Re: [PING] [PATCH v2] gdb: Modify until_break_command to act correctly on SIGTRAMP_FRAME

[Luis Machado <luis.machado@arm.com>]

On 11/28/22 11:48, Tomas Vanek via Gdb-patches wrote:
> On 21/10/2022 13:58, Tomas Vanek wrote:
>> This patch partially depends on
>> gdb/arm: Terminate frame unwinding in M-profile lockup state
>> (without it lockup state is unwound as if it were a normal
>> stack frame).
>>
>> The commands 'advance' and 'until' try to set a breakpoint
>> on the bogus return address derived from Arm M-profile magic
>> address (actually EXC_RETURN or a PC value indicating lockup).
>>
>> The offending breakpoint should be set at the return address in
>> the caller. The magic value 0xffffffff in LR indicates
>> there is no caller (return to this address would lock up the CPU).
>>
>> Similar behaviour of 'advance' and 'until' is observed in
>> an exception handler routine. In this case LR contains e.g.
>> 0xfffffff1 (EXC_RETURN) and GDB tries to set a breakpoint at
>> 0xfffffff0. It should use a return value stacked by the exception
>> instead.
>>
>> Testbench setup:
>> STM32G474, a Cortex-M4 device. Any Cortex-M device can be used.
>> A test application (an ordinary blink) with a standard startup
>> is loaded to the device flash.
>>
>> Steps to reproduce the problem:
>>
>> start GDB server
>>   $ openocd -f interface/cmsis-dap.cfg -f target/stm32g4x.cfg
>>
>> start GDB in second terminal
>>   $ arm-none-eabi-gdb blink.elf
>>
>>   (gdb) target extended-remote localhost:3333
>>
>> Reset the device and halt it:
>>   (gdb) monitor reset halt
>>   target halted due to debug-request, current mode: Thread
>>   xPSR: 0x01000000 pc: 0x08000e14 msp: 0x20020000
>>
>> Step by one instruction to re-read GDB register cache:
>>   (gdb) stepi
>>
>> Check registers, LR should be 0xffffffff after reset:
>>   (gdb) info registers
>>   ...
>>   sp             0x20020000          0x20020000
>>   lr             0xffffffff          -1
>>   pc             0x8000e16           0x8000e16
>>   xPSR           0x1000000           16777216
>>   ...
>>
>>   (gdb) set debug remote
>>
>> Issue 'advance' command:
>>   (gdb) advance main
>>   [remote] Sending packet: $mfffffffe,2#fa
>>   [remote] Packet received: 0000
>>   [remote] Sending packet: $mfffffffe,2#fa
>>   [remote] Packet received: 0000
>>   [remote] Sending packet: $m8000526,2#30
>>   [remote] Packet received: 2046
>>   [remote] Sending packet: $Z1,8000526,2#7a
>>   [remote] Packet received: OK
>>   [remote] packet_ok: Packet Z1 (hardware-breakpoint) is supported
>>   [remote] Sending packet: $Z0,fffffffe,2#43
>>   [remote] Packet received: E0E
>>   [remote] packet_ok: Packet Z0 (software-breakpoint) is supported
>>   Warning:
>>   Cannot insert breakpoint 0.
>>   Cannot access memory at address 0xfffffffe
>>
>>   Command aborted.
>>   (gdb)
>>
>> Relevant messages from OpenOCD:
>>   Error: Failed to read memory at 0xfffff000
>>   Error: can't add breakpoint: unknown reason
>>
>> This patch adds skipping over frames that are not suitable for
>> guarding with a breakpoint inspired by 'finish' command.
>> If no suitable frame is found, a momentary breakpoint is not set.
>>
>> v2: Comment fixes, bug reference.
>>
>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28683
>> Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
>> ---
>>   gdb/breakpoint.c | 22 +++++++++++++++++-----
>>   1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
>> index f6591d4..bb85342 100644
>> --- a/gdb/breakpoint.c
>> +++ b/gdb/breakpoint.c
>> @@ -10467,6 +10467,7 @@ enum async_reply_reason
>>   until_break_command (const char *arg, int from_tty, int anywhere)
>>   {
>>     frame_info_ptr frame;
>> +  frame_info_ptr caller_frame;
>>     struct gdbarch *frame_gdbarch;
>>     struct frame_id stack_frame_id;
>>     struct frame_id caller_frame_id;
>> @@ -10505,7 +10506,17 @@ enum async_reply_reason
>>     frame = get_selected_frame (NULL);
>>     frame_gdbarch = get_frame_arch (frame);
>>     stack_frame_id = get_stack_frame_id (frame);
>> -  caller_frame_id = frame_unwind_caller_id (frame);
>> +
>> +  caller_frame = get_prev_frame_always (frame);
>> +
>> +  while (caller_frame)
>> +    {
>> +      if (get_frame_type (caller_frame) != TAILCALL_FRAME
>> +      && gdbarch_code_of_frame_writable (get_frame_arch (caller_frame), caller_frame))
>> +    break;
>> +
>> +      caller_frame = get_prev_frame_always (caller_frame);
>> +    }
>>     /* Keep within the current frame, or in frames called by the current
>>        one.  */
>> @@ -10514,14 +10525,15 @@ enum async_reply_reason
>>     gdb::optional<delete_longjmp_breakpoint_cleanup> lj_deleter;
>> -  if (frame_id_p (caller_frame_id))
>> +  if (caller_frame)
>>       {
>>         struct symtab_and_line sal2;
>>         struct gdbarch *caller_gdbarch;
>> -      sal2 = find_pc_line (frame_unwind_caller_pc (frame), 0);
>> -      sal2.pc = frame_unwind_caller_pc (frame);
>> -      caller_gdbarch = frame_unwind_caller_arch (frame);
>> +      sal2 = find_pc_line (get_frame_pc (caller_frame), 0);
>> +      sal2.pc = get_frame_pc (caller_frame);
>> +      caller_gdbarch = get_frame_arch (caller_frame);
>> +      caller_frame_id = get_frame_id (caller_frame);
>>         breakpoint_up caller_breakpoint
>>       = set_momentary_breakpoint (caller_gdbarch, sal2,
> 
Would be nice to have some feedback on the generic parts of the above patch. It seems like a useful fix to have.