Re: [PATCH] 2.4.25: BUG(): Use guard page instead of page 0

Bjorn Helgaas
2004-03-04T23:25:00Z

On Wednesday 18 February 2004 6:14 pm, David Mosberger wrote:
> >>>>> On Wed, 18 Feb 2004 14:13:03 -0800, Jason Uhlenkott <jasonuhl@sgi.com> said:
>
> Jason> The ia64 BUG macro deliberately writes to address 0 in order to
> Jason> trigger a page fault and an Oops. This won't work if the process has
> Jason> mapped something into page zero: We'll just print the "kernel BUG"
> Jason> message and continue (after having stomped on whatever user memory was
> Jason> at address 0).
>
> Jason> A solution is to write to the guard page in region 5, which is
> Jason> guaranteed to trigger a page fault.
>
> The 2.6 kernel uses __builtin_trap(), which is even better (when available).

How about the following? I like the idea of using the guard page
instead of address 0, but I sort of hate to add another magic number
(though I guess you could argue that "0" is almost as magic as
"0xa000000000000000"). And I would think most people would be
using gcc 3.x or better by now.

=== include/asm-ia64/page.h 1.9 vs edited ==--- 1.9/include/asm-ia64/page.h Tue Jan 20 13:44:48 2004
+++ edited/include/asm-ia64/page.h Thu Mar 4 16:20:00 2004
@@ -120,7 +120,13 @@
#define is_invalid_hugepage_range(addr, len) 0
#endif

-#define BUG() do { printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); *(int *)0=0; } while (0)
+#if (__GNUC__ > 3) || (__GNUC__ = 3 && __GNUC_MINOR__ >= 1)
+# define ia64_abort() __builtin_trap()
+#else
+# define ia64_abort() (*(volatile int *) 0 = 0)
+#endif
+
+#define BUG() do { printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); ia64_abort(); } while (0)
#define PAGE_BUG(page) do { BUG(); } while (0)

static __inline__ int