linux-user: If loading fails, print error as string, not number

If the attempt to load the guest executable fails, print the
error message as a string, not a number. This requires us to
fix a couple of places in loader_exec() where we were returning
-1 instead of a valid negative errno.

The change allows us to drop the "Unknown binary format" message
because the strerror-enhanced message is now a more self-explanatory
"Error while loading $guest-binary: Exec format error".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/linuxload.c

@@ -140,8 +140,9 @@ int loader_exec(const char * filename, char ** argv, char ** envp,
     bprm->p = TARGET_PAGE_SIZE*MAX_ARG_PAGES-sizeof(unsigned int);
     memset(bprm->page, 0, sizeof(bprm->page));
     retval = open(filename, O_RDONLY);
-    if (retval < 0)
-        return retval;
+    if (retval < 0) {
+        return -errno;
+    }
     bprm->fd = retval;
     bprm->filename = (char *)filename;
     bprm->argc = count(argv);
@@ -165,8 +166,7 @@ int loader_exec(const char * filename, char ** argv, char ** envp,
             retval = load_flt_binary(bprm,regs,infop);
 #endif
         } else {
-            fprintf(stderr, "Unknown binary format\n");
-            return -1;
+            return -ENOEXEC;
         }
     }
 

linux-user/main.c

@@ -3569,7 +3569,7 @@ int main(int argc, char **argv, char **envp)
     ret = loader_exec(filename, target_argv, target_environ, regs,
         info, &bprm);
     if (ret != 0) {
-        printf("Error %d while loading %s\n", ret, filename);
+        printf("Error while loading %s: %s\n", filename, strerror(-ret));
         _exit(1);
     }