I have download gdb from gnu. And compile it with the following command.
./configure --enable-targets=all
make
make install

issue gdb vmlinux vmcore
There is one error information: xxx is not a core dump: File format is ambiguous

Any body can help me fix this issue. thanks in advance.

How does it work on something simpler - like a "hello world; abort();" program ?

Dear linosaurusroot:
thanks your feedback. I wrote one "hello" program to test gdb, it seams that same error information. I have paste all the information here. Could you help check why? thanks.

xinhua@xinhua-ThinkCentre-M81:~/test$ gcc -Wall -g hello.c -c
xinhua@xinhua-ThinkCentre-M81:~/test$ gdb hello.o hello
GNU gdb (GDB) 7.6
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/xinhua/test/hello.o...done.
"/home/xinhua/test/hello" is not a core dump: File format not recognized

Your commands reveal you are in fact running gdb with a second argument that is not a core file.
The above is wrong.

I suggest you try this.
Assuming you are successful with the block just above here then your original problem may be to do with how you called gdb or to do with using it on the kernel as opposed to a userspace program.

and actually ./hello must procude a corefile (named core). Using any other file (including any other core file) will give you an error message, the core and app should belong to each other (the name of the corefile is not relevant)

Dear linosaurusroot and pan64:
Thanks your mention. It seams that gdb enviroment is ok. The follow is my new try. But when i use it for android kernel dump. still has issue. And crash tool is ok.
$ ulimit -a
$ gcc -Wall -g hello.c
there is new file a.out
$ ./a.out -> there will be one new file "core"
$ gdb a.out core
GNU gdb (GDB) 7.6
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/xinhua/test/a.out...done.
[New LWP 28132]

warning: Can't read pathname for load map: Input/output error.
Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
#0 0x0000000000400508 in main () at hello.c:5
5 printf("p=%d\n",*p);

that looks like the normal (expected) behaviour. I can't see your hello.c, but in line 5 printf("p=%d\n",*p); there is an error which caused the segmentation fault. That was reported by gdb.
I do not know what does that warning means:
warning: Can't read pathname for load map: Input/output error.

The follow is hello.c. the printf function used to generate dump.

#include <stdio.h>
int main(void)
{
int *p=NULL;
printf("p=%d\n",*p);
return 0;
}

see here https://www.linuxquestions.org/quest...-error-817820/
and also the link at the bottom of that page: http://www.ffnn.nl/pages/articles/li...gger-intro.php