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Date: Fri, 26 Apr 2019 20:13:29 -0500
From: Rodger Combs <rodger.combs@...il.com>
To: musl@...ts.openwall.com
Subject: [PATCH 3/3] crt: add dcrt1, with support for locating the dynamic loader at runtime

---
 Makefile                |  10 +-
 crt/dcrt1.c             | 362 ++++++++++++++++++++++++++++++++++++++++++++++++
 ldso/dlstart.c          |  57 +++++---
 ldso/dynlink.c          |  13 ++
 ldso/map_library.h      |  59 ++++----
 src/internal/dynlink.h  |   2 +-
 tools/ld.musl-clang.in  |  17 ++-
 tools/musl-clang.in     |   8 ++
 tools/musl-gcc.specs.sh |   4 +-
 9 files changed, 479 insertions(+), 53 deletions(-)
 create mode 100644 crt/dcrt1.c

diff --git a/Makefile b/Makefile
index b46f8ca..163bf03 100644
--- a/Makefile
+++ b/Makefile
@@ -105,13 +105,15 @@ obj/src/internal/version.h: $(wildcard $(srcdir)/VERSION $(srcdir)/.git)
 
 obj/src/internal/version.o obj/src/internal/version.lo: obj/src/internal/version.h
 
-obj/crt/rcrt1.o obj/ldso/dlstart.lo obj/ldso/dynlink.lo: $(srcdir)/src/internal/dynlink.h $(srcdir)/arch/$(ARCH)/reloc.h
+obj/crt/rcrt1.o obj/crt/dcrt1.o obj/ldso/dlstart.lo obj/ldso/dynlink.lo: $(srcdir)/src/internal/dynlink.h $(srcdir)/arch/$(ARCH)/reloc.h
 
-obj/crt/crt1.o obj/crt/scrt1.o obj/crt/rcrt1.o obj/ldso/dlstart.lo: $(srcdir)/arch/$(ARCH)/crt_arch.h
+obj/crt/crt1.o obj/crt/scrt1.o obj/crt/rcrt1.o obj/crt/dcrt1.o obj/ldso/dlstart.lo: $(srcdir)/arch/$(ARCH)/crt_arch.h
 
-obj/crt/rcrt1.o: $(srcdir)/ldso/dlstart.c
+obj/crt/rcrt1.o obj/crt/dcrt1.o: $(srcdir)/ldso/dlstart.c
 
-obj/crt/Scrt1.o obj/crt/rcrt1.o: CFLAGS_ALL += -fPIC
+obj/crt/Scrt1.o obj/crt/rcrt1.o obj/crt/dcrt1.o: CFLAGS_ALL += -fPIC
+
+obj/crt/dcrt1.o: CFLAGS_ALL += -DLDSO_PATHNAME=\"$(LDSO_PATHNAME)\"
 
 OPTIMIZE_SRCS = $(wildcard $(OPTIMIZE_GLOBS:%=$(srcdir)/src/%))
 $(OPTIMIZE_SRCS:$(srcdir)/%.c=obj/%.o) $(OPTIMIZE_SRCS:$(srcdir)/%.c=obj/%.lo): CFLAGS += -O3
diff --git a/crt/dcrt1.c b/crt/dcrt1.c
new file mode 100644
index 0000000..47c6dc2
--- /dev/null
+++ b/crt/dcrt1.c
@@ -0,0 +1,362 @@
+#define SYSCALL_NO_TLS
+
+#include <elf.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <features.h>
+#include <libgen.h>
+#include <sys/mman.h>
+#include <string.h>
+#include <unistd.h>
+#include "atomic.h"
+#include "dynlink.h"
+#include "syscall.h"
+
+extern weak hidden const size_t _DYNAMIC[];
+
+int main();
+weak void _init();
+weak void _fini();
+weak _Noreturn int __libc_start_main(int (*)(), int, char **,
+	void (*)(), void(*)(), void(*)());
+
+#define DLSTART_PROLOGUE __libc_start_main(main, argc, argv, _init, _fini, 0);
+
+#define START "_start"
+#define _dlstart_c _start_c
+#include "../ldso/dlstart.c"
+
+struct dso {
+	unsigned char *base;
+	struct fdpic_loadmap *loadmap;
+	size_t *dynv;
+	Phdr *phdr;
+	int phnum;
+	size_t phentsize;
+	unsigned char *map;
+	size_t map_len;
+};
+
+#ifndef PAGE_SIZE
+#define PAGE_SIZE SYSCALL_MMAP2_UNIT
+#endif
+
+#ifdef SYS_mmap2
+#define mmap(start, len, prot, flags, fd, off) (void*)__syscall(SYS_mmap2, start, len, prot, flags, fd, off/SYSCALL_MMAP2_UNIT)
+#else
+#define mmap(start, len, prot, flags, fd, off) (void*)__syscall(SYS_mmap, start, len, prot, flags, fd, off)
+#endif
+
+#define munmap(ptr, len) __syscall(SYS_munmap, ptr, len)
+
+static inline int crt_mprotect(void *addr, size_t len, int prot)
+{
+	size_t start, end;
+	start = (size_t)addr & -PAGE_SIZE;
+	end = (size_t)((char *)addr + len + PAGE_SIZE-1) & -PAGE_SIZE;
+	return __syscall(SYS_mprotect, start, end-start, prot);
+}
+#define mprotect crt_mprotect
+
+#define read(fd, buf, size) __syscall(SYS_read, fd, buf, size)
+#define pread(fd, buf, size, ofs) __syscall(SYS_pread, fd, buf, size, __SYSCALL_LL_PRW(ofs))
+
+static inline off_t crt_lseek(int fd, off_t offset, int whence)
+{
+#ifdef SYS__llseek
+	off_t result;
+	return __syscall(SYS__llseek, fd, offset>>32, offset, &result, whence) ? -1 : result;
+#else
+	return __syscall(SYS_lseek, fd, offset, whence);
+#endif
+}
+#define lseek crt_lseek
+
+static inline void *map_library_allocz(size_t *size)
+{
+	void *ret;
+	*size = (*size + SYSCALL_MMAP2_UNIT - 1) & -SYSCALL_MMAP2_UNIT;
+	ret = mmap(0, *size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (ret == MAP_FAILED)
+		return NULL;
+	return ret;
+}
+
+static inline void map_library_free(void *ptr, size_t size)
+{
+	munmap(ptr, size);
+}
+
+#define map_library_failed(val) ((unsigned long)val > -4096UL)
+#define map_library_error(ret) (-(long)ret)
+
+#ifdef SYS_readlink
+#define readlink(path, buf, bufsize) __syscall(SYS_readlink, path, buf, bufsize)
+#else
+#define readlink(path, buf, bufsize) __syscall(SYS_readlinkat, AT_FDCWD, path, buf, bufsize)
+#endif
+
+#ifdef SYS_access
+#define access(filename, amode) __syscall(SYS_access, filename, amode)
+#else
+#define access(filename, amode) __syscall(SYS_faccessat, AT_FDCWD, filename, amode, 0)
+#endif
+
+static void *crt_memcpy(void *restrict dest, const void *restrict src, size_t n)
+{
+	unsigned char *d = dest;
+	const unsigned char *s = src;
+	for (; n; n--) *d++ = *s++;
+	return dest;
+}
+
+static void *crt_memset(void *dest, int c, size_t n)
+{
+	unsigned char *s = dest;
+	for (; n; n--, s++) *s = c;
+	return dest;
+}
+#define memset crt_memset
+
+static size_t crt_strlen(const char *s)
+{
+	const char *a = s;
+	for (; *s; s++);
+	return s-a;
+}
+
+static char *crt_strchrnul(const char *s, int c)
+{
+	c = (unsigned char)c;
+	if (!c) return (char *)s + crt_strlen(s);
+	for (; *s && *(unsigned char *)s != c; s++);
+	return (char *)s;
+}
+
+static int crt_strncmp(const char *_l, const char *_r, size_t n)
+{
+	const unsigned char *l=(void *)_l, *r=(void *)_r;
+	if (!n--) return 0;
+	for (; *l && *r && n && *l == *r ; l++, r++, n--);
+	return *l - *r;
+}
+
+static char *crt_getenv(const char *name, char **environ)
+{
+	size_t l = crt_strchrnul(name, '=') - name;
+	if (l && !name[l] && environ)
+		for (char **e = environ; *e; e++)
+			if (!crt_strncmp(name, *e, l) && l[*e] == '=')
+				return *e + l+1;
+	return 0;
+}
+
+#define LD_CRT
+#include "../ldso/map_library.h"
+
+static void decode_vec(size_t *v, size_t *a, size_t cnt)
+{
+	size_t i;
+	for (i=0; i<cnt; i++) a[i] = 0;
+	for (; v[0]; v+=2) if (v[0]-1<cnt-1) {
+		a[0] |= 1UL<<v[0];
+		a[v[0]] = v[1];
+	}
+}
+
+static void get_rpaths(const char **rpath, const char **runpath, const size_t *dyn)
+{
+	/* DT_STRTAB is pre-relocated for us by dlstart */
+	const char *strings = (void*)dyn[DT_STRTAB];
+
+	if (dyn[0] & (1 << DT_RPATH))
+		*rpath = strings + dyn[DT_RPATH];
+	if (dyn[0] & (1 << DT_RUNPATH))
+		*runpath = strings + dyn[DT_RUNPATH];
+}
+
+static size_t find_linker(char *outbuf, size_t bufsize, const char *this_path, size_t thisl, const size_t *dyn, char **environ, int secure)
+{
+	const char *paths[3] = {0}; // rpath, envpath, runpath
+	size_t i;
+	int fd;
+
+	if (thisl)
+		thisl--;
+	while (thisl > 1 && this_path[thisl] == '/')
+		thisl--;
+	while (thisl > 0 && this_path[thisl] != '/')
+		thisl--;
+
+	if (!secure) {
+		const char *envpath = crt_getenv("LD_LOADER_PATH", environ);
+		if (envpath) {
+			size_t envlen = crt_strlen(envpath);
+			if (envlen < bufsize) {
+				crt_memcpy(outbuf, envpath, envlen + 1);
+				return envlen + 1;
+			}
+		}
+	}
+
+	get_rpaths(&paths[0], &paths[2], dyn);
+
+	paths[1] = secure ? NULL : crt_getenv("LD_LIBRARY_PATH", environ);
+
+	for (i = 0; i < 3; i++) {
+		int relative = 0;
+		const char *p = paths[i];
+		char *o = outbuf;
+		if (!p)
+			continue;
+		for (;;) {
+			if (!crt_strncmp(p, "$ORIGIN", 7) ||
+					!crt_strncmp(p, "${ORIGIN}", 9)) {
+				relative = 1;
+				if (o + thisl + 1 < outbuf + bufsize) {
+					crt_memcpy(o, this_path, thisl);
+					o += thisl;
+				} else {
+					o = outbuf + bufsize - 1;
+				}
+				p += (p[1] == '{' ? 9 : 7);
+			} else if (*p == ':' || !*p) {
+#define LDSO_FILENAME "ld-musl-" LDSO_ARCH ".so.1"
+				relative |= outbuf[0] != '/';
+				if ((!secure || !relative) && o + sizeof(LDSO_FILENAME) + 1 < outbuf + bufsize) {
+					*o++ = '/';
+					crt_memcpy(o, LDSO_FILENAME, sizeof(LDSO_FILENAME));
+					if (!access(outbuf, R_OK | X_OK))
+						return (o + sizeof(LDSO_FILENAME)) - outbuf;
+				}
+				if (!*p)
+					break;
+				relative = 0;
+				o = outbuf;
+				p++;
+			} else {
+				if (o < outbuf + bufsize)
+					*o++ = *p;
+				p++;
+			}
+		}
+	}
+
+	// Didn't find a usable loader anywhere, so try the hardcoded path :shrug:
+	crt_memcpy(outbuf, LDSO_PATHNAME, sizeof(LDSO_PATHNAME));
+	return sizeof(LDSO_PATHNAME);
+}
+
+static void final_start_c(long *p)
+{
+	int argc = p[0];
+	char **argv = (void *)(p+1);
+	__libc_start_main(main, argc, argv, _init, _fini, 0);
+}
+
+hidden _Noreturn void __dls2(unsigned char *base, size_t *p, size_t *dyn)
+{
+	int argc = p[0];
+	char **argv = (void *)(p+1);
+	int fd;
+	int secure;
+	int prot = PROT_READ;
+	struct dso loader;
+	Ehdr *loader_hdr;
+	Phdr *new_hdr;
+	void *entry;
+	char this_path[2*NAME_MAX+2] = {0};
+	size_t thisl;
+	char linker_path[2*NAME_MAX+2] = {0};
+	size_t linker_len;
+	size_t i;
+	size_t aux[AUX_CNT];
+	size_t *auxv;
+	char **environ = argv + argc + 1;
+
+	/* Find aux vector just past environ[] and use it to initialize
+	* global data that may be needed before we can make syscalls. */
+	for (i = argc + 1; argv[i]; i++);
+	auxv = (void *)(argv + i + 1);
+	decode_vec(auxv, aux, AUX_CNT);
+	secure = ((aux[0] & 0x7800) != 0x7800 || aux[AT_UID] != aux[AT_EUID]
+		|| aux[AT_GID] != aux[AT_EGID] || aux[AT_SECURE]);
+
+	thisl = readlink("/proc/self/exe", this_path, sizeof this_path);
+	linker_len = find_linker(linker_path, sizeof linker_path, this_path, thisl, dyn, environ, secure);
+
+	fd = __sys_open2(, linker_path, O_RDONLY);
+	if (fd < 0)
+		goto error;
+
+	loader_hdr = map_library(fd, &loader);
+	if (!loader_hdr)
+		goto error;
+
+	__syscall(SYS_close, fd);
+
+	// Copy the program headers into an anonymous mapping
+	new_hdr = mmap(0, (aux[AT_PHENT] * (aux[AT_PHNUM] + 2) + linker_len + PAGE_SIZE - 1) & -PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (map_library_failed(new_hdr))
+		goto error;
+
+	// Point it back at the original kernel-provided base
+	new_hdr->p_type = PT_PHDR;
+	new_hdr->p_vaddr = (size_t)new_hdr - (size_t)base;
+
+	((Phdr*)((char*)new_hdr + aux[AT_PHENT]))->p_type = PT_INTERP;
+	((Phdr*)((char*)new_hdr + aux[AT_PHENT]))->p_vaddr = new_hdr->p_vaddr + aux[AT_PHENT] * (aux[AT_PHNUM] + 2);
+
+	crt_memcpy((char*)new_hdr + aux[AT_PHENT] * (aux[AT_PHNUM] + 2), linker_path, linker_len);
+
+	for (i = 0; i < aux[AT_PHNUM]; i++) {
+		Phdr *hdr = (void*)((char*)aux[AT_PHDR] + aux[AT_PHENT] * i);
+		Phdr *dst = (void*)((char*)new_hdr + aux[AT_PHENT] * (i + 2));
+		if (hdr->p_type == PT_PHDR || hdr->p_type == PT_INTERP) {
+			// Can't have a duplicate
+			dst->p_type = PT_NULL;
+		} else {
+			crt_memcpy(dst, hdr, aux[AT_PHENT]);
+		}
+	}
+
+	if (mprotect(new_hdr, aux[AT_PHENT] * (aux[AT_PHNUM] + 2) + linker_len, PROT_READ))
+		goto error;
+
+	for (i=0; auxv[i]; i+=2) {
+		if (auxv[i] == AT_BASE)
+			auxv[i + 1] = (size_t)loader_hdr;
+		if (auxv[i] == AT_PHDR)
+			auxv[i + 1] = (size_t)new_hdr;
+		if (auxv[i] == AT_PHNUM)
+			auxv[i + 1] += 2;
+	}
+
+	entry = laddr(&loader, loader_hdr->e_entry);
+
+#ifndef LD_FDPIC
+	/* Undo the relocations performed by dlstart */
+
+	if (NEED_MIPS_GOT_RELOCS) {
+		const size_t *dynv = _DYNAMIC;
+		size_t local_cnt = 0;
+		size_t *got = (void *)(base + dyn[DT_PLTGOT]);
+		for (i=0; dynv[i]; i+=2) if (dynv[i]==DT_MIPS_LOCAL_GOTNO)
+			local_cnt = dynv[i+1];
+		for (i=0; i<local_cnt; i++) got[i] -= (size_t)base;
+	}
+
+	size_t *rel = (void *)((size_t)base+dyn[DT_REL]);
+	size_t rel_size = dyn[DT_RELSZ];
+	for (; rel_size; rel+=2, rel_size-=2*sizeof(size_t)) {
+		if (!IS_RELATIVE(rel[1], 0)) continue;
+		size_t *rel_addr = (void *)((size_t)base + rel[0]);
+		*rel_addr -= (size_t)base;
+	}
+#endif
+
+	CRTJMP(entry, argv - 1);
+
+error:
+	for(;;) a_crash();
+}
diff --git a/ldso/dlstart.c b/ldso/dlstart.c
index 20d50f2..7ff79d6 100644
--- a/ldso/dlstart.c
+++ b/ldso/dlstart.c
@@ -21,7 +21,7 @@
 hidden void _dlstart_c(size_t *sp, size_t *dynv)
 {
 	size_t i, aux[AUX_CNT], dyn[DYN_CNT];
-	size_t *rel, rel_size, base;
+	size_t *rel, rel_size, base, loader_phdr;
 
 	int argc = *sp;
 	char **argv = (void *)(sp+1);
@@ -41,6 +41,13 @@ hidden void _dlstart_c(size_t *sp, size_t *dynv)
 		 * space and moving the extra fdpic arguments to the stack
 		 * vector where they are easily accessible from C. */
 		segs = ((struct fdpic_loadmap *)(sp[-1] ? sp[-1] : sp[-2]))->segs;
+		if (aux[AT_BASE]) {
+			Ehdr *eh = (void*)aux[AT_BASE];
+			for (i = 0; eh->e_phoff - segs[i].p_vaddr >= segs[i].p_memsz; i++);
+			loader_phdr = (eh->e_phoff - segs[i].p_vaddr + segs[i].addr);
+		} else {
+			loader_phdr = aux[AT_PHDR];
+		}
 	} else {
 		/* If dynv is null, the entry point was started from loader
 		 * that is not fdpic-aware. We can assume normal fixed-
@@ -55,6 +62,7 @@ hidden void _dlstart_c(size_t *sp, size_t *dynv)
 		segs[0].p_memsz = -1;
 		Ehdr *eh = (void *)base;
 		Phdr *ph = (void *)(base + eh->e_phoff);
+		loader_phdr = (size_t)ph;
 		size_t phnum = eh->e_phnum;
 		size_t phent = eh->e_phentsize;
 		while (phnum-- && ph->p_type != PT_DYNAMIC)
@@ -69,13 +77,38 @@ hidden void _dlstart_c(size_t *sp, size_t *dynv)
 
 #if DL_FDPIC
 	for (i=0; i<DYN_CNT; i++) {
-		if (i==DT_RELASZ || i==DT_RELSZ) continue;
+		if (i==DT_RELASZ || i==DT_RELSZ || i==DT_RPATH || i==DT_RUNPATH) continue;
 		if (!dyn[i]) continue;
 		for (j=0; dyn[i]-segs[j].p_vaddr >= segs[j].p_memsz; j++);
 		dyn[i] += segs[j].addr - segs[j].p_vaddr;
 	}
 	base = 0;
+#else
+	/* If the dynamic linker is invoked as a command, its load
+	 * address is not available in the aux vector. Instead, compute
+	 * the load address as the difference between &_DYNAMIC and the
+	 * virtual address in the PT_DYNAMIC program header. */
+	base = aux[AT_BASE];
+	if (!base) {
+		size_t phnum = aux[AT_PHNUM];
+		size_t phentsize = aux[AT_PHENT];
+		Phdr *ph = (void *)aux[AT_PHDR];
+		for (i=phnum; i--; ph = (void *)((char *)ph + phentsize)) {
+			if (ph->p_type == PT_DYNAMIC) {
+				base = (size_t)dynv - ph->p_vaddr;
+				break;
+			}
+		}
+	}
+	loader_phdr = base + ((Ehdr*)base)->e_phoff;
+#endif
 
+#ifdef DLSTART_PROLOGUE
+	if (aux[AT_PHDR] != loader_phdr)
+		DLSTART_PROLOGUE
+#endif
+
+#if DL_FDPIC
 	const Sym *syms = (void *)dyn[DT_SYMTAB];
 
 	rel = (void *)dyn[DT_RELA];
@@ -97,22 +130,8 @@ hidden void _dlstart_c(size_t *sp, size_t *dynv)
 		}
 	}
 #else
-	/* If the dynamic linker is invoked as a command, its load
-	 * address is not available in the aux vector. Instead, compute
-	 * the load address as the difference between &_DYNAMIC and the
-	 * virtual address in the PT_DYNAMIC program header. */
-	base = aux[AT_BASE];
-	if (!base) {
-		size_t phnum = aux[AT_PHNUM];
-		size_t phentsize = aux[AT_PHENT];
-		Phdr *ph = (void *)aux[AT_PHDR];
-		for (i=phnum; i--; ph = (void *)((char *)ph + phentsize)) {
-			if (ph->p_type == PT_DYNAMIC) {
-				base = (size_t)dynv - ph->p_vaddr;
-				break;
-			}
-		}
-	}
+	/* For convenience in dcrt1, we relocate STRTAB here (as with FDPIC) */
+	dyn[DT_STRTAB] += base;
 
 	/* MIPS uses an ugly packed form for GOT relocations. Since we
 	 * can't make function calls yet and the code is tiny anyway,
@@ -144,5 +163,5 @@ hidden void _dlstart_c(size_t *sp, size_t *dynv)
 
 	stage2_func dls2;
 	GETFUNCSYM(&dls2, __dls2, base+dyn[DT_PLTGOT]);
-	dls2((void *)base, sp);
+	dls2((void *)base, sp, dyn);
 }
diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index d761837..d4b2331 100644
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -153,6 +153,19 @@ extern hidden void (*const __init_array_end)(void), (*const __fini_array_end)(vo
 weak_alias(__init_array_start, __init_array_end);
 weak_alias(__fini_array_start, __fini_array_end);
 
+static inline void *map_library_allocz(size_t *size)
+{
+	return calloc(1, *size);
+}
+
+static inline void map_library_free(void *ptr, size_t size)
+{
+	return free(ptr);
+}
+
+#define map_library_failed(ptr) (ptr == MAP_FAILED)
+#define map_library_error(val) (errno)
+
 #include "map_library.h"
 
 static int dl_strcmp(const char *l, const char *r)
diff --git a/ldso/map_library.h b/ldso/map_library.h
index 3609a0f..27d2a7e 100644
--- a/ldso/map_library.h
+++ b/ldso/map_library.h
@@ -1,11 +1,3 @@
-#include <errno.h>
-#include <features.h>
-#include <string.h>
-#include <sys/mman.h>
-#include <unistd.h>
-#include "dynlink.h"
-#include "pthread_impl.h"
-
 /* Compute load address for a virtual address in a given dso. */
 #if DL_FDPIC
 static inline void *laddr(const struct dso *p, size_t v)
@@ -44,7 +36,7 @@ static void *mmap_fixed(void *p, size_t n, int prot, int flags, int fd, off_t of
 	char *q;
 	if (!no_map_fixed) {
 		q = mmap(p, n, prot, flags|MAP_FIXED, fd, off);
-		if (!DL_NOMMU_SUPPORT || q != MAP_FAILED || errno != EINVAL)
+		if (!DL_NOMMU_SUPPORT || !map_library_failed(q) || map_library_error(q) != EINVAL)
 			return q;
 		no_map_fixed = 1;
 	}
@@ -57,7 +49,7 @@ static void *mmap_fixed(void *p, size_t n, int prot, int flags, int fd, off_t of
 	if (lseek(fd, off, SEEK_SET) < 0) return MAP_FAILED;
 	for (q=p; n; q+=r, off+=r, n-=r) {
 		r = read(fd, q, n);
-		if (r < 0 && errno != EINTR) return MAP_FAILED;
+		if (r < 0 && map_library_error(r) != EINTR) return MAP_FAILED;
 		if (!r) {
 			memset(q, 0, n);
 			break;
@@ -66,6 +58,11 @@ static void *mmap_fixed(void *p, size_t n, int prot, int flags, int fd, off_t of
 	return p;
 }
 
+static inline int map_library_error_check(int ret, int expected)
+{
+	return ret && map_library_error(ret) != expected;
+}
+
 static inline void unmap_library(struct dso *dso)
 {
 	if (dso->loadmap) {
@@ -76,7 +73,7 @@ static inline void unmap_library(struct dso *dso)
 			munmap((void *)dso->loadmap->segs[i].addr,
 				dso->loadmap->segs[i].p_memsz);
 		}
-		free(dso->loadmap);
+		map_library_free(dso->loadmap, dso->map_len);
 	} else if (dso->map && dso->map_len) {
 		munmap(dso->map, dso->map_len);
 	}
@@ -86,8 +83,8 @@ static inline void *map_library(int fd, struct dso *dso)
 {
 	Ehdr buf[(896+sizeof(Ehdr))/sizeof(Ehdr)];
 	void *allocated_buf=0;
+	size_t allocated_buf_size;
 	size_t phsize;
-	size_t ph_allocated_size;
 	size_t addr_min=SIZE_MAX, addr_max=0, map_len;
 	size_t this_min, this_max;
 	size_t nsegs = 0;
@@ -98,7 +95,9 @@ static inline void *map_library(int fd, struct dso *dso)
 	unsigned char *map=MAP_FAILED, *base;
 	size_t dyn=0;
 	size_t i;
+#ifndef LD_CRT
 	size_t tls_image=0;
+#endif
 
 	ssize_t l = read(fd, buf, sizeof buf);
 	eh = buf;
@@ -107,7 +106,8 @@ static inline void *map_library(int fd, struct dso *dso)
 		goto noexec;
 	phsize = eh->e_phentsize * eh->e_phnum;
 	if (phsize > sizeof buf - sizeof *eh) {
-		allocated_buf = malloc(phsize);
+		allocated_buf_size = phsize;
+		allocated_buf = map_library_allocz(&allocated_buf_size);
 		if (!allocated_buf) return 0;
 		l = pread(fd, allocated_buf, phsize, eh->e_phoff);
 		if (l < 0) goto error;
@@ -124,6 +124,7 @@ static inline void *map_library(int fd, struct dso *dso)
 	for (i=eh->e_phnum; i; i--, ph=(void *)((char *)ph+eh->e_phentsize)) {
 		if (ph->p_type == PT_DYNAMIC) {
 			dyn = ph->p_vaddr;
+#ifndef LD_CRT
 		} else if (ph->p_type == PT_TLS) {
 			tls_image = ph->p_vaddr;
 			dso->tls.align = ph->p_align;
@@ -138,6 +139,7 @@ static inline void *map_library(int fd, struct dso *dso)
 					ph->p_memsz < DEFAULT_STACK_MAX ?
 					ph->p_memsz : DEFAULT_STACK_MAX;
 			}
+#endif
 		}
 		if (ph->p_type != PT_LOAD) continue;
 		nsegs++;
@@ -154,8 +156,8 @@ static inline void *map_library(int fd, struct dso *dso)
 	}
 	if (!dyn) goto noexec;
 	if (DL_FDPIC && !(eh->e_flags & FDPIC_CONSTDISP_FLAG)) {
-		dso->loadmap = calloc(1, sizeof *dso->loadmap
-			+ nsegs * sizeof *dso->loadmap->segs);
+		dso->map_len = sizeof *dso->loadmap + nsegs * sizeof *dso->loadmap->segs;
+		dso->loadmap = map_library_allocz(&dso->map_len);
 		if (!dso->loadmap) goto error;
 		dso->loadmap->nsegs = nsegs;
 		for (ph=ph0, i=0; i<nsegs; ph=(void *)((char *)ph+eh->e_phentsize)) {
@@ -166,7 +168,7 @@ static inline void *map_library(int fd, struct dso *dso)
 			map = mmap(0, ph->p_memsz + (ph->p_vaddr & PAGE_SIZE-1),
 				prot, MAP_PRIVATE,
 				fd, ph->p_offset & -PAGE_SIZE);
-			if (map == MAP_FAILED) {
+			if (map_library_failed(map)) {
 				unmap_library(dso);
 				goto error;
 			}
@@ -181,10 +183,10 @@ static inline void *map_library(int fd, struct dso *dso)
 				size_t pgbrk = brk + PAGE_SIZE-1 & -PAGE_SIZE;
 				size_t pgend = brk + ph->p_memsz - ph->p_filesz
 					+ PAGE_SIZE-1 & -PAGE_SIZE;
-				if (pgend > pgbrk && mmap_fixed(map+pgbrk,
+				if (pgend > pgbrk && !map_library_failed(mmap_fixed(map+pgbrk,
 					pgend-pgbrk, prot,
 					MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS,
-					-1, off_start) == MAP_FAILED)
+					-1, off_start)))
 					goto error;
 				memset(map + brk, 0, pgbrk-brk);
 			}
@@ -207,13 +209,15 @@ static inline void *map_library(int fd, struct dso *dso)
 			MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
 		: mmap((void *)addr_min, map_len, prot,
 			MAP_PRIVATE, fd, off_start);
-	if (map==MAP_FAILED) goto error;
+	if (map_library_failed(map)) goto error;
 	dso->map = map;
 	dso->map_len = map_len;
 	/* If the loaded file is not relocatable and the requested address is
 	 * not available, then the load operation must fail. */
 	if (eh->e_type != ET_DYN && addr_min && map!=(void *)addr_min) {
+#ifndef LD_CRT
 		errno = EBUSY;
+#endif
 		goto error;
 	}
 	base = map - addr_min;
@@ -238,33 +242,36 @@ static inline void *map_library(int fd, struct dso *dso)
 			((ph->p_flags&PF_X) ? PROT_EXEC : 0));
 		/* Reuse the existing mapping for the lowest-address LOAD */
 		if ((ph->p_vaddr & -PAGE_SIZE) != addr_min || DL_NOMMU_SUPPORT)
-			if (mmap_fixed(base+this_min, this_max-this_min, prot, MAP_PRIVATE|MAP_FIXED, fd, off_start) == MAP_FAILED)
+			if (map_library_failed(mmap_fixed(base+this_min, this_max-this_min, prot, MAP_PRIVATE|MAP_FIXED, fd, off_start)))
 				goto error;
 		if (ph->p_memsz > ph->p_filesz && (ph->p_flags&PF_W)) {
 			size_t brk = (size_t)base+ph->p_vaddr+ph->p_filesz;
 			size_t pgbrk = brk+PAGE_SIZE-1 & -PAGE_SIZE;
 			memset((void *)brk, 0, pgbrk-brk & PAGE_SIZE-1);
-			if (pgbrk-(size_t)base < this_max && mmap_fixed((void *)pgbrk, (size_t)base+this_max-pgbrk, prot, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) == MAP_FAILED)
+			if (pgbrk-(size_t)base < this_max && map_library_failed(mmap_fixed((void *)pgbrk, (size_t)base+this_max-pgbrk, prot, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0)))
 				goto error;
 		}
 	}
 	for (i=0; ((size_t *)(base+dyn))[i]; i+=2)
 		if (((size_t *)(base+dyn))[i]==DT_TEXTREL) {
-			if (mprotect(map, map_len, PROT_READ|PROT_WRITE|PROT_EXEC)
-			    && errno != ENOSYS)
+			if (map_library_error_check(mprotect(map, map_len, PROT_READ|PROT_WRITE|PROT_EXEC), ENOSYS))
 				goto error;
 			break;
 		}
 done_mapping:
 	dso->base = base;
 	dso->dynv = laddr(dso, dyn);
+#ifndef LD_CRT
 	if (dso->tls.size) dso->tls.image = laddr(dso, tls_image);
-	free(allocated_buf);
+#endif
+	map_library_free(allocated_buf, allocated_buf_size);
 	return map;
 noexec:
+#ifndef LD_CRT
 	errno = ENOEXEC;
+#endif
 error:
-	if (map!=MAP_FAILED) unmap_library(dso);
-	free(allocated_buf);
+	if (!map_library_failed(map)) unmap_library(dso);
+	map_library_free(allocated_buf, allocated_buf_size);
 	return 0;
 }
diff --git a/src/internal/dynlink.h b/src/internal/dynlink.h
index cbe0a6f..7a9560d 100644
--- a/src/internal/dynlink.h
+++ b/src/internal/dynlink.h
@@ -94,7 +94,7 @@ struct fdpic_dummy_loadmap {
 #define AUX_CNT 32
 #define DYN_CNT 32
 
-typedef void (*stage2_func)(unsigned char *, size_t *);
+typedef _Noreturn void (*stage2_func)(unsigned char *, size_t *, size_t *);
 typedef _Noreturn void (*stage3_func)(size_t *);
 
 hidden void *__dlsym(void *restrict, const char *restrict, void *restrict);
diff --git a/tools/ld.musl-clang.in b/tools/ld.musl-clang.in
index 93763d6..02d9893 100644
--- a/tools/ld.musl-clang.in
+++ b/tools/ld.musl-clang.in
@@ -7,6 +7,18 @@ shared=
 userlinkdir=
 userlink=
 
+Scrt="$libc_lib/Scrt1.o"
+dynamic_linker_args="-dynamic-linker \"$ldso\""
+
+for x ; do
+    case "$x" in
+        -l-dni)
+            dynamic_linker_args="-no-dynamic-linker"
+            Scrt="$libc_lib/dcrt1.o"
+            ;;
+    esac
+done
+
 for x ; do
     test "$cleared" || set -- ; cleared=1
 
@@ -42,10 +54,13 @@ for x ; do
             ;;
         -sysroot=*|--sysroot=*)
             ;;
+        $libc_lib/Scrt1.o)
+            set -- "$@" $Scrt
+            ;;
         *)
             set -- "$@" "$x"
             ;;
     esac
 done
 
-exec $($cc -print-prog-name=ld) -nostdlib "$@" -lc -dynamic-linker "$ldso"
+exec $($cc -print-prog-name=ld) -nostdlib "$@" -lc "$dynamic_linker_args"
diff --git a/tools/musl-clang.in b/tools/musl-clang.in
index 623de6f..49cba1b 100644
--- a/tools/musl-clang.in
+++ b/tools/musl-clang.in
@@ -5,14 +5,21 @@ libc_inc="@INCDIR@"
 libc_lib="@LIBDIR@"
 thisdir="`cd "$(dirname "$0")"; pwd`"
 
+cleared=
+
 # prevent clang from running the linker (and erroring) on no input.
 sflags=
 eflags=
+dniflags=
 for x ; do
+    test "$cleared" || set -- ; cleared=1
+
     case "$x" in
+        --dni) dniflags=-l-dni; continue ;;
         -l*) input=1 ;;
         *) input= ;;
     esac
+    set -- "$@" "$x"
     if test "$input" ; then
         sflags="-l-user-start"
         eflags="-l-user-end"
@@ -29,6 +36,7 @@ exec $cc \
     -isystem "$libc_inc" \
     -L-user-start \
     $sflags \
+    $dniflags \
     "$@" \
     $eflags \
     -L"$libc_lib" \
diff --git a/tools/musl-gcc.specs.sh b/tools/musl-gcc.specs.sh
index 3049257..86374f1 100644
--- a/tools/musl-gcc.specs.sh
+++ b/tools/musl-gcc.specs.sh
@@ -17,13 +17,13 @@ cat <<EOF
 libgcc.a%s %:if-exists(libgcc_eh.a%s)
 
 *startfile:
-%{!shared: $libdir/Scrt1.o} $libdir/crti.o crtbeginS.o%s
+%{!shared: %{-dni:$libdir/dcrt1.o;:$libdir/Scrt1.o}} $libdir/crti.o crtbeginS.o%s
 
 *endfile:
 crtendS.o%s $libdir/crtn.o
 
 *link:
--dynamic-linker $ldso -nostdlib %{shared:-shared} %{static:-static} %{rdynamic:-export-dynamic}
+%{-dni:-no-dynamic-linker;:--dynamic-linker $ldso} -nostdlib %{shared:-shared} %{static:-static} %{rdynamic:-export-dynamic}
 
 *esp_link:
 
-- 
2.7.4

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