ARM: fix a cockup in 48be69a02 (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-gabe0308 #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>

2 files changed, 24 insertions, 26 deletions

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 1e6c33d01c05..d03b5bd889c5 100644
--- a/arch/arm/kernel/process.c
+++ b/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;
diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 0f17e06d51e6..39e7105a9b70 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -614,35 +614,32 @@ 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;
-                void *addr;
+        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)
-                        return NULL;
+        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);
-                signal_return_offset = offset;
+        /* Give the signal return code some randomness */
+        offset = 0x200 + (get_random_int() & 0x7fc);
+        signal_return_offset = offset;
 
-                /*
-                 * Copy signal return handlers into the vector page, and
-                 * set sigreturn to be a pointer to these.
-                 */
-                memcpy(addr + offset, sigreturn_codes, sizeof(sigreturn_codes));
+        /*
+         * Copy signal return handlers into the vector page, and
+         * set sigreturn to be a pointer to these.
+         */
+        memcpy(addr + offset, sigreturn_codes, sizeof(sigreturn_codes));
 
-                ptr = (unsigned long)addr + offset;
-                flush_icache_range(ptr, ptr + sizeof(sigreturn_codes));
-        }
+        ptr = (unsigned long)addr + offset;
+        flush_icache_range(ptr, ptr + sizeof(sigreturn_codes));
 
-        return signal_page;
+        return page;
 }