex_offset];143    if (!isRexPrefix(rex) or (rex & 0x08) == 0) return false;144    const opcode_offset = place_offset - 2;145    const group_operation: u8 = switch (image[opcode_offset]) {146        0x03 => 0,147        0x0b => 1,148        0x13 => 2,149        0x1b => 3,150        0x23 => 4,151        0x2b => 5,152        0x33 => 6,153        0x3b => 7,154        else => return false,155    };156    const modrm_offset = place_offset - 1;157    const modrm = image[modrm_offset];158    if (!isRipRelativeMemoryOperand(modrm)) return false;159    const reg = (modrm >> 3) & 0x7;160    const high_reg = (rex >> 2) & 0x1;161    image[rex_offset] = 0x48 | high_reg;162    image[opcode_offset] = 0x81;163    image[modrm_offset] = 0xc0 | (group_operation << 3) | reg;164    return true;165}166167fn isRexPrefix(byte: u8) bool {168    return byte >= 0x40 and byte <= 0x4f;169}170171fn isRipRelativeMemoryOperand(modrm: u8) bool {172    return (modrm & 0xc7) == 0x05;173}

1pub const relax = @import("relax.zig");