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

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>