make dlerror produce informative results

note that dlerror is specified to be non-thread-safe, so no locking is
performed on the error flag or message aside from the rwlock already
held by dlopen or dlsym. if 2 invocations of dlsym are generating
errors at the same time, they could clobber each other's results, but
the resulting string, albeit corrupt, will still be null-terminated.
any use of dlerror in such a situation could not be expected to give
meaningful results anyway.

src/ldso/dynlink.c

@@ -18,6 +18,7 @@
 #include <dlfcn.h>
 
 static int errflag;
+static char errbuf[128];
 
 #ifdef __PIC__
 
@@ -142,8 +143,11 @@ static void do_relocs(unsigned char *base, size_t *rel, size_t rel_size, size_t
 			ctx = IS_COPY(type) ? dso->next : dso;
 			sym_val = (size_t)find_sym(ctx, name, IS_PLT(type));
 			if (!sym_val && sym->st_info>>4 != STB_WEAK) {
+				snprintf(errbuf, sizeof errbuf,
+					"Error relocating %s: %s: symbol not found",
+					dso->name, name);
 				if (runtime) longjmp(rtld_fail, 1);
-				dprintf(2, "%s: symbol not found\n", name);
+				dprintf(2, "%s\n", errbuf);
 				_exit(127);
 			}
 			sym_size = sym->st_size;
@@ -405,9 +409,11 @@ static void load_deps(struct dso *p)
 			if (p->dynv[i] != DT_NEEDED) continue;
 			dep = load_library(p->strings + p->dynv[i+1]);
 			if (!dep) {
-				if (runtime) longjmp(rtld_fail, 1);
+				snprintf(errbuf, sizeof errbuf,
-				dprintf(2, "%s: %m (needed by %s)\n",
+					"Error loading shared library %s: %m (needed by %s)",
 					p->strings + p->dynv[i+1], p->name);
+				if (runtime) longjmp(rtld_fail, 1);
+				dprintf(2, "%s\n", errbuf);
 				_exit(127);
 			}
 			if (runtime) {
@@ -634,9 +640,13 @@ void *dlopen(const char *file, int mode)
 		tail = orig_tail;
 		tail->next = 0;
 		p = 0;
+		errflag = 1;
+		goto end;
 	} else p = load_library(file);
 
 	if (!p) {
+		snprintf(errbuf, sizeof errbuf,
+			"Error loading shared library %s: %m", file);
 		errflag = 1;
 		goto end;
 	}
@@ -694,6 +704,7 @@ static void *do_dlsym(struct dso *p, const char *s, void *ra)
 			return p->deps[i]->base + sym->st_value;
 	}
 	errflag = 1;
+	snprintf(errbuf, sizeof errbuf, "Symbol not found: %s", s);
 	return 0;
 }
 
@@ -720,7 +731,7 @@ char *dlerror()
 {
 	if (!errflag) return 0;
 	errflag = 0;
-	return "unknown error";
+	return errbuf;
 }
 
 int dlclose(void *p)