zig/src/link/Elf/SharedObject.zig

path: []const u8,
data: []const u8,
index: File.Index,

header: ?elf.Elf64_Ehdr = null,
shdrs: std.ArrayListUnmanaged(ElfShdr) = .{},
symtab: []align(1) const elf.Elf64_Sym = &[0]elf.Elf64_Sym{},
strtab: []const u8 = &[0]u8{},
/// Version symtab contains version strings of the symbols if present.
versyms: std.ArrayListUnmanaged(elf.Elf64_Versym) = .{},
verstrings: std.ArrayListUnmanaged(u32) = .{},

dynamic_sect_index: ?u16 = null,
versym_sect_index: ?u16 = null,
verdef_sect_index: ?u16 = null,

symbols: std.ArrayListUnmanaged(Symbol.Index) = .{},
aliases: ?std.ArrayListUnmanaged(u32) = null,

needed: bool,
alive: bool,

output_symtab_size: Elf.SymtabSize = .{},

pub fn isSharedObject(file: std.fs.File) bool {
    const reader = file.reader();
    const header = reader.readStruct(elf.Elf64_Ehdr) catch return false;
    defer file.seekTo(0) catch {};
    if (!mem.eql(u8, header.e_ident[0..4], "\x7fELF")) return false;
    if (header.e_ident[elf.EI_VERSION] != 1) return false;
    if (header.e_type != elf.ET.DYN) return false;
    return true;
}

pub fn deinit(self: *SharedObject, allocator: Allocator) void {
    allocator.free(self.data);
    self.versyms.deinit(allocator);
    self.verstrings.deinit(allocator);
    self.symbols.deinit(allocator);
    if (self.aliases) |*aliases| aliases.deinit(allocator);
    self.shdrs.deinit(allocator);
}

pub fn parse(self: *SharedObject, elf_file: *Elf) !void {
    const gpa = elf_file.base.allocator;
    var stream = std.io.fixedBufferStream(self.data);
    const reader = stream.reader();

    self.header = try reader.readStruct(elf.Elf64_Ehdr);
    const shoff = std.math.cast(usize, self.header.?.e_shoff) orelse return error.Overflow;

    var dynsym_index: ?u16 = null;
    const shdrs = @as(
        [*]align(1) const elf.Elf64_Shdr,
        @ptrCast(self.data.ptr + shoff),
    )[0..self.header.?.e_shnum];
    try self.shdrs.ensureTotalCapacityPrecise(gpa, shdrs.len);

    for (shdrs, 0..) |shdr, i| {
        self.shdrs.appendAssumeCapacity(try ElfShdr.fromElf64Shdr(shdr));
        switch (shdr.sh_type) {
            elf.SHT_DYNSYM => dynsym_index = @as(u16, @intCast(i)),
            elf.SHT_DYNAMIC => self.dynamic_sect_index = @as(u16, @intCast(i)),
            elf.SHT_GNU_VERSYM => self.versym_sect_index = @as(u16, @intCast(i)),
            elf.SHT_GNU_VERDEF => self.verdef_sect_index = @as(u16, @intCast(i)),
            else => {},
        }
    }

    if (dynsym_index) |index| {
        const shdr = self.shdrs.items[index];
        const symtab = self.shdrContents(index);
        const nsyms = @divExact(symtab.len, @sizeOf(elf.Elf64_Sym));
        self.symtab = @as([*]align(1) const elf.Elf64_Sym, @ptrCast(symtab.ptr))[0..nsyms];
        self.strtab = self.shdrContents(@as(u16, @intCast(shdr.sh_link)));
    }

    try self.parseVersions(elf_file);
    try self.initSymtab(elf_file);
}

fn parseVersions(self: *SharedObject, elf_file: *Elf) !void {
    const gpa = elf_file.base.allocator;

    try self.verstrings.resize(gpa, 2);
    self.verstrings.items[elf.VER_NDX_LOCAL] = 0;
    self.verstrings.items[elf.VER_NDX_GLOBAL] = 0;

    if (self.verdef_sect_index) |shndx| {
        const verdefs = self.shdrContents(shndx);
        const nverdefs = self.verdefNum();
        try self.verstrings.resize(gpa, self.verstrings.items.len + nverdefs);

        var i: u32 = 0;
        var offset: u32 = 0;
        while (i < nverdefs) : (i += 1) {
            const verdef = @as(*align(1) const elf.Elf64_Verdef, @ptrCast(verdefs.ptr + offset)).*;
            defer offset += verdef.vd_next;
            if (verdef.vd_flags == elf.VER_FLG_BASE) continue; // Skip BASE entry
            const vda_name = if (verdef.vd_cnt > 0)
                @as(*align(1) const elf.Elf64_Verdaux, @ptrCast(verdefs.ptr + offset + verdef.vd_aux)).vda_name
            else
                0;
            self.verstrings.items[verdef.vd_ndx] = vda_name;
        }
    }

    try self.versyms.ensureTotalCapacityPrecise(gpa, self.symtab.len);

    if (self.versym_sect_index) |shndx| {
        const versyms_raw = self.shdrContents(shndx);
        const nversyms = @divExact(versyms_raw.len, @sizeOf(elf.Elf64_Versym));
        const versyms = @as([*]align(1) const elf.Elf64_Versym, @ptrCast(versyms_raw.ptr))[0..nversyms];
        for (versyms) |ver| {
            const normalized_ver = if (ver & elf.VERSYM_VERSION >= self.verstrings.items.len - 1)
                elf.VER_NDX_GLOBAL
            else
                ver;
            self.versyms.appendAssumeCapacity(normalized_ver);
        }
    } else for (0..self.symtab.len) |_| {
        self.versyms.appendAssumeCapacity(elf.VER_NDX_GLOBAL);
    }
}

fn initSymtab(self: *SharedObject, elf_file: *Elf) !void {
    const gpa = elf_file.base.allocator;

    try self.symbols.ensureTotalCapacityPrecise(gpa, self.symtab.len);

    for (self.symtab, 0..) |sym, i| {
        const hidden = self.versyms.items[i] & elf.VERSYM_HIDDEN != 0;
        const name = self.getString(sym.st_name);
        // We need to garble up the name so that we don't pick this symbol
        // during symbol resolution. Thank you GNU!
        const off = if (hidden) blk: {
            const full_name = try std.fmt.allocPrint(gpa, "{s}@{s}", .{
                name,
                self.versionString(self.versyms.items[i]),
            });
            defer gpa.free(full_name);
            break :blk try elf_file.strtab.insert(gpa, full_name);
        } else try elf_file.strtab.insert(gpa, name);
        const gop = try elf_file.getOrPutGlobal(off);
        self.symbols.addOneAssumeCapacity().* = gop.index;
    }
}

pub fn resolveSymbols(self: *SharedObject, elf_file: *Elf) void {
    for (self.globals(), 0..) |index, i| {
        const esym_index = @as(u32, @intCast(i));
        const this_sym = self.symtab[esym_index];

        if (this_sym.st_shndx == elf.SHN_UNDEF) continue;

        const global = elf_file.symbol(index);
        if (self.asFile().symbolRank(this_sym, false) < global.symbolRank(elf_file)) {
            global.value = this_sym.st_value;
            global.atom_index = 0;
            global.esym_index = esym_index;
            global.version_index = self.versyms.items[esym_index];
            global.file_index = self.index;
        }
    }
}

pub fn resetGlobals(self: *SharedObject, elf_file: *Elf) void {
    for (self.globals()) |index| {
        const global = elf_file.symbol(index);
        const off = global.name_offset;
        global.* = .{};
        global.name_offset = off;
    }
}

pub fn markLive(self: *SharedObject, elf_file: *Elf) void {
    for (self.globals(), 0..) |index, i| {
        const sym = self.symtab[i];
        if (sym.st_shndx != elf.SHN_UNDEF) continue;

        const global = elf_file.symbol(index);
        const file = global.file(elf_file) orelse continue;
        const should_drop = switch (file) {
            .shared_object => |sh| !sh.needed and sym.st_bind() == elf.STB_WEAK,
            else => false,
        };
        if (!should_drop and !file.isAlive()) {
            file.setAlive();
            file.markLive(elf_file);
        }
    }
}

pub fn updateSymtabSize(self: *SharedObject, elf_file: *Elf) void {
    for (self.globals()) |global_index| {
        const global = elf_file.symbol(global_index);
        if (global.file(elf_file)) |file| if (file.index() != self.index) continue;
        if (global.isLocal()) continue;
        global.flags.output_symtab = true;
        self.output_symtab_size.nglobals += 1;
    }
}

pub fn writeSymtab(self: *SharedObject, elf_file: *Elf, ctx: anytype) void {
    var iglobal = ctx.iglobal;
    for (self.globals()) |global_index| {
        const global = elf_file.symbol(global_index);
        if (global.file(elf_file)) |file| if (file.index() != self.index) continue;
        if (!global.flags.output_symtab) continue;
        global.setOutputSym(elf_file, &ctx.symtab[iglobal]);
        iglobal += 1;
    }
}

pub fn globals(self: SharedObject) []const Symbol.Index {
    return self.symbols.items;
}

pub fn shdrContents(self: SharedObject, index: u16) []const u8 {
    const shdr = self.shdrs.items[index];
    return self.data[shdr.sh_offset..][0..shdr.sh_size];
}

pub fn getString(self: SharedObject, off: u32) [:0]const u8 {
    assert(off < self.strtab.len);
    return mem.sliceTo(@as([*:0]const u8, @ptrCast(self.strtab.ptr + off)), 0);
}

pub fn versionString(self: SharedObject, index: elf.Elf64_Versym) [:0]const u8 {
    const off = self.verstrings.items[index & elf.VERSYM_VERSION];
    return self.getString(off);
}

pub fn asFile(self: *SharedObject) File {
    return .{ .shared_object = self };
}

fn dynamicTable(self: *SharedObject) []align(1) const elf.Elf64_Dyn {
    const shndx = self.dynamic_sect_index orelse return &[0]elf.Elf64_Dyn{};
    const raw = self.shdrContents(shndx);
    const num = @divExact(raw.len, @sizeOf(elf.Elf64_Dyn));
    return @as([*]align(1) const elf.Elf64_Dyn, @ptrCast(raw.ptr))[0..num];
}

fn verdefNum(self: *SharedObject) u32 {
    const entries = self.dynamicTable();
    for (entries) |entry| switch (entry.d_tag) {
        elf.DT_VERDEFNUM => return @as(u32, @intCast(entry.d_val)),
        else => {},
    };
    return 0;
}

pub fn soname(self: *SharedObject) []const u8 {
    const entries = self.dynamicTable();
    for (entries) |entry| switch (entry.d_tag) {
        elf.DT_SONAME => return self.getString(@as(u32, @intCast(entry.d_val))),
        else => {},
    };
    return std.fs.path.basename(self.path);
}

pub fn initSymbolAliases(self: *SharedObject, elf_file: *Elf) !void {
    assert(self.aliases == null);

    const SortAlias = struct {
        pub fn lessThan(ctx: *Elf, lhs: Symbol.Index, rhs: Symbol.Index) bool {
            const lhs_sym = ctx.symbol(lhs).elfSym(ctx);
            const rhs_sym = ctx.symbol(rhs).elfSym(ctx);
            return lhs_sym.st_value < rhs_sym.st_value;
        }
    };

    const gpa = elf_file.base.allocator;
    var aliases = std.ArrayList(Symbol.Index).init(gpa);
    defer aliases.deinit();
    try aliases.ensureTotalCapacityPrecise(self.globals().len);

    for (self.globals()) |index| {
        const global = elf_file.symbol(index);
        const global_file = global.file(elf_file) orelse continue;
        if (global_file.index() != self.index) continue;
        aliases.appendAssumeCapacity(index);
    }

    std.mem.sort(u32, aliases.items, elf_file, SortAlias.lessThan);

    self.aliases = aliases.moveToUnmanaged();
}

pub fn symbolAliases(self: *SharedObject, index: u32, elf_file: *Elf) []const u32 {
    assert(self.aliases != null);

    const symbol = elf_file.symbol(index).elfSym(elf_file);
    const aliases = self.aliases.?;

    const start = for (aliases.items, 0..) |alias, i| {
        const alias_sym = elf_file.symbol(alias).elfSym(elf_file);
        if (symbol.st_value == alias_sym.st_value) break i;
    } else aliases.items.len;

    const end = for (aliases.items[start..], 0..) |alias, i| {
        const alias_sym = elf_file.symbol(alias).elfSym(elf_file);
        if (symbol.st_value < alias_sym.st_value) break i + start;
    } else aliases.items.len;

    return aliases.items[start..end];
}

pub fn format(
    self: SharedObject,
    comptime unused_fmt_string: []const u8,
    options: std.fmt.FormatOptions,
    writer: anytype,
) !void {
    _ = self;
    _ = unused_fmt_string;
    _ = options;
    _ = writer;
    @compileError("do not format shared objects directly");
}

pub fn fmtSymtab(self: SharedObject, elf_file: *Elf) std.fmt.Formatter(formatSymtab) {
    return .{ .data = .{
        .shared = self,
        .elf_file = elf_file,
    } };
}

const FormatContext = struct {
    shared: SharedObject,
    elf_file: *Elf,
};

fn formatSymtab(
    ctx: FormatContext,
    comptime unused_fmt_string: []const u8,
    options: std.fmt.FormatOptions,
    writer: anytype,
) !void {
    _ = unused_fmt_string;
    _ = options;
    const shared = ctx.shared;
    try writer.writeAll("  globals\n");
    for (shared.symbols.items) |index| {
        const global = ctx.elf_file.symbol(index);
        try writer.print("    {}\n", .{global.fmt(ctx.elf_file)});
    }
}

const SharedObject = @This();

const std = @import("std");
const assert = std.debug.assert;
const elf = std.elf;
const log = std.log.scoped(.elf);
const mem = std.mem;

const Allocator = mem.Allocator;
const Elf = @import("../Elf.zig");
const ElfShdr = @import("Object.zig").ElfShdr;
const File = @import("file.zig").File;
const Symbol = @import("Symbol.zig");