I am using the Xilinx SDK to debug an application running on a CORA-Z7-10 microcontroller.
When I use the debugger memory view sometimes the view shows the memory as expected, while sometimes it shows a bunch of question marks instead of the actual memory contents.
I know that in that areas of memory there is a binary file and I don't understand why the view shows a bunch of question mark instead. What does these questions marks represent? Why are they there?
To be more clear let's make an example: I put my binaries for the execution at addresses 0x00100000, 0x04000000, 0x08000000 (all in the same DDR3 memory address space). When I stop the execution, let's say at address 0x00101550 (which it is in the first binary), I cannot see memory at 0x04000000 but I can see memory at 0x08000000.
The disassembly view give me an "Memory read error at 0x04000000. Memory read aborted. Fault status 0x8, Domain 0x0". Any clue?
I'm trying to find current flag count in KMines by using gdb. I know that I should look for memory mappings first to avoid non-existent memory locations. So I ran info proc mappings command to see the memory segments. I picked up a random memory gap (0xd27000-0x168b000) from the result and executed the find command like this: find 0x00d27000, 0x0168b000, 10
But I got the warning: Unable to access 1458 bytes of target memory at 0x168aa4f, halting search. error. Although the address 0x168aa4f is between 0xd27000 and 0x168b000, gdb says that it can't access to it. Why does this happen? What can I do to avoid this situation? Or is there a way to ignore unmapped/unaccessible memory locations?
Edit: I tried to set the value of the address 0x168aa4f to 1 and it works, so gdb can actually access that address but gives error when used with the find command. But why?
I guess I have solved my own problem, I can't believe how simple the solution was. The only thing I did was to decrease the 2nd parameter's value by one. So the code should be find 0x00d27000, 0x0168afff, 10 because linux allocates the memory by using maps in [x,y) format, so if the line in root/proc/pid/maps says something like this;
01a03000-0222a000 rw-p
The memory allocated includes 0x01a03000 but not 0x0222a000. Hope this silly mistake of mine helps someone :D
Edit: The root of the problem is the algorithm implemented in target.c(gdb's source code I mean) the algorithm reads and searches the memory as chunks at the size of 16000 bytes. So even if the last byte of the chunk is invalid, gdb will throw the entire chunk into the trash and won't even give any proper information about the invalid byte, it only reports the beginning of the current chunk.
I've got a rather complicated program that does a lot of memory allocation, and today by surprise it started segfaulting in a weird way that gdb couldn't pin-point the location of. Suspecting memory corruption somewhere, I linked it against Electric Fence, but I am baffled as to what it is telling me:
ElectricFence Exiting: mprotect() failed:
Program received signal SIGSEGV, Segmentation fault.
__strlen_sse2 () at ../sysdeps/i386/i686/multiarch/strlen.S:99
99 ../sysdeps/i386/i686/multiarch/strlen.S: No such file or directory.
in ../sysdeps/i386/i686/multiarch/strlen.S
#0 __strlen_sse2 () at ../sysdeps/i386/i686/multiarch/strlen.S:99
#1 0xb7fd6f2d in ?? () from /usr/lib/libefence.so.0
#2 0xb7fd6fc2 in EF_Exit () from /usr/lib/libefence.so.0
#3 0xb7fd6b48 in ?? () from /usr/lib/libefence.so.0
#4 0xb7fd66c9 in memalign () from /usr/lib/libefence.so.0
#5 0xb7fd68ed in malloc () from /usr/lib/libefence.so.0
#6 <and above are frames in my program>
I'm calling malloc with a value of 36, so I'm pretty sure that shouldn't be a problem.
What I don't understand is how it is even possible that I could be trashing the heap in malloc. In reading the manual page a bit more, it appears that maybe I am writing to a free page, or maybe I'm underwriting a buffer. So, I have tried the following environment variables, together and by themselves:
EF_PROTECT_FREE=1
EF_PROTECT_BELOW=1
EF_ALIGNMENT=64
EF_ALIGNMENT=4096
The last two had absolutely no effect.
The first one changed the portions of the stack frame which are in my program (where in my program was executing when malloc was called fatally), but with identical frames once malloc was entered.
The second one changed a bit more; in addition to the crash occurring at a different place in my program, it also occurred in a call to realloc instead of malloc, although realloc is directly calling malloc and otherwise the back trace is identical to above.
I'm not explicitly linking against any other libraries besides fence.
Update: I found several places where it suggests that the message: " mprotect() failed: Cannot allocate memory" means that there is not enough memory on the machine. But I am not seeing the "Cannot allocate memory" part, and ps says I am only using 15% of memory. With such a small allocation (4k+32) could this really be the problem?
I just wasted several hours on the same problem.
It turns out that it is to do with the setting in
/proc/sys/vm/max_map_count
From the kernel documentation:
"This file contains the maximum number of memory map areas a process may have. Memory map areas are used as a side-effect of calling malloc, directly by mmap and mprotect, and also when loading shared libraries.
While most applications need less than a thousand maps, certain programs, particularly malloc debuggers, may consume lots of them, e.g., up to one or two maps per allocation."
So you can 'cat' that file to see what it is set to, and then you can 'echo' a bigger number into it. Like this: echo 165535 > /proc/sys/vm/max_map_count
For me, this allowed electric fence to get past where it was before, and start to find real bugs.
In Windows, stack is implemented as followed: a specified page is followed committed stack pages. It's protection flag is as guarded. So when thead references an address on the guared page, an memory fault rises which makes memory manager commits the guarded page to the stack and clean the page's guarded flag, then it reserves a new page as guarded.
when I allocate an buffer which size is more than one page(4KB), however, an expected error haven't happen. Why?
Excellent question (+1).
There's a trick, and few people know about it (besides driver writers).
When you allocate large buffer on the stack - the compiler automatically adds so-called stack probes. It's an extra code (implemented in CRT usually), which probes the allocated region, page-by-page, in the needed order.
EDIT:
The function is _chkstk.
The fault doesn't reach your program - it is handled by the operating system. Similar thing happens when your program tries to read memory that happens to be written into the swap file - a trap occurs and the operating system unswaps the page and your program continues.
About the application
It runs on Windows XP Professional SP2.
It's built with Microsoft Visual C++ 6.0 with Service Pack 6.
It's MFC based.
It uses several external dlls (e.g. Xerces, ZLib or ACE).
It has high performance requirements.
It does a lot of network and hard disk I/O, but it's also cpu intensive.
It has an exception handling mechanism which generates a minidump when an unhandled exception occurs.
UPDATE: It is a highly multithreaded application and we are using mutexes to protect concurrent access (of course, we might be failing at some place...)
Facts about the crash
It only happens on multiprocessor/multicore machines and under heavy loads of work.
It happens at random (neither we nor our client have found a pattern yet) after some some hours running.
We cannot reproduce the crash on our testing lab. It only happens on some production systems (but always in multicore machines)
It always ends up crashing at the same point, although the complete stack is not always the same. Let me add the stack of the crashing thread (obtained using WinDbg, sorry we don't have symbols)
Exception code: c0000005 ACCESS_VIOLATION
Address : 006a85b9
Access Type : write
Access Address : 2e020fff
Fault address: 006a85b9 01:002a75b9 C:\MyDir\MyApplication.exe
ChildEBP RetAddr Args to Child
WARNING: Stack unwind information not available. Following frames may be wrong.
030af6c8 7c9206eb 77bfc3c9 01a80000 00224bc3 MyApplication+0x2a85b9
030af960 7c91e9c0 7c92901b 00000ab4 00000000 ntdll!RtlAllocateHeap+0xeac (FPO: [Non-Fpo])
030af98c 7c9205c8 00000001 00000000 00000000 ntdll!ZwWaitForSingleObject+0xc (FPO: [3,0,0])
030af9c0 7c920551 01a80898 7c92056d 313adfb0 ntdll!RtlpFreeToHeapLookaside+0x22 (FPO: [2,0,4])
030afa8c 4ba3ae96 000307da 00130005 00040012 ntdll!RtlFreeHeap+0x1e9 (FPO: [Non-Fpo])
030afacc 77bfc2e3 0214e384 3087c8d8 02151030 0x4ba3ae96
030afb00 7c91e306 7c80bfc1 00000948 00000001 msvcrt!free+0xc8 (FPO: [Non-Fpo])
030afb20 0042965b 030afcc0 0214d780 02151218 ntdll!ZwReleaseSemaphore+0xc (FPO: [3,0,0])
030afb7c 7c9206eb 02e6c471 02ea0000 00000008 MyApplication+0x2965b
030afe60 7c9205c8 02151248 030aff38 7c920551 ntdll!RtlAllocateHeap+0xeac (FPO: [Non-Fpo])
030afe74 7c92056d 0210bfb8 02151250 02151250 ntdll!RtlpFreeToHeapLookaside+0x22 (FPO: [2,0,4])
030aff38 77bfc2de 01a80000 00000000 77bfc2e3 ntdll!RtlFreeHeap+0x647 (FPO: [Non-Fpo])
7c92056d c5ffffff ce7c94be ff7c94be 00ffffff msvcrt!free+0xc3 (FPO: [Non-Fpo])
7c920575 ff7c94be 00ffffff 12000000 907c94be 0xc5ffffff
7c920579 00ffffff 12000000 907c94be 90909090 0xff7c94be
*** WARNING: Unable to verify checksum for xerces-c_2_7.dll
*** ERROR: Symbol file could not be found. Defaulted to export symbols for xerces-c_2_7.dll -
7c92057d 12000000 907c94be 90909090 8b55ff8b MyApplication+0xbfffff
7c920581 907c94be 90909090 8b55ff8b 08458bec xerces_c_2_7
7c920585 90909090 8b55ff8b 08458bec 04408b66 0x907c94be
7c920589 8b55ff8b 08458bec 04408b66 0004c25d 0x90909090
7c92058d 08458bec 04408b66 0004c25d 90909090 0x8b55ff8b
The address MyApplication+0x2a85b9 corresponds to a call to erase() of a std::list.
What I have tried so far
Reviewing all the code related to the point where the crash ends happening.
Trying to enable pageheap on our testing lab though nothing useful has been found by now.
We have substituted the std::list for a C array and then it crashes in other part of the code (although it is related code, it's not in the code where the old list resided). Coincidentally, now it crashes in another erase, though this time of a std::multiset. Let me copy the stack contained in the dump:
ntdll.dll!_RtlpCoalesceFreeBlocks#16() + 0x124e bytes
ntdll.dll!_RtlFreeHeap#12() + 0x91f bytes
msvcrt.dll!_free() + 0xc3 bytes
MyApplication.exe!006a4fda()
[Frames below may be incorrect and/or missing, no symbols loaded for MyApplication.exe]
MyApplication.exe!0069f305()
ntdll.dll!_NtFreeVirtualMemory#16() + 0xc bytes
ntdll.dll!_RtlpSecMemFreeVirtualMemory#16() + 0x1b bytes
ntdll.dll!_ZwWaitForSingleObject#12() + 0xc bytes
ntdll.dll!_RtlpFreeToHeapLookaside#8() + 0x26 bytes
ntdll.dll!_RtlFreeHeap#12() + 0x114 bytes
msvcrt.dll!_free() + 0xc3 bytes
c5ffffff()
(12-Apr-2010) I've tried to enable heap free checking (using gflags) but it slows down the application a lot...
Possible solutions (that I'm aware of) which cannot be applied
"Migrate the application to a newer compiler": We are working on this but It's not a solution at the moment.
"Enable pageheap (normal or full)": We can't enable pageheap on production machines as this affects performance heavily.
I think that's all I remember now, if I have forgotten something I'll add it asap. If you can give me some hint or propose some possible solution, don't hesitate to answer!
You can try peppering your code with calls to the debug heap checking routines to see if you can locate the corruption closer to the source (you're using the debug CRT to track down this problem, right?):
http://msdn.microsoft.com/en-us/library/aa271695(VS.60).aspx
Use Application Verifier from debugging tools for windows. Sometimes it helps.
Try to set up VS to download OS debug symbols and make sure that OMIT FRAME POINTERS is off in your application. Perhaps stack trace will be informative.
Highly multithreaded
Long time ago I discovered that there is a limit for thread count per process in WinXP. My test snippet could create only few thoursands of thread. The problem was resolved by thread pool.
EDIT:
For my purposes there was enough just to check “Application Verifier” checkbox in gflags.exe. Unfortunately, I have no experience with other options.
As for thread limit, test snippet was simple:
unsigned __stdcall ThreadProc(LPVOID)
{
_tprintf(_T("Thread started\n"));
return 0;
}
int _tmain(int argc, _TCHAR* argv[])
{
while (TRUE)
{
unsigned threadId = 0;
_tprintf(_T("Start thread\n"));
_beginthreadex( NULL, 0, &ThreadProc, NULL, 0, &threadId);
}
return 0;
}
I didn’t wait long this time, but handle count in Task Manager was increasing very fast. My real world application got this effect only in 12 hours. But must say the issue was not in crashing, new threads just not created.
Can you post what exceptions you are getting?
If this is some memory corruption bug, then the crash occurs sometime after the memory corruption, so that will be challenging to track down the root cause. You should:
Travel (or remotely logon) to the production system, install Visual Studio, have .pdb and .map files ready (and windows' symbols as well), attach debugger to the release-build and wait for the crash. Though if you set it up correctly, you can use the minidump file on your dev machine, where you would already have your app and window's symbols setup. Then you can see which free call is throwing, and try to figure out which object is being freed to see if that object is corrupted somehow and nearby objects in memory.
Somehow find a way to reproduce the bug in your office, can you create high enough volumes to duplicate what the customer is doing?
Your posted callstacks don't look particularly illuminating.
Since you are using VS 6 with SP6, then its STL is OK.
Can you tell if the app on the production system is leaking any resources? Running perfmon can help with this.
Another thing, you're not calling new/delete like very frequently from different threads are you? I've found that if you do this fast enough, you'll crash your app rather quickly (did this on XP). I had to replace new/delete calls in my app with VirtualAlloc (windows Virtual Memory API), that worked great for me. Of course, STL could be allocating from the heap as well.
Use a performance profiler that can hook into CPU events, such as VTune. Set it up in sampling mode and tell it to wait for events related to cache line sharing. These are identified by a HITM event from the SNOOP phase.
If you run this on a multi processor machine with a realistic workload then it will find places in your code where there is active contention between threads for a single piece of data. You will need to analyze the profiler hot spots found this way and try to find something that is not being wrapped in an appropriate mutex.
I'm not an expert on CPU architecture or anything, but my understanding is that when the CPUs are about to access a piece of data the system will check if any other CPUs are accessing the same piece of data, this is done by watching the memory fetches and writes coming out of each CPU, a process called snooping. Snooping makes sure that if TWO or more CPUs have the same data in each of their caches that the duplicated copies of the data are removed when one of them is modified. A HIT-Modified event means that the system detected this situation and had to flush one of the CPUs cache lines.
See this document for more information on using VTune like this
http://software.intel.com/en-us/articles/using-intel-vtune-performance-analyzer-events-ratios-optimizing-applications/
I don't have a copy of VTune in front of me right now so maybe this won't work but it seems like the lowest impact way of getting some data. VTune in sampling mode should not cause a lot of problems with performance.
The key here is that this only happens on multiprocessor machines (Cores are the same as processors)
What happens when a threaded program runs on a single processor is that two threads never execute at the same time. The OS has to time-slice each processor to simulate threads.
In a multiprocessor system multiple threads can operate at the same time.
You are probably accessing shared resources from different threads at the same time now.
These resources can be be connections to external systems and even global variables and data structures even Singleton classes.
Unfortunately you now have one of the hardest problems to find.
If you can find the memory being corrupted then you need to find who else is using it on a different thread and then synchronize the memory (Semaphore or CriticalSection).
Unfortunately there is no easy way to find the problem.
You might be able to set the processor affinity temporarily to only run on one processor until you find the problem. See link
http://msdn.microsoft.com/en-us/library/ms684251(VS.85).aspx
Here is a method to set affinity on
For Windows XP/Vista/7, access Affinity by opening the Windows Task Manager (CTL+ALT+DEL, or right-click on Task Bar), select "Processes" tab, right-click the application process you wish to isolate, then select "Set Affinity." Inside the Processor Affinity dialog, un-check the CPU/cores you do not need to use. This effectively isolates that application to the selected CPUs/cores preventing cashe spanning and reducing process-switching and simplifies your ability to supervise CPU/core allocation for multiple programs.
As your second stack trace shows, your application is corrupting the heap. The header of a heap block is written over and thus the crash occurs in the heap manager when coalescing free blocks, or when going through the free list (in the first stack trace).
The code you identified that is currently freeing memory may be a victim of another code overflowing or underflowing a memory block.
The easiest way to debug this kind of crash is to use the debugging help from windows, through pageheap or appverifier, but depending on the application it may slow down too much, or grow the memory usage too high to be usable, which seems to be the case. You may try to use light pageheap, which will have less impact.
You need to identify what part of the application is overflowing. One way to do this is to look at the information contained in the overflown block. If you have a crash in RtlpCoalesceFreeBlocks, I think I remember one of the registers (#esi) is pointing to the start of the corrupted block (I am not on a windows system at the time of this writing and can not check that). Or if you have a dump, using windbg command !heap -a will dump all memory and display corrupted blocks (better log into a file, since the full heap listing can be long). Once corrupted blocks are known, their content may help to identify the code.
Another help can be to enable the stack backtraces (using gflags). This can be done in production as it is lighter than pageheap. It will add some information to heap blocks and may move the crash to another place in your application, but the stack traces will help to identify what code allocated the blocks that are overflowing.
I would focus on getting the issue to happen on a build for which you have proper debugging symbols, at least for your main application. You seem to gloss over this with "sorry we don't have symbols", but when symbols are applied, the stacktraces may show you more information.
What exactly does this mean: "We can't generate symbols because we're linking with a library which doesn't link if we're using them."? This seems odd.