signal: change sys_kill() to use SEND_SIG_NOINFO

prepare_kill_siginfo(PIDTYPE_TGID) fills si_code = SI_USER and sets si_pid/si_uid in the sender's namespace. Then send_signal_locked() translates si_pid/si_uid to the target's namespace. SEND_SIG_NOINFO exists precisely for the case when si_code == SI_USER and si_pid/si_uid are the sender's ids; this is exactly what sys_kill() does via prepare_kill_siginfo(PIDTYPE_TGID). Change sys_kill() to use it directly. SEND_SIG_NOINFO produces the same result: si_code = SI_USER, and __send_signal_locked() computes si_pid/si_uid directly in the target's namespace. The force computation is also the same: both check if the sender is visible in the target's pid namespace. This is just a cleanup and microoptimization (especially with [1]), this skips the has_si_pid_and_uid() block in send_signal_locked() and offloads the namespace translation logic to __send_signal_locked(SEND_SIG_NOINFO) which uses the simpler computations. NOTE: As a "side effect" this also fixes the kill(pid < 0, sig) case where send_signal_locked() rewrites si_pid/si_uid in the shared siginfo, corrupting it for subsequent recipients. But for other group senders like __kill_pgrp_info() we still need the fix from Bradley Morgan [1] who found this problem. TODO: kill prepare_kill_siginfo() and change other users to use SEND_SIG_NOINFO too. This needs trivial changes in __send_signal_locked() and TP_STORE_SIGINFO().

ONOleg Nesterov committed 4 months ago