glibcVerFromSoFile

fn glibcVerFromSoFile(file: fs.File) !std.SemanticVersion {
    var hdr_buf: [@sizeOf(elf.Elf64_Ehdr)]u8 align(@alignOf(elf.Elf64_Ehdr)) = undefined;
    _ = try preadAtLeast(file, &hdr_buf, 0, hdr_buf.len);
    const hdr32: *elf.Elf32_Ehdr = @ptrCast(&hdr_buf);
    const hdr64: *elf.Elf64_Ehdr = @ptrCast(&hdr_buf);
    if (!mem.eql(u8, hdr32.e_ident[0..4], elf.MAGIC)) return error.InvalidElfMagic;
    const elf_endian: std.builtin.Endian = switch (hdr32.e_ident[elf.EI_DATA]) {
        elf.ELFDATA2LSB => .little,
        elf.ELFDATA2MSB => .big,
        else => return error.InvalidElfEndian,
    };
    const need_bswap = elf_endian != native_endian;
    if (hdr32.e_ident[elf.EI_VERSION] != 1) return error.InvalidElfVersion;

    const is_64 = switch (hdr32.e_ident[elf.EI_CLASS]) {
        elf.ELFCLASS32 => false,
        elf.ELFCLASS64 => true,
        else => return error.InvalidElfClass,
    };
    const shstrndx = elfInt(is_64, need_bswap, hdr32.e_shstrndx, hdr64.e_shstrndx);
    var shoff = elfInt(is_64, need_bswap, hdr32.e_shoff, hdr64.e_shoff);
    const shentsize = elfInt(is_64, need_bswap, hdr32.e_shentsize, hdr64.e_shentsize);
    const str_section_off = shoff + @as(u64, shentsize) * @as(u64, shstrndx);
    var sh_buf: [16 * @sizeOf(elf.Elf64_Shdr)]u8 align(@alignOf(elf.Elf64_Shdr)) = undefined;
    if (sh_buf.len < shentsize) return error.InvalidElfFile;

    _ = try preadAtLeast(file, &sh_buf, str_section_off, shentsize);
    const shstr32: *elf.Elf32_Shdr = @ptrCast(@alignCast(&sh_buf));
    const shstr64: *elf.Elf64_Shdr = @ptrCast(@alignCast(&sh_buf));
    const shstrtab_off = elfInt(is_64, need_bswap, shstr32.sh_offset, shstr64.sh_offset);
    const shstrtab_size = elfInt(is_64, need_bswap, shstr32.sh_size, shstr64.sh_size);
    var strtab_buf: [4096:0]u8 = undefined;
    const shstrtab_len = @min(shstrtab_size, strtab_buf.len);
    const shstrtab_read_len = try preadAtLeast(file, &strtab_buf, shstrtab_off, shstrtab_len);
    const shstrtab = strtab_buf[0..shstrtab_read_len];
    const shnum = elfInt(is_64, need_bswap, hdr32.e_shnum, hdr64.e_shnum);
    var sh_i: u16 = 0;
    const dynstr: struct { offset: u64, size: u64 } = find_dyn_str: while (sh_i < shnum) {
        // Reserve some bytes so that we can deref the 64-bit struct fields
        // even when the ELF file is 32-bits.
        const sh_reserve: usize = @sizeOf(elf.Elf64_Shdr) - @sizeOf(elf.Elf32_Shdr);
        const sh_read_byte_len = try preadAtLeast(
            file,
            sh_buf[0 .. sh_buf.len - sh_reserve],
            shoff,
            shentsize,
        );
        var sh_buf_i: usize = 0;
        while (sh_buf_i < sh_read_byte_len and sh_i < shnum) : ({
            sh_i += 1;
            shoff += shentsize;
            sh_buf_i += shentsize;
        }) {
            const sh32: *elf.Elf32_Shdr = @ptrCast(@alignCast(&sh_buf[sh_buf_i]));
            const sh64: *elf.Elf64_Shdr = @ptrCast(@alignCast(&sh_buf[sh_buf_i]));
            const sh_name_off = elfInt(is_64, need_bswap, sh32.sh_name, sh64.sh_name);
            const sh_name = mem.sliceTo(shstrtab[sh_name_off..], 0);
            if (mem.eql(u8, sh_name, ".dynstr")) {
                break :find_dyn_str .{
                    .offset = elfInt(is_64, need_bswap, sh32.sh_offset, sh64.sh_offset),
                    .size = elfInt(is_64, need_bswap, sh32.sh_size, sh64.sh_size),
                };
            }
        }
    } else return error.InvalidGnuLibCVersion;

    // Here we loop over all the strings in the dynstr string table, assuming that any
    // strings that start with "GLIBC_2." indicate the existence of such a glibc version,
    // and furthermore, that the system-installed glibc is at minimum that version.

    // Empirically, glibc 2.34 libc.so .dynstr section is 32441 bytes on my system.
    // Here I use double this value plus some headroom. This makes it only need
    // a single read syscall here.
    var buf: [80000]u8 = undefined;
    if (buf.len < dynstr.size) return error.InvalidGnuLibCVersion;

    const dynstr_size: usize = @intCast(dynstr.size);
    const dynstr_bytes = buf[0..dynstr_size];
    _ = try preadAtLeast(file, dynstr_bytes, dynstr.offset, dynstr_bytes.len);
    var it = mem.splitScalar(u8, dynstr_bytes, 0);
    var max_ver: std.SemanticVersion = .{ .major = 2, .minor = 2, .patch = 5 };
    while (it.next()) |s| {
        if (mem.startsWith(u8, s, "GLIBC_2.")) {
            const chopped = s["GLIBC_".len..];
            const ver = Target.Query.parseVersion(chopped) catch |err| switch (err) {
                error.Overflow => return error.InvalidGnuLibCVersion,
                error.InvalidVersion => return error.InvalidGnuLibCVersion,
            };
            switch (ver.order(max_ver)) {
                .gt => max_ver = ver,
                .lt, .eq => continue,
            }
        }
    }
    return max_ver;
}