[lldb-dev] lldb fails to examine any variable with the message - Interpreting the expression locally failed: Interpreter couldn't write to memory
Thirumurthi, Ashok
ashok.thirumurthi at intel.com
Fri Apr 26 14:50:01 PDT 2013
Ø 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?
The behavior may have changed in the last couple of weeks. With trunk, during PrepareToExecuteJITExpression, EntityVariable::Materialize calls AddressOf on the ValueObject for the variable being examined. This fails, and I'm currently trying to understand the failure by comparing the operation with the same expression evaluation just prior to the crash. Thanks for the helpful perspective on this thread. I'll try to have a closer look early next week,
- Ashok
From: Jim Ingham [mailto:jingham at apple.com]
Sent: Thursday, April 25, 2013 7:41 PM
To: Thirumurthi, Ashok
Cc: lldb-dev at cs.uiuc.edu; Greg Clayton
Subject: Re: [lldb-dev] lldb fails to examine any variable with the message - Interpreting the expression locally failed: Interpreter couldn't write to memory
IIRC, gdb can call functions and allocate memory (which it needs to pass strings to functions among other things) and the like on a crashed program on Linux. Been a while since I looked at how gdb works on Linux, but if gdb can do that, lldb should be able to as well.
Jim
On Apr 25, 2013, at 2:43 PM, Greg Clayton <gclayton at apple.com<mailto:gclayton at apple.com>> wrote:
On Apr 25, 2013, at 1:47 PM, "Thirumurthi, Ashok" <ashok.thirumurthi at intel.com<mailto: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<mailto: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<mailto: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>
[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<mailto: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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