linux-user: fix QEMU_STRACE=1 segfault

While debugging some issues with QEMU_STRACE I stumbled over segmentation
faults that were pretty reproducible. Turns out we tried to treat a
normal return value as errno, resulting in an access over array boundaries
for the resolution.

Fix this by allowing failure to resolve invalid errnos into strings.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/strace.c

@@ -284,8 +284,13 @@ print_ipc(const struct syscallname *name,
 static void
 print_syscall_ret_addr(const struct syscallname *name, abi_long ret)
 {
-if( ret == -1 ) {
+    char *errstr = NULL;
-        gemu_log(" = -1 errno=%d (%s)\n", errno, target_strerror(errno));
+
+    if (ret == -1) {
+        errstr = target_strerror(errno);
+    }
+    if ((ret == -1) && errstr) {
+        gemu_log(" = -1 errno=%d (%s)\n", errno, errstr);
     } else {
         gemu_log(" = 0x" TARGET_ABI_FMT_lx "\n", ret);
     }
@@ -1515,14 +1520,19 @@ void
 print_syscall_ret(int num, abi_long ret)
 {
     int i;
+    char *errstr = NULL;
 
     for(i=0;i<nsyscalls;i++)
         if( scnames[i].nr == num ) {
             if( scnames[i].result != NULL ) {
                 scnames[i].result(&scnames[i],ret);
             } else {
-                if( ret < 0 ) {
+                if (ret < 0) {
-                    gemu_log(" = -1 errno=" TARGET_ABI_FMT_ld " (%s)\n", -ret, target_strerror(-ret));
+                    errstr = target_strerror(-ret);
+                }
+                if (errstr) {
+                    gemu_log(" = -1 errno=" TARGET_ABI_FMT_ld " (%s)\n",
+                             -ret, errstr);
                 } else {
                     gemu_log(" = " TARGET_ABI_FMT_ld "\n", ret);
                 }

linux-user/syscall.c

@@ -731,6 +731,9 @@ static inline int is_error(abi_long ret)
 
 char *target_strerror(int err)
 {
+    if ((err >= ERRNO_TABLE_SIZE) || (err < 0)) {
+        return NULL;
+    }
     return strerror(target_to_host_errno(err));
 }