Core Dump

The core dump module enables dumping the CPU registers and memory content for offline debugging. This module is called when a fatal error is encountered and prints or stores data according to which backends are enabled.

Configuration

Configure this module using the following options.

DEBUG_COREDUMP: enable the module.

Here are the options to enable output backends for core dump:

DEBUG_COREDUMP_BACKEND_LOGGING: use log module for core dump output.
DEBUG_COREDUMP_BACKEND_FLASH_PARTITION: use flash partition for core dump output.
DEBUG_COREDUMP_BACKEND_NULL: fallback core dump backend if other backends cannot be enabled. All output is sent to null.

Here are the choices regarding memory dump:

DEBUG_COREDUMP_MEMORY_DUMP_MIN: only dumps the stack of the exception thread, its thread struct, and some other bare minimal data to support walking the stack in the debugger. Use this only if absolute minimum of data dump is desired.
DEBUG_COREDUMP_MEMORY_DUMP_THREADS: Dumps the thread struct and stack of all threads and all data required to debug threads.
DEBUG_COREDUMP_MEMORY_DUMP_LINKER_RAM: Dumps the memory region between _image_ram_start[] and _image_ram_end[]. This includes at least data, noinit, and BSS sections. This is the default.

Additional memory can be included in a dump (even with the “DEBUG_COREDUMP_MEMORY_DUMP_MIN” config selected) through one or more coredump devices

Usage

When the core dump module is enabled, during a fatal error, CPU registers and memory content are printed or stored according to which backends are enabled. This core dump data can fed into a custom-made GDB server as a remote target for GDB (and other GDB compatible debuggers). CPU registers, memory content and stack can be examined in the debugger.

This usually involves the following steps:

Get the core dump log from the device depending on enabled backends. For example, if the log module backend is used, get the log output from the log module backend.
Convert the core dump log into a binary format that can be parsed by the GDB server. For example, scripts/coredump/coredump_serial_log_parser.py can be used to convert the serial console log into a binary file.
Start the custom GDB server using the script scripts/coredump/coredump_gdbserver.py with the core dump binary log file, and the Zephyr ELF file as parameters. The GDB server can also be started from within GDB, see below.
Start the debugger corresponding to the target architecture.

Note

Developers for Intel ADSP CAVS 15-25 platforms using ZEPHYR_TOOLCHAIN_VARIANT=zephyr should use the debugger in the xtensa-intel_apl_adsp toolchain of the SDK.

When DEBUG_COREDUMP_BACKEND_FLASH_PARTITION is enabled the core dump data is stored in the flash partition. The flash partition must be defined in the device tree:

&flash0 {
        partitions {
                coredump_partition: partition@255000 {
                        label = "coredump-partition";
                        reg = <0x255000 DT_SIZE_K(4)>;
                };
};

Example

This example uses the log module backend tied to serial console. This was done on QEMU Emulation for X86 where a null pointer was dereferenced.

This is the core dump log from the serial console, and is stored in coredump.log:

Booting from ROM..*** Booting Zephyr OS build zephyr-v2.3.0-1840-g7bba91944a63  ***
Hello World! qemu_x86
E: Page fault at address 0x0 (error code 0x2)
E: Linear address not present in page tables
E:   PDE: 0x0000000000115827 Writable, User, Execute Enabled
E:   PTE: Non-present
E: EAX: 0x00000000, EBX: 0x00000000, ECX: 0x00119d74, EDX: 0x000003f8
E: ESI: 0x00000000, EDI: 0x00101aa7, EBP: 0x00119d10, ESP: 0x00119d00
E: EFLAGS: 0x00000206 CS: 0x0008 CR3: 0x00119000
E: call trace:
E: EIP: 0x00100459
E:      0x00100477 (0x0)
E:      0x00100492 (0x0)
E:      0x001004c8 (0x0)
E:      0x00105465 (0x105465)
E:      0x00101abe (0x0)
E: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
E: Current thread: 0x00119080 (unknown)
E: #CD:BEGIN#
E: #CD:5a4501000100050000000000
E: #CD:4101003800
E: #CD:0e0000000200000000000000749d1100f803000000000000009d1100109d1100
E: #CD:00000000a71a100059041000060200000800000000901100
E: #CD:4d010080901100e0901100
E: #CD:0100000000000000000000000180000000000000000000000000000000000000
E: #CD:00000000000000000000000000000000e364100000000000000000004c9c1100
E: #CD:000000000000000000000000b49911000004000000000000fc03000000000000
E: #CD:4d0100b4991100b49d1100
E: #CD:f8030000020000000200000002000000f8030000fd03000a02000000dc9e1100
E: #CD:149a1160fd03000002000000dc9e1100249a110087201000049f11000a000000
E: #CD:349a11000a4f1000049f11000a9e1100449a11000a8b10000200000002000000
E: #CD:449a1100388b1000049f11000a000000549a1100ad201000049f11000a000000
E: #CD:749a11000a201000049f11000a000000649a11000a201000049f11000a000000
E: #CD:749a1100e8201000049f11000a000000949a1100890b10000a0000000a000000
E: #CD:a49a1100890b10000a0000000a000000f8030000189b11000200000002000000
E: #CD:f49a1100289b11000a000000189b1100049b11009b0710000a000000289b1100
E: #CD:f49a110087201000049f110045000000f49a1100509011000a00000020901100
E: #CD:f49a110060901100049f1100ffffffff0000000000000000049f1100ffffffff
E: #CD:0000000000000000630b1000189b1100349b1100af0b1000630b1000289b1100
E: #CD:55891000789b11000000000020901100549b1100480000004a891000609b1100
E: #CD:649b1100d00b10004a891000709b110000000000609b11000a00000000000000
E: #CD:849b1100709b11004a89100000000000949b1100794a10000000000058901100
E: #CD:20901100c34a10000a00001734020000d001000000000000d49b110038000000
E: #CD:c49b110078481000b49911000004000000000000000000000c9c11000c9c1100
E: #CD:149c110000000000d49b110038000000f49b1100da481000b499110000040000
E: #CD:0e0000000200000000000000744d0100b4991100b49d1100009d1100109d1100
E: #CD:149c110099471000b4991100000400000800000000901100ad861000409c1100
E: #CD:349c1100e94710008090110000000000349c1100b64710008086100045000000
E: #CD:849c11002d53100000000000d09c11008090110020861000f5ffffff8c9c1100
E: #CD:000000000000000000000000a71a1000a49c1100020200008090110000000000
E: #CD:a49c1100020200000800000000000000a49c11001937100000000000d09c1100
E: #CD:0c9d0000bc9c0000b49d1100b4991100c49c1100ae37100000000000d09c1100
E: #CD:0800000000000000c888100000000000109d11005d031000d09c1100009d1100
E: #CD:109d11000000000000000000a71a1000f803000000000000749d110002000000
E: #CD:5904100008000000060200000e0000000202000002020000000000002c9d1100
E: #CD:7704100000000000d00b1000c9881000549d110000000000489d110092041000
E: #CD:00000000689d1100549d11000000000000000000689d1100c804100000000000
E: #CD:c0881000000000007c9d110000000000749d11007c9d11006554100065541000
E: #CD:00000000000000009c9d1100be1a100000000000000000000000000038041000
E: #CD:08000000020200000000000000000000f4531000000000000000000000000000
E: #CD:END#
E: Halting system

Run the core dump serial log converter:

./scripts/coredump/coredump_serial_log_parser.py coredump.log coredump.bin

Start the custom GDB server:

./scripts/coredump/coredump_gdbserver.py build/zephyr/zephyr.elf coredump.bin

Start GDB:

<path to SDK>/x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gdb build/zephyr/zephyr.elf

Inside GDB, connect to the GDB server via port 1234:
```
(gdb) target remote localhost:1234
```

Examine the CPU registers:

(gdb) info registers

Output from GDB:

eax            0x0                 0
ecx            0x119d74            1154420
edx            0x3f8               1016
ebx            0x0                 0
esp            0x119d00            0x119d00 <z_main_stack+844>
ebp            0x119d10            0x119d10 <z_main_stack+860>
esi            0x0                 0
edi            0x101aa7            1055399
eip            0x100459            0x100459 <func_3+16>
eflags         0x206               [ PF IF ]
cs             0x8                 8
ss             <unavailable>
ds             <unavailable>
es             <unavailable>
fs             <unavailable>
gs             <unavailable>

Examine the backtrace:

(gdb) bt

Output from GDB:

#0  0x00100459 in func_3 (addr=0x0) at zephyr/rtos/zephyr/samples/hello_world/src/main.c:14
#1  0x00100477 in func_2 (addr=0x0) at zephyr/rtos/zephyr/samples/hello_world/src/main.c:21
#2  0x00100492 in func_1 (addr=0x0) at zephyr/rtos/zephyr/samples/hello_world/src/main.c:28
#3  0x001004c8 in main () at zephyr/rtos/zephyr/samples/hello_world/src/main.c:42

Starting the GDB server from within GDB

You can use target remote | to start the custom GDB server from inside GDB, instead of in a separate shell.

Start GDB:

<path to SDK>/x86_64-zephyr-elf/bin/x86_64-zephyr-elf-gdb build/zephyr/zephyr.elf

Inside GDB, start the GDB server using the --pipe option:

(gdb) target remote | ./scripts/coredump/coredump_gdbserver.py --pipe build/zephyr/zephyr.elf coredump.bin

File Format

The core dump binary file consists of one file header, one architecture-specific block, zero or one threads metadata block(s), and multiple memory blocks. All numbers in the headers below are little endian.

File Header

The file header consists of the following fields:

Core dump binary file header
Field	Data Type	Description
ID	`char[2]`	`Z`, `E` as identifier of file.
Header version	`uint16_t`	Identify the version of the header. This needs to be incremented whenever the header struct is modified. This allows parser to reject older header versions so it will not incorrectly parse the header.
Target code	`uint16_t`	Indicate which target (e.g. architecture or SoC) so the parser can instantiate the correct register block parser.
Pointer size	‘uint8_t’	Size of `uintptr_t` in power of 2. (e.g. 5 for 32-bit, 6 for 64-bit). This is needed to accommodate 32-bit and 64-bit target in parsing the memory block addresses.
Flags	`uint8_t`
Fatal error reason	`unsigned int`	Reason for the fatal error, as the same in `enum k_fatal_error_reason` defined in include/zephyr/fatal.h

Architecture-specific Block

The architecture-specific block contains the byte stream of data specific to the target architecture (e.g. CPU registers)

Architecture-specific Block
Field	Data Type	Description
ID	`char`	`A` to indicate this is a architecture-specific block.
Header version	`uint16_t`	Identify the version of this block. To be interpreted by the target architecture specific block parser.
Number of bytes	`uint16_t`	Number of bytes following the header which contains the byte stream for target data. The format of the byte stream is specific to the target and is only being parsed by the target parser.
Register byte stream	`uint8_t[]`	Contains target architecture specific data.

Threads Metadata Block

The threads metadata block contains the byte stream of data necessary for debugging threads.

Threads Metadata Block
Field	Data Type	Description
ID	`char`	`T` to indicate this is a threads metadata block.
Header version	`uint16_t`	Identify the version of the header. This needs to be incremented whenever the header struct is modified. This allows parser to reject older header versions so it will not incorrectly parse the header.
Number of bytes	`uint16_t`	Number of bytes following the header which contains the byte stream for target data.
Byte stream	`uint8_t[]`	Contains data necessary for debugging threads.

Memory Block

The memory block contains the start and end addresses and the data within the memory region.

Memory Block
Field	Data Type	Description
ID	`char`	`M` to indicate this is a memory block.
Header version	`uint16_t`	Identify the version of the header. This needs to be incremented whenever the header struct is modified. This allows parser to reject older header versions so it will not incorrectly parse the header.
Start address	`uintptr_t`	The start address of the memory region.
End address	`uintptr_t`	The end address of the memory region.
Memory byte stream	`uint8_t[]`	Contains the memory content between the start and end addresses.

Adding New Target

The architecture-specific block is target specific and requires new dumping routine and parser for new targets. To add a new target, the following needs to be done:

Add a new target code to the enum coredump_tgt_code in include/zephyr/debug/coredump.h.
Implement arch_coredump_tgt_code_get() simply to return the newly introduced target code.
Implement arch_coredump_info_dump() to construct a target architecture block and call coredump_buffer_output() to output the block to core dump backend.
Add a parser to the core dump GDB stub scripts under scripts/coredump/gdbstubs/
1. Extends the gdbstubs.gdbstub.GdbStub class.
2. During __init__, store the GDB signal corresponding to the exception reason in self.gdb_signal.
3. Parse the architecture-specific block from self.logfile.get_arch_data(). This needs to match the format as implemented in step 3 (inside arch_coredump_info_dump()).
4. Implement the abstract method handle_register_group_read_packet where it returns the register group as GDB expected. Refer to GDB’s code and documentation on what it is expecting for the new target.
5. Optionally implement handle_register_single_read_packet for registers not covered in the g packet.
Extend get_gdbstub() in scripts/coredump/gdbstubs/__init__.py to return the newly implemented GDB stub.

API documentation

Coredump APIs

Architecture-specific core dump APIs