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SubjectRe: [PATCH V2] use canary at end of stack to indicate overruns at oops time
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On Tue, 2008-04-22 at 16:38 -0500, Eric Sandeen wrote:

> ... unless the stack overrun is so bad that it corrupts some other
> thread.
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
> Index: linux-2.6.25/arch/x86/mm/fault.c
> ===================================================================
> --- linux-2.6.25.orig/arch/x86/mm/fault.c
> +++ linux-2.6.25/arch/x86/mm/fault.c
> @@ -25,6 +25,7 @@
> #include <linux/kprobes.h>
> #include <linux/uaccess.h>
> #include <linux/kdebug.h>
> +#include <linux/magic.h>
>
> #include <asm/system.h>
> #include <asm/desc.h>
> @@ -581,6 +582,8 @@ void __kprobes do_page_fault(struct pt_r
> unsigned long address;
> int write, si_code;
> int fault;
> + unsigned long *stackend;
> +
> #ifdef CONFIG_X86_64
> unsigned long flags;
> #endif
> @@ -850,6 +853,10 @@ no_context:
>
> show_fault_oops(regs, error_code, address);
>
> + stackend = end_of_stack(tsk);
> + if (*stackend != STACK_END_MAGIC)
> + printk(KERN_ALERT "Thread overran stack, or stack corrupted\n");
> +
> tsk->thread.cr2 = address;
> tsk->thread.trap_no = 14;
> tsk->thread.error_code = error_code;
> Index: linux-2.6.25/include/linux/magic.h
> ===================================================================
> --- linux-2.6.25.orig/include/linux/magic.h
> +++ linux-2.6.25/include/linux/magic.h
> @@ -42,4 +42,5 @@
> #define FUTEXFS_SUPER_MAGIC 0xBAD1DEA
> #define INOTIFYFS_SUPER_MAGIC 0x2BAD1DEA
>
> +#define STACK_END_MAGIC 0x57AC6E9D
> #endif /* __LINUX_MAGIC_H__ */
> Index: linux-2.6.25/kernel/fork.c
> ===================================================================
> --- linux-2.6.25.orig/kernel/fork.c
> +++ linux-2.6.25/kernel/fork.c
> @@ -53,6 +53,7 @@
> #include <linux/tty.h>
> #include <linux/proc_fs.h>
> #include <linux/blkdev.h>
> +#include <linux/magic.h>
>
> #include <asm/pgtable.h>
> #include <asm/pgalloc.h>
> @@ -167,6 +168,8 @@ static struct task_struct *dup_task_stru
> {
> struct task_struct *tsk;
> struct thread_info *ti;
> + unsigned long *stackend;
> +
> int err;
>
> prepare_to_copy(orig);
> @@ -192,6 +195,8 @@ static struct task_struct *dup_task_stru
> }
>
> setup_thread_stack(tsk, orig);
> + stackend = end_of_stack(tsk);
> + *stackend = STACK_END_MAGIC; /* for overflow detection */
>
> #ifdef CONFIG_CC_STACKPROTECTOR
> tsk->stack_canary = get_random_int();
> Index: linux-2.6.25/kernel/exit.c
> ===================================================================
> --- linux-2.6.25.orig/kernel/exit.c
> +++ linux-2.6.25/kernel/exit.c
> @@ -823,12 +823,9 @@ static void check_stack_usage(void)
> {
> static DEFINE_SPINLOCK(low_water_lock);
> static int lowest_to_date = THREAD_SIZE;
> - unsigned long *n = end_of_stack(current);
> unsigned long free;
>
> - while (*n == 0)
> - n++;
> - free = (unsigned long)n - (unsigned long)end_of_stack(current);
> + free = stack_not_used(current);
>
> if (free >= lowest_to_date)
> return;
> Index: linux-2.6.25/kernel/sched.c
> ===================================================================
> --- linux-2.6.25.orig/kernel/sched.c
> +++ linux-2.6.25/kernel/sched.c
> @@ -5188,12 +5188,7 @@ void sched_show_task(struct task_struct
> printk(KERN_CONT " %016lx ", thread_saved_pc(p));
> #endif
> #ifdef CONFIG_DEBUG_STACK_USAGE
> - {
> - unsigned long *n = end_of_stack(p);
> - while (!*n)
> - n++;
> - free = (unsigned long)n - (unsigned long)end_of_stack(p);
> - }
> + free = stack_not_used(p);
> #endif

Maybe remove the #ifdef CONFIG_DEBUG_STACK_USAGE block and move it into
stack_not_used...call it debug_stack_not_used.

> printk(KERN_CONT "%5lu %5d %6d\n", free,
> task_pid_nr(p), task_pid_nr(p->real_parent));
> Index: linux-2.6.25/include/linux/sched.h
> ===================================================================
> --- linux-2.6.25.orig/include/linux/sched.h
> +++ linux-2.6.25/include/linux/sched.h
> @@ -1893,6 +1893,19 @@ static inline unsigned long *end_of_stac
>
> #endif
>
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +static inline unsigned long stack_not_used(struct task_struct *p)
> +{
> + unsigned long *n = end_of_stack(p);
> +
> + do { /* Skip over canary */
> + n++;
> + } while (!*n);
> +
> + return (unsigned long)n - (unsigned long)end_of_stack(p);
> +}
> +#endif
> +

static inline unsigned long debug_stack_not_used(struct task_struct *p)
{
#ifdef CONFIG_DEBUG_STACK_USAGE
unsigned long *n = end_of_stack(p);
do { /* Skip over canary */
n++;
} while (!*n);
return (unsigned long)n - (unsigned long)end_of_stack(p);
#else
return $(large_value)....maybe or just some known value.
#endif
}
Also, do you expect this to ever be used outside of sched.c? Maybe just
leave it as a static function there rather than an inline in the header.

Harvey



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