signal: Ensure every siginfo we send has all bits initialized

Call clear_siginfo to ensure every stack allocated siginfo is properly
initialized before being passed to the signal sending functions.

Note: It is not safe to depend on C initializers to initialize struct
siginfo on the stack because C is allowed to skip holes when
initializing a structure.

The initialization of struct siginfo in tracehook_report_syscall_exit
was moved from the helper user_single_step_siginfo into
tracehook_report_syscall_exit itself, to make it clear that the local
variable siginfo gets fully initialized.

In a few cases the scope of struct siginfo has been reduced to make it
clear that siginfo siginfo is not used on other paths in the function
in which it is declared.

Instances of using memset to initialize siginfo have been replaced
with calls clear_siginfo for clarity.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

arch/arm/kernel/ptrace.c

@@ -205,6 +205,7 @@ void ptrace_break(struct task_struct *tsk, struct pt_regs *regs)
 {
 	siginfo_t info;
 
+	clear_siginfo(&info);
 	info.si_signo = SIGTRAP;
 	info.si_errno = 0;
 	info.si_code  = TRAP_BRKPT;

arch/arm/kernel/swp_emulate.c

@@ -112,6 +112,7 @@ static void set_segfault(struct pt_regs *regs, unsigned long addr)
 {
 	siginfo_t info;
 
+	clear_siginfo(&info);
 	down_read(&current->mm->mmap_sem);
 	if (find_vma(current->mm, addr) == NULL)
 		info.si_code = SEGV_MAPERR;

arch/arm/kernel/traps.c

@@ -439,6 +439,7 @@ asmlinkage void do_undefinstr(struct pt_regs *regs)
 	siginfo_t info;
 	void __user *pc;
 
+	clear_siginfo(&info);
 	pc = (void __user *)instruction_pointer(regs);
 
 	if (processor_mode(regs) == SVC_MODE) {
@@ -537,6 +538,7 @@ static int bad_syscall(int n, struct pt_regs *regs)
 {
 	siginfo_t info;
 
+	clear_siginfo(&info);
 	if ((current->personality & PER_MASK) != PER_LINUX) {
 		send_sig(SIGSEGV, current, 1);
 		return regs->ARM_r0;
@@ -604,6 +606,7 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
 {
 	siginfo_t info;
 
+	clear_siginfo(&info);
 	if ((no >> 16) != (__ARM_NR_BASE>> 16))
 		return bad_syscall(no, regs);
 
@@ -740,6 +743,8 @@ baddataabort(int code, unsigned long instr, struct pt_regs *regs)
 	unsigned long addr = instruction_pointer(regs);
 	siginfo_t info;
 
+	clear_siginfo(&info);
+
 #ifdef CONFIG_DEBUG_USER
 	if (user_debug & UDBG_BADABORT) {
 		pr_err("[%d] %s: bad data abort: code %d instr 0x%08lx\n",