Re: mmap'ed memory in core files ?

[Hidehiro Kawai]

Hello,

Philippe De Muyter wrote:

> Hello Hidehiro,
> 
> On Thu, Jul 03, 2008 at 12:51:33PM +0900, Hidehiro Kawai wrote:
> 
>>Hi,
>>
>>Michael Kerrisk wrote:
>>
>>
>>>[CC+= hidehiro.kawai.ez@xxxxxxxxxxx]
>>>
>>>On Wed, Jul 2, 2008 at 12:50 PM, Philippe De Muyter <phdm@xxxxxxxxx> wrote:
>>>
>>>
>>>>Hi Michael,
>>>>
>>>>On Tue, Jul 01, 2008 at 08:16:11PM +0200, Michael Kerrisk wrote:
>>>>
>>>>
>>>>>On 7/1/08, Philippe De Muyter <phdm@xxxxxxxxx> 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@xxxxxxxxxxx>
>>---
>>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.

I'm afraid I don't know real usages of VM_RESERVED and VM_IO, either.
Allowing everyone to choose whether dump the dma region or not,
perhaps we need to introduce a new VM flag (e.g. VM_DUMPABLE) and
a coredump_filter bit which controls (VM_IO | VM_RESERVED) &&
VM_DUMPABLE area, for example.

I think it is also OK to just add VM_ALWAYSDUMP flag to the dma
region if the device driver knows the region is safely readable
and small enough.

Regards,
-- 
Hidehiro Kawai
Hitachi, Systems Development Laboratory
Linux Technology Center

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