[PATCH] tgkill patch for safe inter-thread signals

This is the updated versions of the patch Ingo sent some time ago to
implement a new tgkill() syscall which specifies the target thread
without any possibility of ambiguity or thread ID wrap races, by passing
in both the thread group _and_ the thread ID as the arguments.

This is really needed since many/most people still run with limited PID
ranges (maybe due to legacy apps breaking) and the PID reuse can cause
problems.

arch/i386/kernel/entry.S

@@ -876,5 +876,6 @@ ENTRY(sys_call_table)
  	.long sys_clock_nanosleep
 	.long sys_statfs64
 	.long sys_fstatfs64	
+	.long sys_tgkill
  
 nr_syscalls=(.-sys_call_table)/4

include/asm-i386/unistd.h

@@ -275,8 +275,9 @@
 #define __NR_clock_nanosleep	(__NR_timer_create+8)
 #define __NR_statfs64		268
 #define __NR_fstatfs64		269
+#define __NR_tgkill		270
 
-#define NR_syscalls 270
+#define NR_syscalls 271
 
 /* user-visible error numbers are in the range -1 - -124: see <asm-i386/errno.h> */
 

kernel/signal.c

@@ -579,7 +579,7 @@ static int rm_from_queue(unsigned long mask, struct sigpending *s)
 /*
  * Bad permissions for sending the signal
  */
-static inline int check_kill_permission(int sig, struct siginfo *info,
+static int check_kill_permission(int sig, struct siginfo *info,
 				 struct task_struct *t)
 {
 	int error = -EINVAL;
@@ -2088,6 +2088,52 @@ sys_kill(int pid, int sig)
 	return kill_something_info(sig, &info, pid);
 }
 
+/**
+ *  sys_tkill - send signal to one specific thread
+ *  @tgid: the thread group ID of the thread
+ *  @pid: the PID of the thread
+ *  @sig: signal to be sent
+ *
+ *  This syscall also checks the tgid and returns -ESRCH even if the PID
+ *  exists but it's not belonging to the target process anymore. This
+ *  method solves the problem of threads exiting and PIDs getting reused.
+ */
+asmlinkage long sys_tgkill(int tgid, int pid, int sig)
+{
+	struct siginfo info;
+	int error;
+	struct task_struct *p;
+
+	/* This is only valid for single tasks */
+	if (pid <= 0 || tgid <= 0)
+		return -EINVAL;
+
+	info.si_signo = sig;
+	info.si_errno = 0;
+	info.si_code = SI_TKILL;
+	info.si_pid = current->tgid;
+	info.si_uid = current->uid;
+
+	read_lock(&tasklist_lock);
+	p = find_task_by_pid(pid);
+	error = -ESRCH;
+	if (p && (p->tgid == tgid)) {
+		error = check_kill_permission(sig, &info, p);
+		/*
+		 * The null signal is a permissions and process existence
+		 * probe.  No signal is actually delivered.
+		 */
+		if (!error && sig && p->sighand) {
+			spin_lock_irq(&p->sighand->siglock);
+			handle_stop_signal(sig, p);
+			error = specific_send_sig_info(sig, &info, p);
+			spin_unlock_irq(&p->sighand->siglock);
+		}
+	}
+	read_unlock(&tasklist_lock);
+	return error;
+}
+
 /*
  *  Send a signal to only one task, even if it's a CLONE_THREAD task.
  */