add stubs for dlopen, dlsym, etc.

This adds weak exports for the POSIX `dlopen`, `dlsym`, `dlclose`, and `dlerror`
functions, allowing code which uses those features to compile.  The
implementations are stubs which always fail since there is currently no official
standard for runtime dynamic linking.

Since the symbols are weak, they can be overriden with useful, runtime-specific
implementations, e.g. based on host functions or statically-generated tables
(see https://github.com/dicej/component-linking-demo for an example of the
latter).

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Makefile

@@ -90,6 +90,7 @@ LIBC_TOP_HALF_MUSL_SOURCES = \
         misc/a64l.c \
         misc/basename.c \
         misc/dirname.c \
+        misc/dl.c \
         misc/ffs.c \
         misc/ffsl.c \
         misc/ffsll.c \
@@ -445,7 +446,6 @@ MUSL_OMIT_HEADERS += \
     "netdb.h" \
     "resolv.h" \
     "pty.h" \
-    "dlfcn.h" \
     "setjmp.h" \
     "ulimit.h" \
     "sys/xattr.h" \

expected/wasm32-wasi-threads/defined-symbols.txt

@@ -573,6 +573,10 @@ difftime
 dirfd
 dirname
 div
+dlclose
+dlerror
+dlopen
+dlsym
 dprintf
 drand48
 drem

expected/wasm32-wasi-threads/include-all.c

@@ -75,6 +75,7 @@
 #include <crypt.h>
 #include <ctype.h>
 #include <dirent.h>
+#include <dlfcn.h>
 #include <endian.h>
 #include <err.h>
 #include <errno.h>

expected/wasm32-wasi-threads/predefined-macros.txt

@@ -1499,6 +1499,15 @@
 #define RRFIXEDSZ NS_RRFIXEDSZ
 #define RRQ 01
 #define RS_HIPRI 0x01
+#define RTLD_DEFAULT ((void *)0)
+#define RTLD_DI_LINKMAP 2
+#define RTLD_GLOBAL 256
+#define RTLD_LAZY 1
+#define RTLD_LOCAL 0
+#define RTLD_NEXT ((void *)-1)
+#define RTLD_NODELETE 4096
+#define RTLD_NOLOAD 4
+#define RTLD_NOW 2
 #define RUSAGE_CHILDREN 2
 #define RUSAGE_SELF 1
 #define R_OK (4)
@@ -2044,6 +2053,7 @@
 #define _Complex_I (0.0f+1.0fi)
 #define _DIRENT_H 
 #define _DIRENT_HAVE_D_TYPE 
+#define _DLFCN_H
 #define _ENDIAN_H 
 #define _ERRNO_H 
 #define _ERR_H 

expected/wasm32-wasi/defined-symbols.txt

@@ -509,6 +509,10 @@ difftime
 dirfd
 dirname
 div
+dlclose
+dlerror
+dlopen
+dlsym
 dprintf
 drand48
 drem

expected/wasm32-wasi/include-all.c

@@ -75,6 +75,7 @@
 #include <crypt.h>
 #include <ctype.h>
 #include <dirent.h>
+#include <dlfcn.h>
 #include <endian.h>
 #include <err.h>
 #include <errno.h>

expected/wasm32-wasi/predefined-macros.txt

@@ -1467,6 +1467,15 @@
 #define RRFIXEDSZ NS_RRFIXEDSZ
 #define RRQ 01
 #define RS_HIPRI 0x01
+#define RTLD_DEFAULT ((void *)0)
+#define RTLD_DI_LINKMAP 2
+#define RTLD_GLOBAL 256
+#define RTLD_LAZY 1
+#define RTLD_LOCAL 0
+#define RTLD_NEXT ((void *)-1)
+#define RTLD_NODELETE 4096
+#define RTLD_NOLOAD 4
+#define RTLD_NOW 2
 #define RUSAGE_CHILDREN 2
 #define RUSAGE_SELF 1
 #define R_OK (4)
@@ -2010,6 +2019,7 @@
 #define _Complex_I (0.0f+1.0fi)
 #define _DIRENT_H 
 #define _DIRENT_HAVE_D_TYPE 
+#define _DLFCN_H
 #define _ENDIAN_H 
 #define _ERRNO_H 
 #define _ERR_H 

libc-top-half/musl/src/misc/dl.c

@@ -0,0 +1,31 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <dlfcn.h>
+
+#define EXPORT __attribute__((visibility("default"))) __attribute__((used))
+#define WEAK __attribute__((__weak__))
+
+static const char *error = 0;
+
+EXPORT WEAK int dlclose(void *library) {
+    error = "dlclose not implemented";
+    return -1;
+}
+
+EXPORT WEAK char *dlerror(void) {
+    const char *var = error;
+    error = 0;
+    return (char*) var;
+}
+
+EXPORT WEAK void *dlopen(const char *name, int flags) {
+    error = "dlopen not implemented";
+    return 0;
+}
+
+EXPORT WEAK void *dlsym(void *library, const char *name) {
+    error = "dlsym not implemented";
+    return 0;
+}