[lldb-dev] lldb fails to examine any variable with the message - Interpreting the expression locally failed: Interpreter couldn't write to memory

[Thirumurthi, Ashok]

Greg, Sean,

Is there a way to rework the lldb interpreter to read variables after a crash?  Currently, lldb injects a variable to store the result of expression evaluation.  One alternative is to use ptrace to handle a pure read on Linux...

- Ashok

-----Original Message-----
From: Samuel Jacob [mailto:samueldotj at gmail.com] 
Sent: Wednesday, April 24, 2013 2:51 PM
To: Thirumurthi, Ashok
Cc: lldb-dev at cs.uiuc.edu
Subject: Re: [lldb-dev] lldb fails to examine any variable with the message - Interpreting the expression locally failed: Interpreter couldn't write to memory

http://llvm.org/bugs/show_bug.cgi?id=15784

I am writing frontend using lldb+python and completely blocked on this.
I guess all Linux user/developers would be blocked if they are using trunk.
Somebody please fix this issue.

Thanks
Samuel

On Thu, Apr 18, 2013 at 10:24 AM, Thirumurthi, Ashok <ashok.thirumurthi at intel.com> wrote:
> FYI
>
> -----Original Message-----
> From: Thirumurthi, Ashok
> Sent: Thursday, April 18, 2013 11:37 AM
> To: 'Samuel Jacob'
> Subject: RE: [lldb-dev] lldb fails to examine any variable with the 
> message - Interpreting the expression locally failed: Interpreter 
> couldn't write to memory
>
> Hi Samuel,
>
> Thanks for the test case.  I can reproduce this using trunk with lldb/tests/functionalities/inferior-crashing, and confirmed that there is no existing bug report.  For instance, the issue is distinct from http://llvm.org/bugs/show_bug.cgi?id=15671.
>
> After the crash, the back-trace is correct, and "register read -a" dumps the register set.  The expression parser should generate IR that the LLDB interpreter can use to read from the address of argc in the inferior.  So, there is no design requirement to inject code into the inferior to evaluate the expression.  I suspect that the write is related to a temporary that is used to store the result of the read, but it may be possible to rework the expression interpreter to eliminate the write.
>
> The attached patch modifies the existing test case to reproduce the issue that you've raised (and also checks the register read).  If you don't mind, I'll log a bugzilla for this in order to cross-reference it with the failing test.  I've marked the test as xfail on Darwin as well.  I'll let folks chime in if it's an xpass...
>
> FYI, when run under our test harness, this test fails with 'use of 
> undeclared identifier 'argc''.  I can reproduce this second issue from 
> the command line by issuing 'expr argc' twice, at which point the 
> process is no longer in a limbo state.  Thanks again,
>
> - Ashok
>
> -----Original Message-----
> From: lldb-dev-bounces at cs.uiuc.edu 
> [mailto:lldb-dev-bounces at cs.uiuc.edu] On Behalf Of Samuel Jacob
> Sent: Wednesday, April 17, 2013 11:53 PM
> To: lldb-dev; Malea, Daniel
> Subject: Re: [lldb-dev] lldb fails to examine any variable with the 
> message - Interpreting the expression locally failed: Interpreter 
> couldn't write to memory
>
> Hi Dan,
>
> Can you please check this?
>
> Thanks
> Samuel
>
> On Wed, Apr 17, 2013 at 4:21 PM, Samuel Jacob <samueldotj at gmail.com> wrote:
>> lldb build from trunk running on Ubuntu 12.04 is not able examine any variable.
>>
>> $cat test1.c
>> int main(int argc, char argv[])
>> {
>>     char *crash=0;
>>
>>     *crash = 0;
>>     return 0;
>> }
>>
>> $gcc -O0 -g3 ./test1.c
>>
>> $lldb ~/a.out
>> Current executable set to '/mts/home3/jacobs/a.out' (x86_64).
>>
>> (lldb) run
>> Process 16615 launched: '/mts/home3/jacobs/a.out' (x86_64) Process
>> 16615 stopped
>> * thread #1: tid = 0x40e7, 0x00000000004004cb a.out`main(argc=1,
>> argv=0x00007fff336ad2a8) + 23 at test1.c:5, stop reason = invalid 
>> address
>>     frame #0: 0x00000000004004cb a.out`main(argc=1,
>> argv=0x00007fff336ad2a8) + 23 at test1.c:5
>>    2    {
>>    3        char *crash=0;
>>    4
>> -> 5        *crash = 0;
>>    6        return 0;
>>    7    }
>> (lldb) p argc
>> error: Interpreting the expression locally failed: Interpreter 
>> couldn't write to memory
>>
>> But before crashing If a breakpoint was setup, lldb stops at the 
>> breakpoint and works fine.
>>
>> Is it a known issue or should a file a bug report?
>>
>> Samuel
> _______________________________________________
> lldb-dev mailing list
> lldb-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
> _______________________________________________
> lldb-dev mailing list
> lldb-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>