Introduce dlopen() support

Every program built using Cosmopolitan is statically-linked. However
there are some cases, e.g. GUIs and video drivers, where linking the
host platform libraries is desirable. So what we do in such cases is
launch a stub executable using the host platform's libc, and longjmp
back into this executable. The stub executable passes back to us the
platform-specific dlopen() implementation, which we shall then wrap.

Here's the list of platforms that are supported so far:

- x86-64 Linux w/ Glibc
- x86-64 Linux w/ Musl Libc
- x86-64 FreeBSD
- x86-64 Windows
- aarch64 Linux w/ Glibc
- aarch64 MacOS

What this means is your Cosmo programs can call foreign functions on
your host operating system. However, it's important to note that any
foreign library you link won't have the ability to call functions in
your Cosmopolitan program. For example it's now technically possible
that Lua can load a module, however that almost certainly won't work
since the Lua module won't have access to Cosmo's Lua API.

Kudos to @jacereda for figuring out how to do this.

Makefile

@@ -176,6 +176,7 @@ include libc/calls/calls.mk			#─┐
 include libc/irq/irq.mk				# ├──SYSTEMS RUNTIME
 include third_party/nsync/nsync.mk		# │  You can issue system calls
 include libc/runtime/runtime.mk			# │
+include libc/dlopen/dlopen.mk			# │
 include third_party/double-conversion/dc.mk	# │
 include libc/crt/crt.mk				# │
 include third_party/dlmalloc/dlmalloc.mk	#─┘

ape/ape-m1.c

@@ -31,10 +31,11 @@
 #include <sys/uio.h>
 #include <time.h>
 #include <unistd.h>
+#include <dlfcn.h>
 
 #define pagesz         16384
 #define SYSLIB_MAGIC   ('s' | 'l' << 8 | 'i' << 16 | 'b' << 24)
-#define SYSLIB_VERSION 5
+#define SYSLIB_VERSION 6
 
 struct Syslib {
   int magic;
@@ -91,6 +92,11 @@ struct Syslib {
   long (*sem_trywait)(int *);
   long (*getrlimit)(int, struct rlimit *);
   long (*setrlimit)(int, const struct rlimit *);
+  // v6 (2023-11-03)
+  void *(*dlopen)(const char *, int);
+  void *(*dlsym)(void *, const char *);
+  int (*dlclose)(void *);
+  char *(*dlerror)(void);
 };
 
 #define ELFCLASS32  1
@@ -943,6 +949,10 @@ int main(int argc, char **argv, char **envp) {
   M->lib.sem_trywait = sys_sem_trywait;
   M->lib.getrlimit = sys_getrlimit;
   M->lib.setrlimit = sys_setrlimit;
+  M->lib.dlopen = dlopen;
+  M->lib.dlsym = dlsym;
+  M->lib.dlclose = dlclose;
+  M->lib.dlerror = dlerror;
 
   /* getenv("_") is close enough to at_execfn */
   execfn = argc > 0 ? argv[0] : 0;

examples/dlopen.c

@@ -0,0 +1,55 @@
+#if 0
+/*─────────────────────────────────────────────────────────────────╗
+│ To the extent possible under law, Justine Tunney has waived      │
+│ all copyright and related or neighboring rights to this file,    │
+│ as it is written in the following disclaimers:                   │
+│   • http://unlicense.org/                                        │
+│   • http://creativecommons.org/publicdomain/zero/1.0/            │
+╚─────────────────────────────────────────────────────────────────*/
+#endif
+#include "libc/calls/calls.h"
+#include "libc/dlopen/dlfcn.h"
+#include "libc/fmt/itoa.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/nt/thunk/msabi.h"
+#include "libc/runtime/runtime.h"
+
+/**
+ * @fileoverview cosmopolitan dynamic runtime linking demo
+ *
+ * Our dlopen() interface currently supports:
+ *
+ * - x86-64 Linux w/ Glibc
+ * - x86-64 Linux w/ Musl Libc
+ * - x86-64 FreeBSD
+ * - x86-64 Windows
+ * - aarch64 Linux w/ Glibc
+ * - aarch64 MacOS
+ *
+ */
+
+int main(int argc, char **argv, char **envp) {
+
+  // open the host system's zlib library
+  void *libc = dlopen("libz.so", RTLD_LAZY);
+  if (!libc) {
+    tinyprint(2, dlerror(), "\n", NULL);
+    exit(1);
+  }
+
+  // load crc() function address
+  unsigned (*crc32)(unsigned, void *, int) = dlsym(libc, "crc32");
+  if (!crc32) {
+    tinyprint(2, dlerror(), "\n", NULL);
+    exit(1);
+  }
+
+  // compute a checksum and print the result
+  char ibuf[12];
+  FormatInt32(ibuf, crc32(0, "hi", 2));
+  tinyprint(1, "crc(hi) = ", ibuf, "\n", NULL);
+
+  // mop up
+  dlclose(libc);
+  exit(0);
+}

examples/examples.mk

@@ -43,6 +43,7 @@ EXAMPLES_DIRECTDEPS =								\
 	DSP_SCALE								\
 	DSP_TTY									\
 	LIBC_CALLS								\
+	LIBC_DLOPEN								\
 	LIBC_DNS								\
 	LIBC_FMT								\
 	LIBC_INTRIN								\

libc/runtime/dlfcn.h → libc/dlopen/dlfcn.h

@@ -1,5 +1,5 @@
-#ifndef COSMOPOLITAN_LIBC_RUNTIME_DLFCN_H_
-#define COSMOPOLITAN_LIBC_RUNTIME_DLFCN_H_
+#ifndef COSMOPOLITAN_LIBC_DLFCN_H_
+#define COSMOPOLITAN_LIBC_DLFCN_H_
 
 #define RTLD_LOCAL  0
 #define RTLD_LAZY   1
@@ -20,4 +20,4 @@ int dl_iterate_phdr(int (*)(void *, size_t, void *), void *);
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
-#endif /* COSMOPOLITAN_LIBC_RUNTIME_DLFCN_H_ */
+#endif /* COSMOPOLITAN_LIBC_DLFCN_H_ */