linux-user: permit sendto() with NULL buf and 0 len

If you pass sendto() a NULL buffer, this is usually an error
(causing an EFAULT return); however if you pass a 0 length then
we should not try to validate the buffer provided. Instead we
skip the copying of the user data and possible processing
through fd_trans_target_to_host_data, and call the host syscall
with NULL, 0.

(unlock_user() permits a NULL buffer pointer for "do nothing"
so we don't need to special case the unlock code.)

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3102
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20251028142001.3011630-1-peter.maydell@linaro.org>

linux-user/syscall.c

@@ -3581,7 +3581,7 @@ static abi_long do_sendto(int fd, abi_ulong msg, size_t len, int flags,
                           abi_ulong target_addr, socklen_t addrlen)
 {
     void *addr;
-    void *host_msg;
+    void *host_msg = NULL;
     void *copy_msg = NULL;
     abi_long ret;
 
@@ -3589,16 +3589,19 @@ static abi_long do_sendto(int fd, abi_ulong msg, size_t len, int flags,
         return -TARGET_EINVAL;
     }
 
-    host_msg = lock_user(VERIFY_READ, msg, len, 1);
-    if (!host_msg)
-        return -TARGET_EFAULT;
-    if (fd_trans_target_to_host_data(fd)) {
-        copy_msg = host_msg;
-        host_msg = g_malloc(len);
-        memcpy(host_msg, copy_msg, len);
-        ret = fd_trans_target_to_host_data(fd)(host_msg, len);
-        if (ret < 0) {
-            goto fail;
+    if (len != 0) {
+        host_msg = lock_user(VERIFY_READ, msg, len, 1);
+        if (!host_msg) {
+            return -TARGET_EFAULT;
+        }
+        if (fd_trans_target_to_host_data(fd)) {
+            copy_msg = host_msg;
+            host_msg = g_malloc(len);
+            memcpy(host_msg, copy_msg, len);
+            ret = fd_trans_target_to_host_data(fd)(host_msg, len);
+            if (ret < 0) {
+                goto fail;
+            }
         }
     }
     if (target_addr) {