Re: mmap'ed memory in core files ?

Philippe De Muyter
2008-07-03T12:29:20Z

Hello Hidehiro,

On Thu, Jul 03, 2008 at 12:51:33PM +0900, Hidehiro Kawai wrote:
> Hi,
>
> Michael Kerrisk wrote:
>
> > [CC+= hidehiro.kawai.ez@hitachi.com]
> >
> > On Wed, Jul 2, 2008 at 12:50 PM, Philippe De Muyter <phdm@macqel.be> wrote:
> >
> >>Hi Michael,
> >>
> >>On Tue, Jul 01, 2008 at 08:16:11PM +0200, Michael Kerrisk wrote:
> >>
> >>>On 7/1/08, Philippe De Muyter <phdm@macqel.be> wrote:
> >>>
> >>>>Hello everybody,
> >>>>
> >>>> I develop video acquisition software using the video1394 interface.
> >>>> The images grabbed by the camera and iee1394 bus are kept in kernel
> >>>> memory and made available to the user program through a mmap call done
> >>>> in the libdc1394 library :
> >>>>
> >>>> dma_ring_buffer= mmap(0, vmmap.nb_buffers * vmmap.buf_size,
> >>>> PROT_READ|PROT_WRITE,MAP_SHARED, craw->capture.dma_fd, 0);
> >>>>
> >>>> Sometimes, my program crashes and produces a core file :) It seems to
> >>>> me that the core file does not contain the mmap'ed memory and hence
> >>>> I cannot replay my program with the same image for debugging purpose.
> >>>>
> >>>> Is it possible to configure the kernel through /proc, or through the mmap
> >>>> system call to have that mmapped segment in the core file, or do I need
> >>>> to modify the kernel itself to obtain the behaviour I want ? If I
> >>>> need to modify the kernel, can some kind soul provide me some pointers ?
> >>>
> >>>
> >>>Have a look at the section "Controlling which mappings are written to
> >>>the core dump" in a recent core.5 man page:
> >>>http://www.kernel.org/doc/man-pages/online/pages/man5/core.5.html
> >>
> >>thanks for the info. I didn't know about /proc/PID/coredump_filter.
> >>
> >>that part was promising :
> >>
> >> bit 2 Dump file-backed private mappings.
> >> bit 3 Dump file-backed shared mappings.
> >>
> >> The default value of coredump_filter is 0x3; this reflects traditional
> >> Linux behavior and means that only anonymous memory segments are dumped.
> >>
> >>Unfortunately, the part that applies to me (I have tested it) is the next one :
> >>
> >> Memory-mapped I/O pages such as frame buffer are never dumped, [...],
> >> regardless of the coredump_filter value.
> >>
> >>Is that a design decision, or a mere finding of the way it is implemented
> >>now ?
>
> MMIO pages have been not dumped since a long time ago, and I didn't target
> them for the coredump_filter feature because I thought it was generally
> danger to read MMIO pages. As for frame buffer we would be able to
> safely access to it, but there is no way to tell it from other MMIO
> mappings, AFAIK.
>
> >>So, back to my original question :
> >>
> >>Can some kind soul provide me some pointers to the way I should modify
> >>the kernel to make the inclusion of the video1394 mmapped segment in
> >>core files possible ?
> >
> >
> > Perhaps Hidehiro, who wrote the coredump_filter feature, can provide insight.
>
> The following patch may help. To dump MMIO pages, set bit 5 of
> coredump_filter.
>
> $ echo 0x23 > /proc/<PID>/coredump_filter
>
>
> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
> ---
> This patch is not intended to be merged to the upstream kernel
> because the safeness of reading VM_IO mappings has not been
> proved.
>
> Index: linux-2.6.26-rc5-mm3/fs/binfmt_elf.c
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/fs/binfmt_elf.c
> +++ linux-2.6.26-rc5-mm3/fs/binfmt_elf.c
> @@ -1141,11 +1141,18 @@ static unsigned long vma_dump_size(struc
> if (vma->vm_flags & VM_ALWAYSDUMP)
> goto whole;
>
> - /* Do not dump I/O mapped devices or special mappings */
> - if (vma->vm_flags & (VM_IO | VM_RESERVED))
> +#define FILTER(type) (mm_flags & (1UL << MMF_DUMP_##type))
> +
> + /* By default, do not dump memory mapped I/O mappings */
> + if (vma->vm_flags & VM_IO) {
> + if (FILTER(MMIO))
> + goto whole;
> return 0;
> + }
>
> -#define FILTER(type) (mm_flags & (1UL << MMF_DUMP_##type))
> + /* Do not dump special mappings */
> + if (vma->vm_flags & VM_RESERVED)
> + return 0;
>
> /* By default, dump shared memory if mapped from an anonymous file. */
> if (vma->vm_flags & VM_SHARED) {
> Index: linux-2.6.26-rc5-mm3/include/linux/sched.h
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/include/linux/sched.h
> +++ linux-2.6.26-rc5-mm3/include/linux/sched.h
> @@ -403,8 +403,9 @@ extern int get_dumpable(struct mm_struct
> #define MMF_DUMP_MAPPED_PRIVATE 4
> #define MMF_DUMP_MAPPED_SHARED 5
> #define MMF_DUMP_ELF_HEADERS 6
> +#define MMF_DUMP_MMIO 7
> #define MMF_DUMP_FILTER_SHIFT MMF_DUMPABLE_BITS
> -#define MMF_DUMP_FILTER_BITS 5
> +#define MMF_DUMP_FILTER_BITS 6
> #define MMF_DUMP_FILTER_MASK \
> (((1 << MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
> #define MMF_DUMP_FILTER_DEFAULT \
>
>
>
> Regards,

Thanks for your patch, but it will not help here. Before applying it blindly
I asked myself if the mmapped region was tagged VM_IO, because it is actually
a simple ram zone, not an I/O zone, and the answer is it is not a VM_IO zone.
Details :

drivers/ieee1394/video1394.c:
static int video1394_mmap(struct file *file, struct vm_area_struct *vma)
{
[...]
return dma_region_mmap(&ctx->current_ctx->dma, file, vma);
}

drivers/ieee1394/dma.c:
int dma_region_mmap(struct dma_region *dma, struct file *file,
struct vm_area_struct *vma)
{
[...]
vma->vm_ops = &dma_region_vm_ops;
vma->vm_private_data = dma;
vma->vm_file = file;
vma->vm_flags |= VM_RESERVED;

return 0;
}

So, actually the zone I would like to get dumped in the core file is tagged
VM_RESERVED.

I see the following ways to solve my problem :
- do not tag the zone as VM_RESERVED in ieee1394::dma_region_mmap
- tag the zone as VM_ALWAYSDUMP in ieee1394::dma_region_mmap
- add a bit in coredump_filter to dump the VM_RESERVED zones.

As I don't know the real meaning of VM_RESERVED, I do not know which choice
is the best one for the official kernel tree, but locally I'll go for
adding VM_ALWAYSDUMP in ieee1394::dma_region_mmap.

Philippe