2.4.25: BUG(): Use guard page instead of page 0

The ia64 BUG macro deliberately writes to address 0 in order to trigger a page fault and an Oops. This won't work if the process has mapped something into page zero: We'll just print the "kernel BUG" message and continue (after having stomped on whatever user memory was at address 0).

A solution is to write to the guard page in region 5, which is guaranteed to trigger a page fault.

@@ -120,7 +120,7 @@
 #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)
+#define BUG() do { printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); *(int *)0xa000000000000000=0; } while (0)
 #define PAGE_BUG(page) do { BUG(); } while (0)
 
 static __inline__ int

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).

	--david

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

Why not use 'break 0' for gcc < 3.1? No need to worry about which pages are protected.

        switch (break_num) {
              case 0: /* unknown error (used by GCC for __builtin_abort()) */
                die_if_kernel("bugcheck!", regs, break_num);

Sounds reasonable to me, but I'm too chicken. Propose a patch and get David to take it, and I'll put it in 2.4.

Bjorn> Sounds reasonable to me, but I'm too chicken. Propose a Bjorn> patch and get David to take it, and I'll put it in 2.4.

I don't think it's worth changing (2.9x is out for 2.6 anyhow, and I doubt anyone is using GCC 3.0 for real work; if you do, you're probably in lots of trouble for other reasons!). Besides, one disadvantage of using "break 0" is that it will make Ski stop immediately, which isn't what you want.

	--david

David> I don't think it's worth changing (2.9x is out for 2.6 David> anyhow, and I doubt anyone is using GCC 3.0 for real work; if David> you do, you're probably in lots of trouble for other David> reasons!). Besides, one disadvantage of using "break 0" is David> that it will make Ski stop immediately, which isn't what you David> want.

Never mind. The Ski argument is BS of course... (since that's what __builtin_trap() uses... ;-)

	--david