Re: Error in save_stack_trace() on x86_64?

Vegard Nossum
2008-05-11T20:46:31Z

On Sun, May 11, 2008 at 9:58 PM, Arjan van de Ven <arjan@linux.intel.com> wrote:
> Vegard Nossum wrote: in userspace this trace looks correct.
>
> > No, it is happening from kernel code. As you can see from the original
> > backtrace, the regs->ip (RIP) (regs taken from the very same
> > do_page_fault()) points at add_uevent_var, which is a kernel function.
> >
> >
> > > if it happens in kernel space... I wonder if the separate exception
> stack
> > > thing
> > > is hurting us with the stacks not being properly connected...
> > > (but oopses and the like seem to come out just fine so I kinda doubt
> you're
> > > hitting that)
> > >
> > >
> >
> > Thanks for looking into this.
>
> do you happen to have something that I maybe can reproduce?
> (if you have that it would save me a ton of time in reproducing)
>

Here's a very dirty hack that reproduces it for me. I'm sorry I don't have
the logs to show it, but it works correctly on 32-bit, but not on 64-bit.

I guess you should also make sure that:

CONFIG_DEBUG_INFO=y
CONFIG_FRAME_POINTER=y
CONFIG_STACKTRACE=y

Thanks.

Vegard

--
"The animistic metaphor of the bug that maliciously sneaked in while the programmer was not looking is intellectually dishonest as it disguises that the error is the programmer's own creation."
-- E. W. Dijkstra, EWD1036

arch/x86/mm/fault.c | 17 +++++++++++++++++
init/main.c | 12 ++++++++++++
lib/Kconfig.debug | 1 +
3 files changed, 30 insertions(+), 0 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index fd7e179..559a5f3 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -25,6 +25,7 @@
#include <linux/kprobes.h>
#include <linux/uaccess.h>
#include <linux/kdebug.h>
+#include <linux/stacktrace.h>

#include <asm/system.h>
#include <asm/desc.h>
@@ -602,6 +603,22 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
if (notify_page_fault(regs))
return;

+ dump_stack();
+
+ struct stack_trace trace;
+ unsigned long entries[16];
+ trace.nr_entries = 0;
+ trace.entries = entries;
+ trace.max_entries = 16;
+ trace.skip = 0;
+ printk(KERN_EMERG "saved stack trace:\n");
+ save_stack_trace(&trace);
+ print_stack_trace(&trace, 0);
+ printk(KERN_EMERG "end saved stack trace\n");
+
+ while(1)
+ halt();
+
/*
* We fault-in kernel-space virtual memory on-demand. The
* 'reference' page table is init_mm.pgd.
diff --git a/init/main.c b/init/main.c
index ddada7a..d9fb240 100644
--- a/init/main.c
+++ b/init/main.c
@@ -531,6 +531,16 @@ void __init __weak thread_info_cache_init(void)
{
}

+void noinline trigger_page_fault(void) {
+ struct page *p = alloc_pages(GFP_KERNEL, 0);
+ unsigned long addr = page_address(p);
+ set_memory_4k(addr, 0);
+ int level;
+ pte_t *pte = lookup_address(addr, &level);
+ set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
+ *(char*) addr = 0;
+}
+
asmlinkage void __init start_kernel(void)
{
char * command_line;
@@ -680,6 +690,8 @@ asmlinkage void __init start_kernel(void)

acpi_early_init(); /* before LAPIC and SMP init */

+ trigger_page_fault();
+
/* Do the rest non-__init'ed, we're now alive */
rest_init();
}
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d2099f4..3fc1247 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -419,6 +419,7 @@ config DEBUG_LOCKING_API_SELFTESTS

config STACKTRACE
bool
+ default y
depends on DEBUG_KERNEL
depends on STACKTRACE_SUPPORT

--
1.5.4.1