* GDB remote debugging stub: Question about memory read
@ 2010-09-27 8:54 Niklas Quarfot Nielsen
2010-09-27 9:16 ` Pedro Alves
0 siblings, 1 reply; 3+ messages in thread
From: Niklas Quarfot Nielsen @ 2010-09-27 8:54 UTC (permalink / raw)
To: gdb
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Hi everyone,
I just subscribed to this list, so if I do not follow your mailing list conventions - please let me know.
I am implementing a remote debugging stub for a research operating system at the technical university of Denmark.
GDB can connect and if I disable memory reads, single stepping and hardware breakpointing works fine. So serial communication and initialization should be in order.
The problem arises when GDB requests memory read of the stack (right after the g-packet as been received).
The registers contains the following:
rax 0x0
rbx 0xb816a
rcx 0x6e
rdx 0x8
rsi 0xffff8000000b8000
rdi 0xb8000
rbp 0xffffffff6efffff0
rsp 0xffffffff6effffd0
r8 0x5
r9 0x78bfbff
r10 0x0
r11 0x0
r12 0x0
r13 0x0
r14 0x0
r15 0x0
rip 0xffffffff80200293 <system_start+223>
eflags 0x46 [ PF ZF ]
cs 0x18
ss 0x0
ds 0x0
es 0x0
fs 0x0
GDB tries to read 0x40 bytes starting at address 0xffffffff6effffc0, which (I guess) is from the RSP.
Packet: mffffffff6effffc0,40
My question is:
Why does GDB try to read 0x40 bytes, when there is only 0x20 bytes on the stack (RBP-RSP)?
The architecture is an AMD64 and the version of GDB is 7.1(x86_64-gnu-linux).
I appreciate any answer and/or clue to why GDB behaves like this.
I have tried to look through the remote debugging source code of GDB, but this has not given me any answers.
If needed, I can post debugging information from the target code in GDB.
Best regards
Niklas Quarfot Nielsen
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: GDB remote debugging stub: Question about memory read
2010-09-27 8:54 GDB remote debugging stub: Question about memory read Niklas Quarfot Nielsen
@ 2010-09-27 9:16 ` Pedro Alves
2010-09-27 9:20 ` Niklas Quarfot Nielsen
0 siblings, 1 reply; 3+ messages in thread
From: Pedro Alves @ 2010-09-27 9:16 UTC (permalink / raw)
To: gdb; +Cc: Niklas Quarfot Nielsen
On Monday 27 September 2010 09:54:04, Niklas Quarfot Nielsen wrote:
> GDB tries to read 0x40 bytes starting at address 0xffffffff6effffc0, which (I guess) is from the RSP.
> Packet: mffffffff6effffc0,40
>
> My question is:
> Why does GDB try to read 0x40 bytes, when there is only 0x20 bytes on the stack (RBP-RSP)?
>
> The architecture is an AMD64 and the version of GDB is 7.1(x86_64-gnu-linux).
>
> I appreciate any answer and/or clue to why GDB behaves like this.
> I have tried to look through the remote debugging source code of GDB, but this has not given me any answers.
> If needed, I can post debugging information from the target code in GDB.
>
0x40 bytes is GDB's stack cache line size (dcache.c).
Try "(gdb) set stack-cache off".
--
Pedro Alves
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: GDB remote debugging stub: Question about memory read
2010-09-27 9:16 ` Pedro Alves
@ 2010-09-27 9:20 ` Niklas Quarfot Nielsen
0 siblings, 0 replies; 3+ messages in thread
From: Niklas Quarfot Nielsen @ 2010-09-27 9:20 UTC (permalink / raw)
To: Pedro Alves; +Cc: gdb
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That worked! Thank you so much - you saved my day. (Probably a couple of days to be more exact).
Cheers,
Niklas
On Sep 27, 2010, at 11:16 , Pedro Alves wrote:
> On Monday 27 September 2010 09:54:04, Niklas Quarfot Nielsen wrote:
>> GDB tries to read 0x40 bytes starting at address 0xffffffff6effffc0, which (I guess) is from the RSP.
>> Packet: mffffffff6effffc0,40
>>
>> My question is:
>> Why does GDB try to read 0x40 bytes, when there is only 0x20 bytes on the stack (RBP-RSP)?
>>
>> The architecture is an AMD64 and the version of GDB is 7.1(x86_64-gnu-linux).
>>
>> I appreciate any answer and/or clue to why GDB behaves like this.
>> I have tried to look through the remote debugging source code of GDB, but this has not given me any answers.
>> If needed, I can post debugging information from the target code in GDB.
>>
>
> 0x40 bytes is GDB's stack cache line size (dcache.c).
> Try "(gdb) set stack-cache off".
>
> --
> Pedro Alves
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2010-09-27 8:54 GDB remote debugging stub: Question about memory read Niklas Quarfot Nielsen
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