ww/selfhost/cmd/w6l/dynout.ww

// selfhost/cmd/w6l/dynout.ww — port of cmd/w6l/dynout.c.
//
// Emit a dynamic-linked ELF executable. The shape is the simplest
// valid one: PT_INTERP + PT_DYNAMIC + DT_BIND_NOW so the loader
// resolves every PLT slot at startup (no lazy binding, no PLT0
// trampoline). SysV .hash, not .gnu.hash. Non-PIE, fixed base.
//
// Layout:
//   [0]            Ehdr
//   [64]           Phdrs (PT_LOAD R+X, PT_LOAD R+W, PT_INTERP, PT_DYNAMIC)
//   [interp_off]   "/lib64/ld-linux-x86-64.so.2\0"
//   [dynstr_off]   .dynstr
//   [dynsym_off]   .dynsym
//   [hash_off]     .hash
//   [versym_off]   .gnu.version
//   [verneed_off]  .gnu.version_r
//   [relaplt_off]  .rela.plt
//   [pad to 0x1000]
//   [text_off]     .text
//   [plt_off]      .plt
//   [pad to next page]
//   [gotplt_off]   .got.plt   (writable; mapped by PT_LOAD #2)
//   [dynamic_off]  .dynamic   (writable; covered by PT_DYNAMIC)

package w6l;

import os;
import rt;
import mem;
import sym;

// ELF constants
def ET_EXEC_D:     u16 = 2u16;
def EM_X86_64_D:   u16 = 62u16;
def EV_CURRENT_D:  u32 = 1u32;
def ELFCLASS64_D:  u8  = 2u8;
def ELFDATA2LSB_D: u8  = 1u8;

def PT_LOAD_D:     u32 = 1u32;
def PT_DYNAMIC_D:  u32 = 2u32;
def PT_INTERP_D:   u32 = 3u32;
def PF_X_D:        u32 = 1u32;
def PF_W_D:        u32 = 2u32;
def PF_R_D:        u32 = 4u32;

def DT_NULL:       i64 = 0i64;
def DT_NEEDED:     i64 = 1i64;
def DT_PLTRELSZ:   i64 = 2i64;
def DT_PLTGOT:     i64 = 3i64;
def DT_HASH:       i64 = 4i64;
def DT_STRTAB:     i64 = 5i64;
def DT_SYMTAB:     i64 = 6i64;
def DT_STRSZ:      i64 = 10i64;
def DT_SYMENT:     i64 = 11i64;
def DT_PLTREL:     i64 = 20i64;
def DT_RELA:       i64 = 7i64;
def DT_JMPREL:     i64 = 23i64;
def DT_BIND_NOW:   i64 = 24i64;
def DT_VERSYM:     i64 = 1879048176i64;	// 0x6ffffff0
def DT_VERNEED:    i64 = 1879048190i64;	// 0x6ffffffe
def DT_VERNEEDNUM: i64 = 1879048191i64;	// 0x6fffffff

def VER_NDX_LOCAL_D:  u16 = 0u16;
def VER_NDX_GLOBAL_D: u16 = 1u16;

def R_X86_64_PC32_D:        i32 = 2;
def R_X86_64_PLT32_D:       i32 = 4;
def R_X86_64_JUMP_SLOT_D:   u32 = 7u32;

def STB_GLOBAL_D: u8 = 1u8;
def STT_FUNC_D:   u8 = 2u8;

def PLT_STUB_BYTES_D: u64 = 8u64;
def PAGE: u64 = 4096u64;

def INTERP: str = "/lib64/ld-linux-x86-64.so.2";

// ---- byte writers ------------------------------------------------------

fn dwr8(buf: *u8, off: u64, v: u8) void = {
	buf[off] = v;
};

fn dwr16(buf: *u8, off: u64, v: u16) void = {
	buf[off]         = (v & 255u16): u8;
	buf[off + 1u64]  = ((v >> 8u16) & 255u16): u8;
};

fn dwr32(buf: *u8, off: u64, v: u32) void = {
	buf[off]         = (v & 255u32): u8;
	buf[off + 1u64]  = ((v >> 8u32)  & 255u32): u8;
	buf[off + 2u64]  = ((v >> 16u32) & 255u32): u8;
	buf[off + 3u64]  = ((v >> 24u32) & 255u32): u8;
};

fn dwr64(buf: *u8, off: u64, v: u64) void = {
	dwr32(buf, off,        (v & 4294967295u64): u32);
	dwr32(buf, off + 4u64, ((v >> 32u64) & 4294967295u64): u32);
};

fn dwri64(buf: *u8, off: u64, v: i64) void = {
	dwr64(buf, off, v: u64);
};

fn dwri32(buf: *u8, off: u64, v: i32) void = {
	dwr32(buf, off, v: u32);
};

// ---- byte readers ------------------------------------------------------

fn drdu16(p: *u8, off: u64) u16 = {
	let b0: u16 = p[off]: u16;
	let b1: u16 = p[off + 1u64]: u16;
	return b0 | (b1 << 8u16);
};

fn drdu32(p: *u8, off: u64) u32 = {
	let b0: u32 = p[off]: u32;
	let b1: u32 = p[off + 1u64]: u32;
	let b2: u32 = p[off + 2u64]: u32;
	let b3: u32 = p[off + 3u64]: u32;
	return b0 | (b1 << 8u32) | (b2 << 16u32) | (b3 << 24u32);
};

fn drdi32(p: *u8, off: u64) i32 = {
	return drdu32(p, off): i32;
};

fn dbcopy(dst: *u8, off: u64, src: *u8, n: u64) void = {
	let i: u64 = 0u64;
	for (i < n) {
		dst[off + i] = src[i];
		i += 1u64;
	};
};

// elfhash — SysV ELF hash. Used for .gnu.version_r's vna_hash.
fn elfhash(name: str) u32 = {
	let h: u32 = 0u32;
	let i: i32 = 0;
	for (i < name.len) {
		let c: u32 = (name[i]: u8): u32;
		h = (h << 4u32) + c;
		let g: u32 = h & 4026531840u32;	// 0xf0000000
		if (g != 0u32) { h = h ^ (g >> 24u32); };
		h = h & ~g;
		i += 1;
	};
	return h;
};

fn alignup(off: u64, a: u64) u64 = {
	return (off + a - 1u64) & ~(a - 1u64);
};

fn streqd(a: str, b: str) bool = {
	if (a.len != b.len) { return false; };
	let i: i32 = 0;
	for (i < a.len) {
		if (a[i] != b[i]) { return false; };
		i += 1;
	};
	return true;
};

// ---- main entry --------------------------------------------------------

export fn emitdynelf(l: *lnk, fd: i32, base: u64, entry: u64) i32 = {
	// .data shares the R+W PT_LOAD with .got.plt and .dynamic.
	// Placed after .dynamic so the segment is one contiguous run;
	// relocate runs from here so the dyn layout's datava lands in
	// patched offsets.
	let a: *arena = l.a;
	let n: i32 = l.dynn;
	let nu: u64 = n: u64;

	// ---- collect dyn syms into a plt_idx-indexed array ----
	let dynsyms: []*lsym = alloc([], nu)!;
	let s: *lsym = l.syms;
	for (s != nil) {
		if (s.isdyn != 0) {
			if (s.pltidx >= 0) {
				if (s.pltidx < n) {
					dynsyms[s.pltidx] = s;
				};
			};
		};
		s = s.snext;
	};
	let i: i32 = 0;
	for (i < n) {
		if (dynsyms[i] == nil) {
			os.write(2, "w6l: dynout: no sym for plt_idx\n".ptr, 31u64);
			return 1;
		};
		i += 1;
	};

	// ---- collect used .so's (in l.sos order) ----
	let maxsos: i32 = 0;
	let so: *lso = l.sos;
	for (so != nil) { maxsos += 1; so = so.sonext; };
	let sosused: []*lso = alloc([], maxsos: u64)!;
	let nsos: i32 = 0;
	so = l.sos;
	for (so != nil) {
		let used: i32 = 0;
		let j: i32 = 0;
		for (j < n) {
			let dsm: *lsym = dynsyms[j];
			let dl: *lso = dsm.dynlib;
			if (dl == so) { used = 1; j = n; }
			else { j += 1; };
		};
		if (used != 0) {
			sosused[nsos] = so;
			nsos += 1;
		};
		so = so.sonext;
	};

	// ---- build flat version table grouped by vlib ----
	// vlib_sos_idx[k] = sos_used index for vlib k.
	// vlib_first[k]   = ver index of first version under vlib k.
	// vlib_count[k]   = number of versions under vlib k.
	// ver_lib_idx[v]  = vlib index that version v belongs to.
	// ver_name_ptr_arr[v] = name's *u8 (interned in the .so's verdef strings).
	// ver_name_len_buf[v] = name length (i32).
	// ver_dynstr_off[v]   = offset within .dynstr (assigned after layout).
	// ver_vna_other[v]    = versym index (starting at 2).

	let vlibsosidxbuf: []u8 = alloc([], (maxsos: u64) * 4u64)!;
	let vlibfirstbuf:   []u8 = alloc([], (maxsos: u64) * 4u64)!;
	let vlibcountbuf:   []u8 = alloc([], (maxsos: u64) * 4u64)!;
	let nvlibs: i32 = 0;

	let verlibidxbuf:  []u8 = alloc([], nu * 4u64)!;
	let vernameptrarr: []*u8 = alloc([], nu)!;
	let vernamelenbuf: []u8 = alloc([], nu * 4u64)!;
	let verdynstroff:   []u8 = alloc([], nu * 4u64)!;
	let vervnaother:    []u8 = alloc([], nu * 2u64)!;
	let nvers: i32 = 0;

	let si: i32 = 0;
	for (si < nsos) {
		let curso: *lso = sosused[si];
		let has: i32 = 0;
		let j: i32 = 0;
		for (j < n) {
			let dsm: *lsym = dynsyms[j];
			let dl: *lso = dsm.dynlib;
			if (dl == curso) {
				let nm0: str = dsm.name;
				let dv: str = soversion(curso, nm0);
				if (dv.len > 0) {
					has = 1; j = n;
				} else { j += 1; };
			} else { j += 1; };
		};
		if (has != 0) {
			dwr32(vlibsosidxbuf.ptr, (nvlibs: u64) * 4u64, si: u32);
			dwr32(vlibfirstbuf.ptr,   (nvlibs: u64) * 4u64, nvers: u32);
			let added: i32 = 0;
			let jj: i32 = 0;
			for (jj < n) {
				let dsm2: *lsym = dynsyms[jj];
				let dl2: *lso = dsm2.dynlib;
				if (dl2 == curso) {
					let nm2: str = dsm2.name;
					let vname: str = soversion(curso, nm2);
					if (vname.len > 0) {
						let seen: i32 = 0;
						let k: i32 = 0;
						for (k < added) {
							let kk: i32 = nvers - added + k;
							let existing: str;
							existing.ptr = vernameptrarr[kk];
							existing.len = drdi32(vernamelenbuf.ptr, (kk: u64) * 4u64);
							if (streqd(existing, vname)) {
								seen = 1; k = added;
							} else { k += 1; };
						};
						if (seen == 0) {
							dwr32(verlibidxbuf.ptr, (nvers: u64) * 4u64, nvlibs: u32);
							vernameptrarr[nvers] = vname.ptr;
							dwri32(vernamelenbuf.ptr, (nvers: u64) * 4u64, vname.len);
							nvers += 1;
							added += 1;
						};
					};
				};
				jj += 1;
			};
			dwr32(vlibcountbuf.ptr, (nvlibs: u64) * 4u64, added: u32);
			nvlibs += 1;
		};
		si += 1;
	};

	// Assign vna_other indices starting at 2, walking vlib then per-version.
	let nextvna: u16 = 2u16;
	let vi: i32 = 0;
	for (vi < nvlibs) {
		let first: i32 = drdi32(vlibfirstbuf.ptr, (vi: u64) * 4u64);
		let cnt:   i32 = drdi32(vlibcountbuf.ptr, (vi: u64) * 4u64);
		let k: i32 = 0;
		for (k < cnt) {
			dwr16(vervnaother.ptr, ((first + k): u64) * 2u64, nextvna);
			nextvna += 1u16;
			k += 1;
		};
		vi += 1;
	};

	// ---- compute dynstr size ----
	let dynstrsz: u64 = 1u64;	// leading NUL
	let pi: i32 = 0;
	for (pi < nsos) {
		let so4: *lso = sosused[pi];
		dynstrsz += so4.soname.len: u64;
		dynstrsz += 1u64;
		pi += 1;
	};
	pi = 0;
	for (pi < n) {
		let dsm4: *lsym = dynsyms[pi];
		dynstrsz += dsm4.name.len: u64;
		dynstrsz += 1u64;
		pi += 1;
	};
	pi = 0;
	for (pi < nvers) {
		let nmlen: i32 = drdi32(vernamelenbuf.ptr, (pi: u64) * 4u64);
		dynstrsz += nmlen: u64;
		dynstrsz += 1u64;
		pi += 1;
	};

	// ---- fill dynstr ----
	let dynstr: []u8 = alloc([], dynstrsz)!;
	let dynstrpos: u64 = 1u64;	// past leading NUL

	let sonamestr: []u8 = alloc([], (nsos: u64) * 4u64)!;
	pi = 0;
	for (pi < nsos) {
		dwr32(sonamestr.ptr, (pi: u64) * 4u64, dynstrpos: u32);
		let so2: *lso = sosused[pi];
		let snm: str = so2.soname;
		dbcopy(dynstr.ptr, dynstrpos, snm.ptr, snm.len: u64);
		dynstrpos += snm.len: u64;
		dynstr[dynstrpos] = 0u8;
		dynstrpos += 1u64;
		pi += 1;
	};
	let symnamestr: []u8 = alloc([], nu * 4u64)!;
	pi = 0;
	for (pi < n) {
		dwr32(symnamestr.ptr, (pi: u64) * 4u64, dynstrpos: u32);
		let dsm: *lsym = dynsyms[pi];
		let snm: str = dsm.name;
		dbcopy(dynstr.ptr, dynstrpos, snm.ptr, snm.len: u64);
		dynstrpos += snm.len: u64;
		dynstr[dynstrpos] = 0u8;
		dynstrpos += 1u64;
		pi += 1;
	};
	pi = 0;
	for (pi < nvers) {
		dwr32(verdynstroff.ptr, (pi: u64) * 4u64, dynstrpos: u32);
		let nmp: *u8 = vernameptrarr[pi];
		let nmlen: i32 = drdi32(vernamelenbuf.ptr, (pi: u64) * 4u64);
		dbcopy(dynstr.ptr, dynstrpos, nmp, nmlen: u64);
		dynstrpos += nmlen: u64;
		dynstr[dynstrpos] = 0u8;
		dynstrpos += 1u64;
		pi += 1;
	};

	// ---- per-dyn-sym versym index ----
	let versymfor: []u8 = alloc([], nu * 2u64)!;
	pi = 0;
	for (pi < n) {
		let dsm3: *lsym = dynsyms[pi];
		let dl3: *lso = dsm3.dynlib;
		let nm3: str = dsm3.name;
		let vname: str = soversion(dl3, nm3);
		if (vname.len == 0) {
			dwr16(versymfor.ptr, (pi: u64) * 2u64, VER_NDX_GLOBAL_D);
		} else {
			let matched: i32 = 0;
			let vk: i32 = 0;
			for (vk < nvers) {
				let vlx:  i32 = drdi32(verlibidxbuf.ptr, (vk: u64) * 4u64);
				let sosx: i32 = drdi32(vlibsosidxbuf.ptr, (vlx: u64) * 4u64);
				if (sosused[sosx] == dl3) {
					let exi: str;
					exi.ptr = vernameptrarr[vk];
					exi.len = drdi32(vernamelenbuf.ptr, (vk: u64) * 4u64);
					if (streqd(exi, vname)) {
						let other: u16 = drdu16(vervnaother.ptr, (vk: u64) * 2u64);
						dwr16(versymfor.ptr, (pi: u64) * 2u64, other);
						matched = 1;
						vk = nvers;
					} else { vk += 1; };
				} else { vk += 1; };
			};
			if (matched == 0) {
				dwr16(versymfor.ptr, (pi: u64) * 2u64, VER_NDX_GLOBAL_D);
			};
		};
		pi += 1;
	};

	// ---- compute byte sizes ----
	let ehdrsz: u64 = 64u64;
	let nphdrs: u64 = 4u64;
	let phdrsz: u64 = nphdrs * 56u64;
	let interpsz: u64 = (INTERP.len: u64) + 1u64;

	let nsymstotal: u64 = 1u64 + nu;
	let dynsymsz: u64 = nsymstotal * 24u64;

	let nbuckets: u32 = 1u32;
	let nchain: u32 = nsymstotal: u32;
	let hashsz: u64 = (2u64 + (nbuckets: u64) + (nchain: u64)) * 4u64;

	let relapltsz: u64 = nu * 24u64;
	let pltsz: u64 = nu * PLT_STUB_BYTES_D;
	let gotpltsz: u64 = (3u64 + nu) * 8u64;
	let versymsz: u64 = nsymstotal * 2u64;

	let verneedsz: u64 = 0u64;
	let vli: i32 = 0;
	for (vli < nvlibs) {
		let cnt: i32 = drdi32(vlibcountbuf.ptr, (vli: u64) * 4u64);
		verneedsz += 16u64 + 16u64 * (cnt: u64);
		vli += 1;
	};

	let withver: i32 = 0;
	if (nvlibs > 0) { withver = 1; };
	let extra: u64 = 0u64;
	if (withver != 0) { extra = 3u64; };
	let ndyn: u64 = (nsos: u64) + 11u64 + extra;
	let dynamicsz: u64 = ndyn * 16u64;

	// ---- compute file offsets ----
	let off: u64 = ehdrsz + phdrsz;
	let interpoff: u64 = off;       off += interpsz;
	off = alignup(off, 8u64);
	let dynstroff: u64 = off;       off += dynstrsz;
	off = alignup(off, 8u64);
	let dynsymoff: u64 = off;       off += dynsymsz;
	let hashoff: u64 = off;         off += hashsz;
	off = alignup(off, 2u64);
	let versymoff: u64 = off;       off += versymsz;
	off = alignup(off, 4u64);
	let verneedoff: u64 = off;      off += verneedsz;
	off = alignup(off, 8u64);
	let relapltoff: u64 = off;      off += relapltsz;

	let textoff: u64 = alignup(off, PAGE);
	let pltoff: u64 = textoff + l.textlen;
	let rxend: u64 = pltoff + pltsz;

	let gotpltoff: u64 = alignup(rxend, PAGE);
	let dynamicoff: u64 = gotpltoff + gotpltsz;
	let dataoff: u64 = dynamicoff + dynamicsz;
	let fileend: u64 = dataoff + l.datalen;

	let interpva: u64 = base + interpoff;
	let dynstrva: u64 = base + dynstroff;
	let dynsymva: u64 = base + dynsymoff;
	let hashva: u64 = base + hashoff;
	let versymva: u64 = base + versymoff;
	let verneedva: u64 = base + verneedoff;
	let relapltva: u64 = base + relapltoff;
	let textva: u64 = base + textoff;
	let pltva: u64 = base + pltoff;
	let gotpltva: u64 = base + gotpltoff;
	let dynamicva: u64 = base + dynamicoff;
	let datava: u64 = base + dataoff;

	// Apply relocations now that the dyn layout's textva/datava are
	// pinned. main.ww defers this so each path uses its own VAs.
	if (relocate(l, textva, datava) != 0) { return 1; };

	// BSS optimisation — same trailing-zero scan as out.ww.
	let bsslen: u64 = 0u64;
	if (l.datalen > 0u64) {
		for (bsslen < l.datalen) {
			let b: u8 = l.data[l.datalen - 1u64 - bsslen];
			if (b != 0u8) { break; };
			bsslen += 1u64;
		};
	};
	let datafilelen: u64 = l.datalen - bsslen;
	let filedataend: u64 = dataoff + datafilelen;

	// ---- build .dynsym ----
	let dynsymbuf: []u8 = alloc([], dynsymsz)!;
	pi = 0;
	for (pi < n) {
		let eoff: u64 = (1u64 + (pi: u64)) * 24u64;
		dwr32(dynsymbuf.ptr, eoff + 0u64, drdu32(symnamestr.ptr, (pi: u64) * 4u64));
		dwr8(dynsymbuf.ptr, eoff + 4u64, (STB_GLOBAL_D << 4u8) | (STT_FUNC_D & 15u8));
		dwr8(dynsymbuf.ptr, eoff + 5u64, 0u8);
		dwr16(dynsymbuf.ptr, eoff + 6u64, 0u16);
		dwr64(dynsymbuf.ptr, eoff + 8u64, 0u64);
		dwr64(dynsymbuf.ptr, eoff + 16u64, 0u64);
		pi += 1;
	};

	// ---- build .hash (SysV, 1 bucket) ----
	let hashbuf: []u8 = alloc([], hashsz)!;
	dwr32(hashbuf.ptr, 0u64, nbuckets);
	dwr32(hashbuf.ptr, 4u64, nchain);
	let bucket0: u32 = 0u32;
	if (nsymstotal > 1u64) { bucket0 = 1u32; };
	dwr32(hashbuf.ptr, 8u64, bucket0);
	let ci: u64 = 1u64;
	for (ci < nsymstotal) {
		let nxt: u32 = 0u32;
		if (ci + 1u64 < nsymstotal) { nxt = (ci + 1u64): u32; };
		dwr32(hashbuf.ptr, 8u64 + (nbuckets: u64) * 4u64 + ci * 4u64, nxt);
		ci += 1u64;
	};

	// ---- build .rela.plt ----
	let relapltbuf: []u8 = alloc([], relapltsz)!;
	pi = 0;
	for (pi < n) {
		let roff: u64 = (pi: u64) * 24u64;
		dwr64(relapltbuf.ptr, roff + 0u64, gotpltva + (3u64 + (pi: u64)) * 8u64);
		let info: u64 = ((1u64 + (pi: u64)) << 32u64) | (R_X86_64_JUMP_SLOT_D: u64);
		dwr64(relapltbuf.ptr, roff + 8u64, info);
		dwri64(relapltbuf.ptr, roff + 16u64, 0i64);
		pi += 1;
	};

	// ---- build .gnu.version (u16 per dynsym entry) ----
	let versymbuf: []u8 = alloc([], versymsz)!;
	dwr16(versymbuf.ptr, 0u64, VER_NDX_LOCAL_D);
	pi = 0;
	for (pi < n) {
		dwr16(versymbuf.ptr, 2u64 + (pi: u64) * 2u64, drdu16(versymfor.ptr, (pi: u64) * 2u64));
		pi += 1;
	};

	// ---- build .gnu.version_r ----
	let verneedbuf: []u8 = alloc([], verneedsz)!;
	if (verneedsz > 0u64) {
		let vnoff: u64 = 0u64;
		vli = 0;
		for (vli < nvlibs) {
			let sosidx: i32 = drdi32(vlibsosidxbuf.ptr, (vli: u64) * 4u64);
			let first:   i32 = drdi32(vlibfirstbuf.ptr,   (vli: u64) * 4u64);
			let cnt:     i32 = drdi32(vlibcountbuf.ptr,   (vli: u64) * 4u64);
			let vnstart: u64 = vnoff;
			dwr16(verneedbuf.ptr, vnoff + 0u64, 1u16);
			dwr16(verneedbuf.ptr, vnoff + 2u64, cnt: u16);
			dwr32(verneedbuf.ptr, vnoff + 4u64, drdu32(sonamestr.ptr, (sosidx: u64) * 4u64));
			dwr32(verneedbuf.ptr, vnoff + 8u64, 16u32);
			vnoff += 16u64;
			let k: i32 = 0;
			for (k < cnt) {
				let vk: i32 = first + k;
				let nm: str;
				nm.ptr = vernameptrarr[vk];
				nm.len = drdi32(vernamelenbuf.ptr, (vk: u64) * 4u64);
				let h: u32 = elfhash(nm);
				dwr32(verneedbuf.ptr, vnoff + 0u64, h);
				dwr16(verneedbuf.ptr, vnoff + 4u64, 0u16);
				dwr16(verneedbuf.ptr, vnoff + 6u64, drdu16(vervnaother.ptr, (vk: u64) * 2u64));
				dwr32(verneedbuf.ptr, vnoff + 8u64, drdu32(verdynstroff.ptr, (vk: u64) * 4u64));
				let nxt: u32 = 0u32;
				if (k + 1 < cnt) { nxt = 16u32; };
				dwr32(verneedbuf.ptr, vnoff + 12u64, nxt);
				vnoff += 16u64;
				k += 1;
			};
			let vnnxt: u32 = 0u32;
			if (vli + 1 < nvlibs) { vnnxt = (vnoff - vnstart): u32; };
			dwr32(verneedbuf.ptr, vnstart + 12u64, vnnxt);
			vli += 1;
		};
	};

	// ---- build .plt ----
	let pltbuf: []u8 = alloc([], pltsz)!;
	pi = 0;
	for (pi < n) {
		let poff: u64 = (pi: u64) * PLT_STUB_BYTES_D;
		let stubva: u64 = pltva + poff;
		let nextip: u64 = stubva + 6u64;
		let slotva: u64 = gotpltva + (3u64 + (pi: u64)) * 8u64;
		let disp: i64 = (slotva: i64) - (nextip: i64);
		dwr8(pltbuf.ptr, poff + 0u64, 255u8);
		dwr8(pltbuf.ptr, poff + 1u64, 37u8);
		dwr32(pltbuf.ptr, poff + 2u64, (disp: i32): u32);
		pi += 1;
	};

	// ---- build .got.plt ----
	let gotpltbuf: []u8 = alloc([], gotpltsz)!;
	dwr64(gotpltbuf.ptr, 0u64, dynamicva);

	// ---- build .dynamic ----
	let dynamicbuf: []u8 = alloc([], dynamicsz)!;
	let dk: u64 = 0u64;
	pi = 0;
	for (pi < nsos) {
		dwri64(dynamicbuf.ptr, dk * 16u64 + 0u64, DT_NEEDED);
		dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, drdu32(sonamestr.ptr, (pi: u64) * 4u64): u64);
		dk += 1u64;
		pi += 1;
	};
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_HASH);     dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, hashva);     dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_STRTAB);   dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, dynstrva);   dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_SYMTAB);   dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, dynsymva);   dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_STRSZ);    dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, dynstrsz);   dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_SYMENT);   dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, 24u64);       dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_PLTGOT);   dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, gotpltva);   dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_PLTRELSZ); dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, relapltsz);  dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_PLTREL);   dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, DT_RELA: u64);dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_JMPREL);   dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, relapltva);  dk += 1u64;
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_BIND_NOW); dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, 0u64);        dk += 1u64;
	if (withver != 0) {
		dwri64(dynamicbuf.ptr, dk * 16u64, DT_VERSYM);     dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, versymva);   dk += 1u64;
		dwri64(dynamicbuf.ptr, dk * 16u64, DT_VERNEED);    dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, verneedva);  dk += 1u64;
		dwri64(dynamicbuf.ptr, dk * 16u64, DT_VERNEEDNUM); dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, nvlibs: u64);dk += 1u64;
	};
	dwri64(dynamicbuf.ptr, dk * 16u64, DT_NULL); dwr64(dynamicbuf.ptr, dk * 16u64 + 8u64, 0u64); dk += 1u64;
	if (dk != ndyn) {
		os.write(2, "w6l: dynamic entry count mismatch\n".ptr, 33u64);
		return 1;
	};

	// ---- patch .text relocs targeting dynamic syms ----
	let r: *lrel = l.rels;
	for (r != nil) {
		if (r.sym != nil) {
			let rsym: *lsym = r.sym;
			if (rsym.isdyn != 0) {
				if (r.kind != R_X86_64_PC32_D) {
					if (r.kind != R_X86_64_PLT32_D) {
						os.write(2, "w6l: dynamic reloc kind unsupported\n".ptr, 35u64);
						return 1;
					};
				};
				let site: u64 = textva + r.off;
				let stub: u64 = pltva + (rsym.pltidx: u64) * PLT_STUB_BYTES_D;
				let disp: i64 = (stub: i64) - (site: i64) + r.addend;
				dwr32(l.text, r.off, (disp: i32): u32);
			};
		};
		r = r.rnext;
	};

	// ---- assemble file buffer ----
	let filebuf: []u8 = alloc([], fileend)!;
	filebuf.len = fileend: i32;

	// Ehdr
	dwr8(filebuf.ptr, 0u64, 127u8);
	dwr8(filebuf.ptr, 1u64, 69u8);
	dwr8(filebuf.ptr, 2u64, 76u8);
	dwr8(filebuf.ptr, 3u64, 70u8);
	dwr8(filebuf.ptr, 4u64, ELFCLASS64_D);
	dwr8(filebuf.ptr, 5u64, ELFDATA2LSB_D);
	dwr8(filebuf.ptr, 6u64, EV_CURRENT_D: u8);
	dwr16(filebuf.ptr, 16u64, ET_EXEC_D);
	dwr16(filebuf.ptr, 18u64, EM_X86_64_D);
	dwr32(filebuf.ptr, 20u64, EV_CURRENT_D);
	dwr64(filebuf.ptr, 24u64, entry);
	dwr64(filebuf.ptr, 32u64, ehdrsz);
	dwr64(filebuf.ptr, 40u64, 0u64);
	dwr32(filebuf.ptr, 48u64, 0u32);
	dwr16(filebuf.ptr, 52u64, ehdrsz: u16);
	dwr16(filebuf.ptr, 54u64, 56u16);
	dwr16(filebuf.ptr, 56u64, nphdrs: u16);
	dwr16(filebuf.ptr, 58u64, 0u16);
	dwr16(filebuf.ptr, 60u64, 0u16);
	dwr16(filebuf.ptr, 62u64, 0u16);

	// Phdrs at offset 64.
	let p0: u64 = 64u64;
	dwr32(filebuf.ptr, p0 + 0u64,  PT_LOAD_D);
	dwr32(filebuf.ptr, p0 + 4u64,  PF_R_D | PF_X_D);
	dwr64(filebuf.ptr, p0 + 8u64,  0u64);
	dwr64(filebuf.ptr, p0 + 16u64, base);
	dwr64(filebuf.ptr, p0 + 24u64, base);
	dwr64(filebuf.ptr, p0 + 32u64, rxend);
	dwr64(filebuf.ptr, p0 + 40u64, rxend);
	dwr64(filebuf.ptr, p0 + 48u64, PAGE);

	let p1: u64 = 64u64 + 56u64;
	dwr32(filebuf.ptr, p1 + 0u64,  PT_LOAD_D);
	dwr32(filebuf.ptr, p1 + 4u64,  PF_R_D | PF_W_D);
	dwr64(filebuf.ptr, p1 + 8u64,  gotpltoff);
	dwr64(filebuf.ptr, p1 + 16u64, gotpltva);
	dwr64(filebuf.ptr, p1 + 24u64, gotpltva);
	// filesz trims the .data trailing zeros (BSS); memsz covers
	// .got.plt + .dynamic + the full .data so the loader zero-fills.
	dwr64(filebuf.ptr, p1 + 32u64, filedataend - gotpltoff);
	dwr64(filebuf.ptr, p1 + 40u64, fileend - gotpltoff);
	dwr64(filebuf.ptr, p1 + 48u64, PAGE);

	let p2: u64 = 64u64 + 112u64;
	dwr32(filebuf.ptr, p2 + 0u64,  PT_INTERP_D);
	dwr32(filebuf.ptr, p2 + 4u64,  PF_R_D);
	dwr64(filebuf.ptr, p2 + 8u64,  interpoff);
	dwr64(filebuf.ptr, p2 + 16u64, interpva);
	dwr64(filebuf.ptr, p2 + 24u64, interpva);
	dwr64(filebuf.ptr, p2 + 32u64, interpsz);
	dwr64(filebuf.ptr, p2 + 40u64, interpsz);
	dwr64(filebuf.ptr, p2 + 48u64, 1u64);

	let p3: u64 = 64u64 + 168u64;
	dwr32(filebuf.ptr, p3 + 0u64,  PT_DYNAMIC_D);
	dwr32(filebuf.ptr, p3 + 4u64,  PF_R_D | PF_W_D);
	dwr64(filebuf.ptr, p3 + 8u64,  dynamicoff);
	dwr64(filebuf.ptr, p3 + 16u64, dynamicva);
	dwr64(filebuf.ptr, p3 + 24u64, dynamicva);
	dwr64(filebuf.ptr, p3 + 32u64, dynamicsz);
	dwr64(filebuf.ptr, p3 + 40u64, dynamicsz);
	dwr64(filebuf.ptr, p3 + 48u64, 8u64);

	// Sections.
	dbcopy(filebuf.ptr, interpoff, INTERP.ptr, INTERP.len: u64);
	dwr8(filebuf.ptr, interpoff + (INTERP.len: u64), 0u8);
	dbcopy(filebuf.ptr, dynstroff, dynstr.ptr, dynstrsz);
	dbcopy(filebuf.ptr, dynsymoff, dynsymbuf.ptr, dynsymsz);
	dbcopy(filebuf.ptr, hashoff, hashbuf.ptr, hashsz);
	dbcopy(filebuf.ptr, versymoff, versymbuf.ptr, versymsz);
	if (verneedsz > 0u64) {
		dbcopy(filebuf.ptr, verneedoff, verneedbuf.ptr, verneedsz);
	};
	dbcopy(filebuf.ptr, relapltoff, relapltbuf.ptr, relapltsz);
	if (l.textlen > 0u64) {
		dbcopy(filebuf.ptr, textoff, l.text, l.textlen);
	};
	dbcopy(filebuf.ptr, pltoff, pltbuf.ptr, pltsz);
	dbcopy(filebuf.ptr, gotpltoff, gotpltbuf.ptr, gotpltsz);
	dbcopy(filebuf.ptr, dynamicoff, dynamicbuf.ptr, dynamicsz);
	if (datafilelen > 0u64) {
		dbcopy(filebuf.ptr, dataoff, l.data, datafilelen);
	};

	let wr: (i64 | os.oserror) = os.writeall(fd, filebuf.ptr, filedataend);
	match (wr) {
	case let v: i64 => { if (v != filedataend: i64) { return 1; }; };
	case let e: os.oserror => return 1;
	};
	return 0;
};