[PATCH] makedumpfile: fix a segmentation fault when pfn exceeds 2G boundary

[HATAYAMA Daisuke]

From: Jingbai Ma <jingbai.ma at hp.com>
Subject: [PATCH] makedumpfile: fix a segmentation fault when pfn exceeds 2G boundary
Date: Mon, 14 Apr 2014 18:35:44 +0800

> This patch intend to fix a segmentation fault when pfn exceeds 2G boundary.
> 
> In function is_on(), if the pfn (i) is greater than 2G, it will be a negative
> value and will cause a segmentation fault.
> is_on(char *bitmap, int i)
> {
>         return bitmap[i>>3] & (1 << (i & 7));
> }
> 
> 
> Signed-off-by: Jingbai Ma <jingbai.ma at hp.com>
> ---
>  makedumpfile.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/makedumpfile.h b/makedumpfile.h
> index 3d270c6..03d35a8 100644
> --- a/makedumpfile.h
> +++ b/makedumpfile.h
> @@ -1591,7 +1591,7 @@ int get_xen_info_ia64(void);
>  #endif	/* s390x */
>  
>  static inline int
> -is_on(char *bitmap, int i)
> +is_on(char *bitmap, unsigned long long i)
>  {
>  	return bitmap[i>>3] & (1 << (i & 7));
>  }
> 

is_on is used at the following two places.

static inline int
is_dumpable(struct dump_bitmap *bitmap, unsigned long long pfn)
{
        off_t offset;
        if (pfn == 0 || bitmap->no_block != pfn/PFN_BUFBITMAP) {
                offset = bitmap->offset + BUFSIZE_BITMAP*(pfn/PFN_BUFBITMAP);
                lseek(bitmap->fd, offset, SEEK_SET);
                read(bitmap->fd, bitmap->buf, BUFSIZE_BITMAP);
                if (pfn == 0)
                        bitmap->no_block = 0;
                else
                        bitmap->no_block = pfn/PFN_BUFBITMAP;
        }
        return is_on(bitmap->buf, pfn%PFN_BUFBITMAP);
}

PFN_BUFBTIMAP is constant 32 K. So, it seems that the 4 byte byte
length came here.

But right shift to signed integer is implementation defined. We should
not use right shift to signed integer. it looks gcc performs
arithmetic shift and this bahaviour is buggy in case of is_on().

static inline int
is_dumpable_cyclic(char *bitmap, unsigned long long pfn, struct cycle *cycle)
{
        if (pfn < cycle->start_pfn || cycle->end_pfn <= pfn)
                return FALSE;
        else
                return is_on(bitmap, pfn - cycle->start_pfn);
}

Simply, (pfn - cycle->start_pfn) could be (info->max_mapnr - 0). It's
possible to pass more than 2 Gi by using system with more than 8 TiB
physical memory space.

So,

- i must be unsigned in order to make right shift operation
  meaningful, and

- i must have 8 byte for systems with more than 8 TiB physical memory
  space.

BTW, to make this situation happen in cyclic mode, there needs to be
16 GiB free memory for bitmap to become large enough. I guess you
don't see this segmentation in the kdump 2nd kernel. Is this right?

Thanks.
HATAYAMA, Daisuke