Re: [PATCH] makedumpfile: inhibit predefined macros when cross building

[Wang Nan <wangnan0@huawei.com>]

On 2014/4/4 16:32, Atsushi Kumagai wrote:
>> On 2014/4/4 11:59, Atsushi Kumagai wrote:
>>> Hello Wang,
>>>
>>>> On 2014/4/1 12:28, Wang Nan wrote:
>>>>> When cross building makedumpfile, for example, build x86_64 exec for
>>>>> dealing with arm vmcore, makefile passes a "-D__arm__" to gcc, but gcc
>>>>> predefined macros still take effect, defines "__x86_64__". Which makes
>>>>> definitions for x86_64 and arm mixed together in makedumpfile.h, causes
>>>>> many problems.
>>>
>>> I'm curious to know is it possible to analyze arm vmcores with
>>> x86_64 binaries even if using the logic for arm ?
>>> In such cases, is it not needed to build an arm binary with a cross
>>> compiler ? I worry about endian issues.
>>>
>>>
>>> Thanks
>>> Atsushi Kumagai
>>>
>>
>> I'm working on cross analysising arm vmcore on x86. I know that crash support
>> cross analysing (at least for analysing little endian arm vmcore on x86). I also notice
>> that makedumpfile has been designed for cross analysing (see commit 96f24dc77dc48e4c8f4c3527f4f5b4d27845393c
>> by Suzuki K. Poulose <suzuki@in.ibm.com>), but it doesn't work correctly for arm vmcore.
> 
> You may misunderstand, makedumpfile doesn't support cross analyzing.
> According to the patch, I'm sure that Suzuki sent the patch to enable
> *cross compile* (e.g. building arm binary on x86 machine) by setting
> a cross compiler to CC variable:
> 
>   (README)
>     5.Build for a different architecture than the host :
>       # make TARGET=<arch> ; make install
>       where <arch> is the 'uname -m' of the target architecture.
>       The user has to set the environment variable CC to appropriate
>       compiler for the target architecture.
> 
> When CC is the appropriate compiler for the target architecture,
> the mismatch predefining macros will not happen.
> So I think there is no problem in the current code.
> 
>> What I'm trying to do is to enable the whole kdump analysing stuff crossly (I know it is hard).
>> It is useful in some special cases, for example, for some boards with strictly limited memory
>> and storage space.
> 
> It sounds reasonable if you mentioned crash.
> 
> The mission of makedumpfile is reducing dump size, so it's best to work
> during capturing dump. Otherwise, you have to save full dump on the
> *limited storage space*.

I think I didn't explain my case very clearly.

Our situation is that, our device has very short storage space which can hold busybox only
(but with large memory). Most of its work is done in kernel (so kdump is important). When crash
happens, What it can do is only scp /proc/vmcore to a x86 machine via network.
Although using crash on the full vmcore on a x86 machine is affordable, makedumpfile is also useful
because we want to backup those vmcore files for furture use, so its size must be reduced.

> As for makedumpfile, I don't have any ideas about the advantage of the
> cross analyzing you said.
> 
>> With this patch, at least makedumpfile can be compiled to x86_64 binary, and can extract
>> symbol information correctly for a vmcore file dumpped from a qemu virtual machine (little
>> endian).
> 
> Your patch sounds inconsistent to me since makedumpfile doesn't support
> cross analyzing. The case you said will go well, but that's just a lucky
> situation.
> 
> 
> Thanks
> Atsushi Kumagai
> 
>>>>> This patch changes Makefile: if host arch and target arch are different,
>>>>> passes "-U__$(HOST_ARCH)__" to gcc.
>>>>>
>>>>> I have tested by running following command on x86_64 machine:
>>>>>
>>>>> make ARCH=arm
>>>>
>>>> Sorry, should be TARGET=arm. I have tested and build okay.
>>>>>
>>>>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>>>>> Cc: Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>
>>>>> Cc: kexec@lists.infradead.org
>>>>> Cc: Geng Hui <hui.geng@huawei.com>
>>>>> ---
>>>>>  Makefile | 12 +++++++++---
>>>>>  1 file changed, 9 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/Makefile b/Makefile
>>>>> index 2f4845c..f85cc21 100644
>>>>> --- a/Makefile
>>>>> +++ b/Makefile
>>>>> @@ -15,10 +15,11 @@ CFLAGS_ARCH	= -g -O2 -Wall -D_FILE_OFFSET_BITS=64 \
>>>>>  		    -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
>>>>>  # LDFLAGS = -L/usr/local/lib -I/usr/local/include
>>>>>
>>>>> +HOST_ARCH := $(shell uname -m)
>>>>>  # Use TARGET as the target architecture if specified.
>>>>>  # Defaults to uname -m
>>>>>  ifeq ($(strip($TARGET)),)
>>>>> -TARGET := $(shell uname -m)
>>>>> +TARGET := $(HOST_ARCH)
>>>>>  endif
>>>>>
>>>>>  ARCH := $(shell echo ${TARGET}  | sed -e s/i.86/x86/ -e s/sun4u/sparc64/ \
>>>>> @@ -26,8 +27,13 @@ ARCH := $(shell echo ${TARGET}  | sed -e s/i.86/x86/ -e s/sun4u/sparc64/ \
>>>>>  			       -e s/s390x/s390/ -e s/parisc64/parisc/ \
>>>>>  			       -e s/ppc64/powerpc64/ -e s/ppc/powerpc32/)
>>>>>
>>>>> -CFLAGS += -D__$(ARCH)__
>>>>> -CFLAGS_ARCH += -D__$(ARCH)__
>>>>> +CROSS :=
>>>>> +ifneq ($(TARGET), $(HOST_ARCH))
>>>>> +CROSS := -U__$(HOST_ARCH)__
>>>>> +endif
>>>>> +
>>>>> +CFLAGS += -D__$(ARCH)__ $(CROSS)
>>>>> +CFLAGS_ARCH += -D__$(ARCH)__ $(CROSS)
>>>>>
>>>>>  ifeq ($(ARCH), powerpc64)
>>>>>  CFLAGS += -m64
>>>>>
>>>>
>>
> 



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