GDB: Assertion while unwinding stack

[rojer]

When unwinding stack, GDB fails with internal assertion if next frame pointer happens to be 0. In this case, this is a legitimate value - it's the end of the stack of this particular task and GDB should stop unwinding. Instead, this is what happens:

[rojer@nba ~/go/src/cesanta.com/fw/platforms/esp8266 mos1]$ make debug_coredump CONSOLE_LOG=/tmp/console.log
Waiting for gdb on 1234
GNU gdb (crosstool-NG crosstool-ng-1.22.0-55-gecfc19a) 7.10
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Reading symbols from .build/mongoose-os.elf...done.
Remote debugging using 127.0.0.1:1234
Found core at 1202 - 177340
Mapping DRAM: 98304 @ 0x3ffe8000
Mapping IRAM: 32768 @ 0x40100000
Mapping IROM: 460784 @ 0x40211000
Mapping /mongoose-os/common/platforms/esp8266/rom/rom.bin at 0x40000000
Loaded core dump from last snippet in  /console.log
>> qSupported:multiprocess+;swbreak+;hwbreak+;qRelocInsn+
>> Hg0
>> qTStatus
>> ?
>> qfThreadInfo
>> Hc-1
>> qC
>> qAttached
>> qOffsets
>> g
>> qfThreadInfo
>> m4023f1d1,4
<< 39 02 01 e7
>> m4023f0e8,20
<< 12 c1 e0 09 71 c9 61 d9 51 29 01 45 59 02 85 be 08 56 52 0e 0c 13 21 ec ff 32 42 00 c1 dd ff 28
>> m3fff5d2c,4
<< 47 f2 23 40
>> m4023f247,4
<< cd 02 0c 13
>> m4023f218,20
<< 12 c1 e0 09 71 c9 61 d9 51 e9 41 45 07 d4 3d 02 21 f8 ff 01 85 47 c0 00 00 2c 02 0c 83 0c 04 05
>> m3fff5d4c,4
<< 00 00 00 00
0x4023f1d1 in esp_mgos_init2 (
/opt/Espressif/esp-open-sdk/crosstool-NG/.build/src/gdb-7.10/gdb/inline-frame.c:171: internal-error: inline_frame_this_id: Assertion `!frame_id_eq (*this_id, outer_frame_id)' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) n

here >> denote the GDB commands received by our core dump server and << are responses to the m commands. Fetch of 0x3fff5d4c is GDB making the next step up, the trace but there is nothing there (legitimately) and it crashes and burns. If i patch up CD server to return non-zero, unwind finishes correctly:

Loaded core dump from last snippet in  /console.log
>> qSupported:multiprocess+;swbreak+;hwbreak+;qRelocInsn+
>> Hg0
>> qTStatus
>> ?
>> qfThreadInfo
>> Hc-1
>> qC
>> qAttached
>> qOffsets
>> g
>> qfThreadInfo
>> m4023f1d1,4
<< 39 02 01 e7
>> m4023f0e8,20
<< 12 c1 e0 09 71 c9 61 d9 51 29 01 45 59 02 85 be 08 56 52 0e 0c 13 21 ec ff 32 42 00 c1 dd ff 28
>> m3fff5d2c,4
<< 47 f2 23 40
>> m4023f247,4
<< cd 02 0c 13
>> m4023f218,20
<< 12 c1 e0 09 71 c9 61 d9 51 e9 41 45 07 d4 3d 02 21 f8 ff 01 85 47 c0 00 00 2c 02 0c 83 0c 04 05
>> m3fff5d4c,4
<< 01 00 00 00
>> m3fff5d10,4
<< 84 64 ff 3f
0x4023f1d1 in esp_mgos_init2 (bcfg=bcfg@entry=0x3fff6484)
    at /mongoose-os/fw/platforms/esp8266/src/esp_main.c:127
127       *((int *) 0x123) = 1;
>> qSymbol::
add symbol table from file "/mongoose-os/common/platforms/esp8266/rom/rom.elf" at
        .text_addr = 0x40000000
Reading symbols from /mongoose-os/common/platforms/esp8266/rom/rom.elf...(no debugging symbols found)...done.
>> qSymbol::
>> m4023f1d1,4
<< 39 02 01 e7
>> m4023f0e8,20
<< 12 c1 e0 09 71 c9 61 d9 51 29 01 45 59 02 85 be 08 56 52 0e 0c 13 21 ec ff 32 42 00 c1 dd ff 28
>> m3fff5d2c,4
<< 47 f2 23 40
(gdb) bt
>> m4023f247,4
<< cd 02 0c 13
>> m4023f218,20
<< 12 c1 e0 09 71 c9 61 d9 51 e9 41 45 07 d4 3d 02 21 f8 ff 01 85 47 c0 00 00 2c 02 0c 83 0c 04 05
>> m3fff5d4c,4
<< 01 00 00 00
>> m3fff5d10,4
<< 84 64 ff 3f
#0  0x4023f1d1 in esp_mgos_init2 (bcfg=bcfg@entry=0x3fff6484)
    at /mongoose-os/fw/platforms/esp8266/src/esp_main.c:127
#1  0x4023f247 in esp_mgos_init ()
    at /mongoose-os/fw/platforms/esp8266/src/esp_main.c:158
>> m1,4
Unmapped addr 0x1
<< 01 00 00 00
>> m3fff5d4c,4
<< 01 00 00 00
#2  mgos_task (arg=<optimized out>)
    at /mongoose-os/fw/platforms/esp8266/src/esp_main.c:201
#3  0x00000001 in ?? ()
>> m1,4
Unmapped addr 0x1
<< 01 00 00 00
>> m3fff5d4c,4
<< 01 00 00 00
Backtrace stopped: previous frame identical to this frame (corrupt stack?)

here i told CD to return 0x1 instead and GDB was able to proceed (although it tried to fetch 0x1 and that failed). backtrace is correct: mgos_task is the main function of the task and there's nothing beyond it.