Re: Error in save_stack_trace() on x86_64?

Vegard Nossum
2008-05-11T19:56:50Z

On Sun, May 11, 2008 at 9:44 PM, Arjan van de Ven <arjan@linux.intel.com> wrote:
> Vegard Nossum wrote:
>
> > Hi,
> >
> > I am having a problem with v2.6.26-rc1 on x86_64. It seems that
> > save_stack_trace() is not able to follow page fault boundaries, since
> > all my saved traces look like this:
> >
> > RIP: 0010:[<ffffffff8039b004>] [<ffffffff8039b004>]
> add_uevent_var+0xb4/0x160
> > ...
> > [<ffffffff80221f97>] kmemcheck_read+0x127/0x1e0
> > [<ffffffff80222269>] kmemcheck_access+0x179/0x1d0
> > [<ffffffff8022231f>] kmemcheck_fault+0x5f/0x80
> > [<ffffffff8061cd1e>] do_page_fault+0x4de/0x8d0
> > [<ffffffff8061a7d9>] error_exit+0x0/0x51
> > [<ffffffffffffffff>] 0xffffffffffffffff
> >
> > I have this in my .config:
> >
> > CONFIG_STACKTRACE_SUPPORT=y
> > CONFIG_STACKTRACE=y
> > ...
> > CONFIG_FRAME_POINTER=y
> > ...
> > CONFIG_DEBUG_INFO=y
> >
> >
> > On 32-bit, I am able to see the calls leading up to the page fault as
> > well. Did I miss something here?
> >
>
> can you give an example?

This is a similarly saved 32-bit backtrace:

[<c0119101>] kmemcheck_read+0xd1/0x160
[<c01192c6>] kmemcheck_access+0x136/0x1a0
[<c04bb206>] do_page_fault+0x5e6/0x690
[<c04b925a>] error_code+0x72/0x78
[<c012d751>] sysctl_set_parent+0x21/0x40
[<c012d751>] sysctl_set_parent+0x21/0x40
[<c012d751>] sysctl_set_parent+0x21/0x40
[<c012d751>] sysctl_set_parent+0x21/0x40
[<c012e9c8>] __register_sysctl_paths+0xb8/0x120
[<c0497cdf>] register_net_sysctl_table+0x4f/0x60
[<c040ba36>] neigh_sysctl_register+0x1a6/0x290
[<c0695734>] arp_init+0x54/0x60
[<c0695ba7>] inet_init+0x107/0x340
[<c066f5c7>] kernel_init+0x127/0x290
[<c0104cc7>] kernel_thread_helper+0x7/0x10
[<ffffffff>] 0xffffffff

>
> if a pagefault happens 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.