ARM: fix a cockup in 48be69a0 (ARM: move signal handlers into a vdso-like page)

Unfortunately, I never committed the fix to a nasty oops which can
occur as a result of that commit:

------------[ cut here ]------------
kernel BUG at /home/olof/work/batch/include/linux/mm.h:414!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 0 PID: 490 Comm: killall5 Not tainted 3.11.0-rc3-00288-gabe03080 #53
task: e90acac0 ti: e9be8000 task.ti: e9be8000
PC is at special_mapping_fault+0xa4/0xc4
LR is at __do_fault+0x68/0x48c

This doesn't show up unless you do quite a bit of testing; a simple
boot test does not do this, so all my nightly tests were passing fine.

The reason for this is that install_special_mapping() expects the
page array to stick around, and as this was only inserting one page
which was stored on the kernel stack, that's why this was blowing up.
Reported-by: Olof Johansson <olof@lixom.net>
Tested-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/process.c

@@ -471,17 +471,18 @@ const char *arch_vma_name(struct vm_area_struct *vma)
 		 "[sigpage]" : NULL;
 }
 
+static struct page *signal_page;
 extern struct page *get_signal_page(void);
 
 int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 {
 	struct mm_struct *mm = current->mm;
-	struct page *page;
 	unsigned long addr;
 	int ret;
 
-	page = get_signal_page();
-	if (!page)
+	if (!signal_page)
+		signal_page = get_signal_page();
+	if (!signal_page)
 		return -ENOMEM;
 
 	down_write(&mm->mmap_sem);
@@ -493,7 +494,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	ret = install_special_mapping(mm, addr, PAGE_SIZE,
 		VM_READ | VM_EXEC | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
-		&page);
+		&signal_page);
 
 	if (ret == 0)
 		mm->context.sigpage = addr;

arch/arm/kernel/signal.c

@@ -614,21 +614,19 @@ do_work_pending(struct pt_regs *regs, unsigned int thread_flags, int syscall)
 	return 0;
 }
 
-static struct page *signal_page;
-
 struct page *get_signal_page(void)
 {
-	if (!signal_page) {
 	unsigned long ptr;
 	unsigned offset;
+	struct page *page;
 	void *addr;
 
-		signal_page = alloc_pages(GFP_KERNEL, 0);
+	page = alloc_pages(GFP_KERNEL, 0);
 
-		if (!signal_page)
+	if (!page)
 		return NULL;
 
-		addr = page_address(signal_page);
+	addr = page_address(page);
 
 	/* Give the signal return code some randomness */
 	offset = 0x200 + (get_random_int() & 0x7fc);
@@ -642,7 +640,6 @@ struct page *get_signal_page(void)
 
 	ptr = (unsigned long)addr + offset;
 	flush_icache_range(ptr, ptr + sizeof(sigreturn_codes));
-	}
 
-	return signal_page;
+	return page;
 }