I wrote a C program in which I did some pretty heavy stack allocation, around 2 MiB. Since I use the poor man's IDE* I was automatically running the program via make in order to test it, each time I compiled.
I had pretty much wrapped everything up, but for some reason, during some of the final optimization, I ran it directly from the shell. Instant segfault! Running it with make still worked, and running it by hand always produced the same segfault.
I eventually reduced the amount of stack allocation I was doing to 256 KiB, which solved the problem. My rationale was that make was probably exec-ing the process, and thus it was inheriting some weird parameters that allowed it to use more stack space.
Although everything is fine now, I have no way of testing my theory. Can anyone confirm or deny, or suggest some way of testing?
* zsh, vim, gcc, gdb, and some nutty makefiles
You can try setting the maximum stack size with ulimit(1) and see if it works:
# Limit stack to 1024 KiB
ulimit -s 1024; ./myprogram
# Now no limit
ulimit -s unlimited; ./myprogram
Personally, my first step would be to try to locate where in the code the segfault occurred, either with gdb or debugging printfs or whatever. (This is why you always check return values from malloc, it cuts down on the possible sources of segfaults ;-p) For one thing, finding the exact source of the problem could give you evidence for or against your theory about stack allocation; also, it'll let you insert error-checking code so the program can exit gracefully with an informative error message rather than segfaulting.
More information is needed to diagnose this. Namely, it'd be nice to see the makefile and the shell scripts which you were using.
Related
I have written a program on C and now I am running it on gcc, with valgrind (the program that detects memory loses).
The thing is, when I run it without valgrind, it works much faster than with valgrind. I have tried it on several inputs and the result is that when the input is pretty high, is even can not end with valgrind, but without it it takes several seconds.
My program has a lot of calls to malloc in it, Can it be related?
Unfortunately I can not post my code, because it is a part of an assignment, and I have to keep it discrete. This assignment will probably be checked with valgrind, so I have to solve it.
A general answer and possible solutions could help very much.
Thanks
This is completely normal. Valgrind emulates your code, keeping trace of allocation, frees, memory access and so on.
From The Valgrind Quick Start Guide:
Your program will run much slower (eg. 20 to 30 times) than normal, and use a lot more memory.
Valgrind is among other thing intercepting calls to malloc and free to gather its statistic. This interception is slowing down the call. There is nothing to do against it.
I think I am experiencing a stack overflow issue when running my unit-tests through gdb within emacs on Windows.
I run exactly the same unit-tests on linux with no problems
I am also aware that I am using a horrendously memory inefficient stack-based .ini file parser within these unit-tests so it would seem to be a reasonable possibility that stack overflow is occuring.
I have noticed a few unit tests faling on Windows that passed on Linux. Further investigation reveals a (stack-based) counter in a for loop resetting itself to zero at random points in the for loops execution and the (stack-based) values in the arrays that the for loop is inspecting change for the same index value
I noticed that gdb seems to be allocated its own thread of execution under Windows - is there any way of finding out how much stack space the thread is allocated?
One of the differences between Linux and Windows is that on Windows the stack-size has to be set at compile-time (there are two sizes, initial and a predefined reserved limit). Not sure what the default is with the compiler you are using, but you might try to increase it using --stack parameter (gcc).
On Linux the stack-size is dynamic up to the limits, generally set by the sysadmin.
Well then, perhaps Windows has tighter restrictions on the maximum amount of stack per process than Linux does?
This page details how to debug a stack overflow in Windows. It's not based on gdb, but perhaps you can still infer something.
I remember reading a long time ago that if I want to test for buffer overflows on my linux box that I need to set something in the system to allow it to happen. I can't remember exactly what it was for, but I was hoping some one knew what I was talking about.
I want to be able to test my programs for vulnerabilities, and see if the registers are overwritten.
EDIT: I am running ubuntu 10.04
One option is to use a memory debugger such as Valgrind. Note, however, that Valgrind only tracks for buffer overflows on dynamically-allocated memory.
If you have the option to use C++ instead of C, then you can switch to using containers rather than raw arrays, and harness GCC's "checked container" mode (see GCC STL bound checking). I'm sure other compilers offer similar tools.
Another hint (in addition of Oli's answer), when chasing memory bugs with the gdb debugger, is to disable address space layout randomization, with e.g.
echo 0 > /proc/sys/kernel/randomize_va_space
After doing that, two consecutive runs of the same deterministic program will usually mmap regions at the same addresses (from one run to another), and this helps a lot debugging with gdb (because then malloc usually gives the same result from one run to another, at the same given location in the run).
You can also use the watch command of gdb. In particular, if in a first run (with ASLR disabled) you figure that the location 0x123456 is changing unexepectedly, you could give gdb the following command in its second run:
watch * (void**) 0x123456
Then gdb will break when this location changes (sadly, it has to be mmap-ed already).
I'm writing a C application which is run across a compute cluster (using condor). I've tried many methods to reveal the offending code but to no avail.
Clues:
On Average when I run the code on 15 machines for 2 days, I get two or three segfaults (signal 11).
When I run the code locally I do not get a segfault. I ran it for nearly 3 weeks on my home machine.
Attempts:
I ran the code in valGrind for four days locally with no memory errors.
I captured the segfault signal by defining my own signal handler so that I can output some of the program state.
Now when a segfault happens I can print out the current stack using backtrace.
I can print out variable values.
I created a variable which is set to the current line number.
Have also tried commenting chunks of the code out, hoping that if the problem goes away I will discover the segfault.
Sadly the line number outputted is fairly random. I'm not entirely sure what I can do with the stacktrace. Am I correct in assuming that it only records the address of the function in which the segfault occurs?
Suspicions:
I suspect that the check pointing system which condor uses to move jobs across machines is more sensitive to memory corruption and this is why I don't see it locally.
That indices are being corrupted by the bug, and that these indices are causing the segfault. This would explain the fact that the segfaults are occurring on fairly random line numbers.
UPDATE
Researching this some more I've found the following links:
LibSegFault - a library for automatically catching and printing state data about segfaults.
Stack unwinding (stack trace) with GCC tutorial on catching segfaults and get the line numbers of the offending instructions.
UPDATE 2
Greg suggested looking at the condor log and to 'correlate the segfaults to when condor restarts the executable from a checkpoint'. Looking at the logs the segfaults all occur immediately after a restart. All of the failures appear to occur when a job switches from one type of machine to another type.
UPDATE 3
The segfault was being caused by differences between hosts, by setting the 'requiremets' field in the condor submit file to problem completely disappeared.
One can set individual machines:
requirements = machine == "hostname1" || machine == "hostname2"
or an entire class of machines:
requirements = classOfMachinesName
See requirements example here
if you can, compile with debugging, and run under gdb.
alternatively, get core dumped and load that into debugger.
mpich has built-in debugger, or you can buy commercial parallel debugger.
Then you can step through the code to see what happening in debugger
http://nmi.cs.wisc.edu/node/1610
http://nmi.cs.wisc.edu/node/1611
Can you create a core dump when your segfault happens? You can then debug this dump to try to figure out the state of the code when it crashed.
Look at what instruction caused the fault. Was it even a valid instruction or are you trying to execute data? If valid, what memory is it trying to access? Where did this pointer come from. You need to narrow down the location of your fault (stack corruption, heap corruption, uninitialized pointer, accessing invalid memory). If it's a corruption, see if if there's any tell-tale data in the corrupted area (pointers to symbols, data that looks like something in your structures, ...). Your memory allocator may already have built in features to debug some corruption (see MALLOC_CHECK_ on Linux or MallocGuardEdges on Mac OS). A common case for these is using memory that has been free()'d, so logging your malloc() / free() pairs might help.
If you have used the condor_compile tool to relink your code with the condor checkpointing code, it does a few things differently than a normal link. Most importantly, it statically links your code, and uses it's own malloc. Another big difference is that condor will then run it on a foreign machine, where the environment may be different enough from what you expect to cause problems.
The executable generated by condor_compile is runnable as a standalone binary outside of the condor system. If you run the binary emitted from condor_compile locally, outside of condor, do you still see the segfaults?
If it doesn't, can you correlate the segfaults to when condor restarts the executable from a checkpoint (the user log will tell you when this happens).
You've tried most of what I'd think of. The only other thing I'd suggest is start adding a lot of logging code and hope you can narrow down where the error is happening.
The one thing you do not say is how much flexibility you have to solve the problem.
Can you, for example, have the system come to a halt and just run your application?
Also how important are these crashes to solve?
I am assuming that for the most part you do. This may require a lot of resources.
The short term step is to put tons of "asserts" ( semi handwritten ) of each variable
to make sure it hasn't changed when you don't want it to. This can ccontinue to work as you go through the long term process.
Long term-- try running it on a cluster of two ( maybe your home computer and a VM ).
Do you still see the segfaults. If not increase the cluster size until you start seeing segfaults.
Run it on a minimum configuration ( to get segfaults ) and record all your inputs till a crash. Automate running the system with the inputs that you recorded, tweaking them until you can consistent get a crash with minimal input.
At that point look around. If you still can't find the bug, then you will have to ask again with some extra data you gathered with those runs.
I have built a plain C code on Linux (Fedora) using code-sorcery tool-chain. This is for ARM Cortex-A8 target. This code is running on a Cortex A8 board, running embedded Linux.
When I run this code for some test case, which does dynamic memory allocation (malloc) for some large size (10MB), it crashes after some time giving error message as below:
select 1 (init), adj 0, size 61, to kill
select 1030 (syslogd), adj 0, size 64, to kill
select 1032 (klogd), adj 0, size 74, to kill
select 1227 (bash), adj 0, size 378, to kill
select 1254 (ppp), adj 0, size 1069, to kill
select 1255 (TheoraDec_Corte), adj 0, size 1159, to kill
send sigkill to 1255 (TheoraDec_Corte), adj 0, size 1159
Program terminated with signal SIGKILL, Killed.
Then, when I debug this code for the same test case using gdb built for the target, the point where this dynamic memory allocation happens, code fails to allocate that memory and malloc returns NULL. But during normal stand-alone run, I believe malloc should be failing to allocate but it strangely might not be returning NULL, but it crashes and the OS kills my process.
Why is this behaviour different when run under gdb and when without debugger?
Why would malloc fails yet not return a NULL. Could this be possible, or the reason for the error message I am getting is else?
How do I fix this?
thanks,
-AD
So, for this part of the question, there is a surefire answer:
Why would malloc fails yet not return a NULL. Could this be possible, or the reason for the error message i am getting is else?
In Linux, by default the kernel interfaces for allocating memory almost never fail outright. Instead, they set up your page table in such a way that on the first access to the memory you asked for, the CPU will generate a page fault, at which point the kernel handles this and looks for physical memory that will be used for that (virtual) page. So, in an out-of-memory situation, you can ask the kernel for memory, it will "succeed", and the first time you try to touch that memory it returned back, this is when the allocation actually fails, killing your process. (Or perhaps some other unfortunate victim. There are some heuristics for that, which I'm not incredibly familiar with. See "oom-killer".)
Some of your other questions, the answers are less clear for me.
Why is this behaviour different when run under gdb and when without debugger?It could be (just a guess really) that GDB has its own malloc, and is tracking your allocations somehow. On a somewhat related point, I've actually frequently found that heap bugs in my code often aren't reproducible under debuggers. This is frustrating and makes me scratch my head, but it's basically something I've pretty much figured one has to live with...
How do i fix this?
This is a bit of a sledgehammer solution (that is, it changes the behavior for all processes rather than just your own, and it's generally not a good idea to have your program alter global state like that), but you can write the string 2 to /proc/sys/vm/overcommit_memory. See this link that I got from a Google search.
Failing that... I'd just make sure you're not allocating more than you expect to.
By definition running under a debugger is different than running standalone. Debuggers can and do hide many of the bugs. If you compile for debugging you can add a fair amount of code, similar to compiling completely unoptimized (allowing you to single step or watch variables for example). Where compiling for release can remove debugging options and remove code that you needed, there are many optimization traps you can fall into. I dont know from your post who is controlling the compile options or what they are.
Unless you plan to deliver the product to be run under the debugger you should do your testing standalone. Ideally do your development without the debugger as well, saves you from having to do everything twice.
It sounds like a bug in your code, slowly re-read your code using new eyes as if you were explaining it to someone, or perhaps actually explain it to someone, line by line. There may be something right there that you cannot see because you have been looking at it the same way for too long. It is amazing how many times and how well that works.
I could also be a compiler bug. Doing things like printing out the return value, or not can cause the compiler to generate different code. Adding another variable and saving the result to that variable can kick the compiler to do something different. Try changing the compiler options, reduce or remove any optimization options, reduce or remove the debugger compiler options, etc.
Is this a proven system or are you developing on new hardware? Try running without any of the caches enabled for example. Working in a debugger and not in standalone, if not a compiler bug can be a timing issue, single stepping flushes the pipline, mixes the cache up differently, gives the cache and memory system an eternity to come up with a result which it doesnt have in real time.
In short there is a very long list of reasons why running under a debugger hides bugs that you cannot find until you test in the final deliverable like environment, I have only touched on a few. Having it work in the debugger and not in standalone is not unexpected, it is simply how the tools work. It is likely your code, the hardware, or your tools based on the description you have given so far.
The fastest way to eliminate it being your code or the tools is to disassemble the section and inspect how the passed values and return values are handled. If the return value is optimized out there is your answer.
Are you compiling for a shared C library or static? Perhaps compile for static...