Re: [PATCH] Put each per-cpu kdump ELF notes into a single page

Petr Tesarik
2014-09-11T19:37:49Z

Hi all,

is anything wrong with the patch below? Are there questions? I thought
this would be an easy one-line bugfix...

Regards,
Petr Tesarik

On Fri, 5 Sep 2014 18:33:14 +0200
Petr Tesarik <ptesarik@suse.cz> wrote:

> On architectures that use percpu-vm, the percpu region is not guaranteed
> to be contiguous in physical space. However, fs/proc/vmcore.c expects
> all ELF notes to be contiguous. If the ELF note happens to occupy
> two non-adjacent physical pages, part of the note may be read from an
> incorrect memory location by the kdump kernel, resulting in failure to
> initialize /proc/vmcore (if the content of the following physical page,
> incorrectly interpreted as an ELF note specifies a large number), wrong
> register values or other apparent random memory corruption.
>
> There is currently no mechanism to pass the virtual-to-physical mapping
> of the percpu allocation to the kdump kernel. So, instead, I'm changing
> the alignment of the ELF note buffer. Since sizeof(note_buf_t) is less
> than PAGE_SIZE, aligning the buffer to the nearest higher power of 2
> is enough to make sure that the buffer cannot cross a page boundary,
> effectively ensuring that the whole buffer is contiguous in physical
> space.
>
> Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
> ---
> kernel/kexec.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 2bee072..cdab59d 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1610,7 +1610,8 @@ void crash_save_cpu(struct pt_regs *regs, int cpu)
> static int __init crash_notes_memory_init(void)
> {
> /* Allocate memory for saving cpu registers. */
> - crash_notes = alloc_percpu(note_buf_t);
> + crash_notes = __alloc_percpu(sizeof(note_buf_t),
> + roundup_pow_of_two(sizeof(note_buf_t)));
> if (!crash_notes) {
> pr_warn("Kexec: Memory allocation for saving cpu register states failed\n");
> return -ENOMEM;