signal/ptrace: Add force_sig_ptrace_errno_trap and use it where needed

There are so many places that build struct siginfo by hand that at
least one of them is bound to get it wrong.  A handful of cases in the
kernel arguably did just that when using the errno field of siginfo to
pass no errno values to userspace.  The usage is limited to a single
si_code so at least does not mess up anything else.

Encapsulate this questionable pattern in a helper function so
that the userspace ABI is preserved.

Update all of the places that use this pattern to use the new helper
function.

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

arch/arm/kernel/ptrace.c

@@ -390,7 +390,6 @@ static void ptrace_hbptriggered(struct perf_event *bp,
 	struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
 	long num;
 	int i;
-	siginfo_t info;
 
 	for (i = 0; i < ARM_MAX_HBP_SLOTS; ++i)
 		if (current->thread.debug.hbp[i] == bp)
@@ -398,12 +397,7 @@ static void ptrace_hbptriggered(struct perf_event *bp,
 
 	num = (i == ARM_MAX_HBP_SLOTS) ? 0 : ptrace_hbp_idx_to_num(i);
 
-	info.si_signo	= SIGTRAP;
-	info.si_errno	= (int)num;
-	info.si_code	= TRAP_HWBKPT;
-	info.si_addr	= (void __user *)(bkpt->trigger);
-
-	force_sig_info(SIGTRAP, &info, current);
+	force_sig_ptrace_errno_trap((int)num, (void __user *)(bkpt->trigger));
 }
 
 /*

arch/arm64/kernel/ptrace.c

@@ -190,21 +190,23 @@ static void ptrace_hbptriggered(struct perf_event *bp,
 
 #ifdef CONFIG_COMPAT
 	if (is_compat_task()) {
+		int si_errno = 0;
 		int i;
 
 		for (i = 0; i < ARM_MAX_BRP; ++i) {
 			if (current->thread.debug.hbp_break[i] == bp) {
-				info.si_errno = (i << 1) + 1;
+				si_errno = (i << 1) + 1;
 				break;
 			}
 		}
 
 		for (i = 0; i < ARM_MAX_WRP; ++i) {
 			if (current->thread.debug.hbp_watch[i] == bp) {
-				info.si_errno = -((i << 1) + 1);
+				si_errno = -((i << 1) + 1);
 				break;
 			}
 		}
+		force_sig_ptrace_errno_trap(si_errno, (void __user *)bkpt->trigger);
 	}
 #endif
 	force_sig_info(SIGTRAP, &info, current);

arch/powerpc/kernel/process.c

@@ -603,19 +603,14 @@ EXPORT_SYMBOL(flush_all_to_thread);
 void do_send_trap(struct pt_regs *regs, unsigned long address,
 		  unsigned long error_code, int breakpt)
 {
-	siginfo_t info;
-
 	current->thread.trap_nr = TRAP_HWBKPT;
 	if (notify_die(DIE_DABR_MATCH, "dabr_match", regs, error_code,
 			11, SIGSEGV) == NOTIFY_STOP)
 		return;
 
 	/* Deliver the signal to userspace */
-	info.si_signo = SIGTRAP;
-	info.si_errno = breakpt;	/* breakpoint or watchpoint id */
-	info.si_code = TRAP_HWBKPT;
-	info.si_addr = (void __user *)address;
-	force_sig_info(SIGTRAP, &info, current);
+	force_sig_ptrace_errno_trap(breakpt, /* breakpoint or watchpoint id */
+				    (void __user *)address);
 }
 #else	/* !CONFIG_PPC_ADV_DEBUG_REGS */
 void do_break (struct pt_regs *regs, unsigned long address,

arch/xtensa/kernel/ptrace.c

@@ -278,7 +278,6 @@ static void ptrace_hbptriggered(struct perf_event *bp,
 				struct pt_regs *regs)
 {
 	int i;
-	siginfo_t info;
 	struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
 
 	if (bp->attr.bp_type & HW_BREAKPOINT_X) {
@@ -293,12 +292,7 @@ static void ptrace_hbptriggered(struct perf_event *bp,
 		i = (i << 1) | 1;
 	}
 
-	info.si_signo = SIGTRAP;
-	info.si_errno = i;
-	info.si_code = TRAP_HWBKPT;
-	info.si_addr = (void __user *)bkpt->address;
-
-	force_sig_info(SIGTRAP, &info, current);
+	force_sig_ptrace_errno_trap(i, (void __user *)bkpt->address);
 }
 
 static struct perf_event *ptrace_hbp_create(struct task_struct *tsk, int type)

include/linux/sched/signal.h

@@ -311,6 +311,8 @@ int send_sig_mceerr(int code, void __user *, short, struct task_struct *);
 int force_sig_bnderr(void __user *addr, void __user *lower, void __user *upper);
 int force_sig_pkuerr(void __user *addr, u32 pkey);
 
+int force_sig_ptrace_errno_trap(int errno, void __user *addr);
+
 extern int send_sig_info(int, struct siginfo *, struct task_struct *);
 extern int force_sigsegv(int, struct task_struct *);
 extern int force_sig_info(int, struct siginfo *, struct task_struct *);

kernel/signal.c

@@ -1599,6 +1599,21 @@ int force_sig_pkuerr(void __user *addr, u32 pkey)
 }
 #endif
 
+/* For the crazy architectures that include trap information in
+ * the errno field, instead of an actual errno value.
+ */
+int force_sig_ptrace_errno_trap(int errno, void __user *addr)
+{
+	struct siginfo info;
+
+	clear_siginfo(&info);
+	info.si_signo = SIGTRAP;
+	info.si_errno = errno;
+	info.si_code  = TRAP_HWBKPT;
+	info.si_addr  = addr;
+	return force_sig_info(info.si_signo, &info, current);
+}
+
 int kill_pgrp(struct pid *pid, int sig, int priv)
 {
 	int ret;