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

[Greg Clayton]

On Apr 25, 2013, at 1:47 PM, "Thirumurthi, Ashok" <ashok.thirumurthi at intel.com> wrote:

> Greg, Sean,
> 
> Is there a way to rework the lldb interpreter to read variables after a crash?  

Is this a problem because the expression parser can't allocate memory after you have crashed?

LLDB does like to place a copy of the result in the program memory, but it doesn't have to. We could change that.

> Currently, lldb injects a variable to store the result of expression evaluation.  

Do you mean injects memory into the inferior?

> One alternative is to use ptrace to handle a pure read on Linux...

The IR interpreter will not run code for anything that it can handle (like memory and register reads). It might be that we are missing a common IR opcode that linux uses which forced JIT'ed code to run more often?

To verify: is your question regarding that fact that we can't allocate memory when crashed? Can't run code when crashed? What is the real issue?

Greg

> 
> - 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
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