Inside a KEXT, I need to do some processing where I would get either a proc_t or just a pid.
If I go the pid route, I would do a sysctl() of sorts.
Unfortunately, I can't do either. proc_t is undefined and sysctl() isn't either. sysctlbyname() can be called but kinfo_proc isn't defined. If I try to use proc_t, the compiler complains about forward definition of [struct proc]
I'm assuming that sysctl() is there to be used in user mode but is there any way I can use proc_t?
I tried to use the XNU/osfmk/bsd include dir but then it won't compile because of redefinitions and other errors.
It's a little disconcerting and I'm still trying to wrap my head around what I can and cannot do.
Surely this can be done but I just don't quite know how.
OK, I'm going to try and take a stab at the question I think you're asking.
As you've discovered, a proc_t is a pointer to an opaque struct proc. Don't write it off though, as there are various functions that operate on such pointers, so you don't need to gain direct access to the struct (which helps maintain binary compatibility). Most of these are declared in sys/proc.h in the Kernel.framework - i.e. /System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/proc.h. You mention PID and parent PID, for which there are the following:
/* returns the pid of the given process */
extern int proc_pid(proc_t);
/* returns the pid of the parent of a given process */
extern int proc_ppid(proc_t);
There are also functions for going the other way - getting the proc_t for a PID etc.
Note that these functions are part of the BSD portion of the kernel, so your kext needs to declare a dependency on the BSD KPI bundle in its info.plist. (look up the kextlibs tool if you haven't come across this yet)
Coming from Windows, you'll probably have to get used to reading header files and source codes instead of documentation. Much of the OSX kernel API is undocumented.
Related
I am working with IJVM and trying to use the GOTO instruction using a local variable in place of a static offset (or label). It won't work. I suppose it is simply treating the variable name as a label and trying to branch to it, but no such label exists. Is there any way I can force it to read the contents of the variable (which contains an offset), or some other solution?
Thanks in advance.
For security reasons, JVM bytecode doesn't let you jump to arbitrary instructions based on the contents of a variable. This restriction makes it possible for the JVM to verify various security properties of the bytecode by statically enumerating all control paths through a particular method. If you were able to jump anywhere, the static analyzer couldn't prove that all necessary program invariants held.
If you do need to jump to an arbitrary index, consider looking into the tableswitch or lookupswitch instructions, which would let you enumerate possible destinations in advance. It's not exactly what you're looking for, but to the best of my knowledge the sort of arbitrary jump you're trying to make isn't possible in JVM bytecode.
Hope this helps!
The GOTO instruction is implemented in MIC1. It interprets the 2 bytes after the opcode as an offset to the PC at the start of the instruction.
I think that the assignment must be asking you to write a new GOTO in MIC1 that interprets the byte after the opcode as the offset to a local variable that contains the branch offset.
My questions are:
Is there a way to conclusively determine if a function is async-signal-safe if you don't have access to its implementation?
If not, is there a way to test if function would be async-signal-safe enough to call from a signal handler?
If you reads the man pages of signal() or sigaction(), you get a list of async-signal-safe functions (functions that can be safely called inside a signal handler). However, I believe that this list is not exhaustive. For example, the following page http://linux.die.net/man/7/signal, under the Async-signal-safe functions header, reads:
POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2) requires an implementation to guarantee that the following functions can be safely called inside a signal handler:
And then it proceeds to list the normal async-signal-safe functions listed in the man pages above. As I read it, it says "it requires", not "these are the only ones".
For example, this site says that back_trace_symbols_fd() is async-signal safe. That function obtains is data from dladdr() and it doesn't use malloc() like back_trace_symbols(), so it looks like it may be safe. Also, I did some testing, and the output struct of dladdr() contains char* variables, but these are NOT malloc'ed at runtime. The char string they point to exists at run-time even before dladdr() is called.
Any thoughts or ideas that can point me in the right direction are appreciated.
If you don't have access to the function's implementation, you can look at the manual page. If the manual page doesn't say it is async-safe, and the POSIX standard doesn't say it is async-safe, the only safe conclusion is "it is not async-safe" (coupled with "do not use it").
There is no 100% reliable way to test whether a function is async-safe. Remember, testing can only show the presence of bugs, not their absence (Dijkstra). The mere fact that you don't manage to tickle the function into misbehaving under test may simply mean that your testing is not adequate (but rest assured, the important customer who you can't afford to offend will immediately and accidentally devise a devastatingly effective test that demonstrates that the function is not async-safe almost as soon as you release the code with the faulty assumption).
What are you hoping to achieve in the signal handler? You should consider whether it is the right place for it. It is probably best to follow the advice of the man page:
In general, signal handlers should do little more
than set a flag; most other actions are not safe.
So let's say I have a C API that looks like this:
// configure various parameters
int set_option(const char* name, const char* value);
// callback invoked during long running operation
typedef int (*callback_t)(void* whatever);
// start a long running operation
int some_long_operation(callback_t callback);
These functions are documented as not being reentrant. Should I attempt to enforce this (e.g. by setting a flag somewhere and returning an appropriate error code) or should I just write if off as undefined behavior? Most of the code I've seen does not attempt to do this; I'm just wondering if that's a conscious decision or if it's just pragmatism (or perhaps laziness). It seems to me that if you wanted to write really robust code you might try something like this; on the other hand, if you're not following the documentation then maybe all bets should be off.
As a member of a team responsible for maintaining a SDK, my experience has been scary at times. Granted our documentation is not great, but I have seen some pretty strange things in OEM support requests. A good number of those stem from calling functions in the context of a callback invoked by another function. Most of these are documented as being not supported, but we don't have any code in place to prevent it; in fact, most instances of this will appear to work but some may actually trash our internal state. It just seems like sometimes it might be easier to enforce proper API usage rather than rely on people to read the documentation.
I could not find any good document on internet about STM32 programming. STM's own documents do not explain anything more than register functions. I will greatly appreciate if anyone can explain my following questions?
I noticed that in all example programs that STM provides, local variables for main() are always defined outside of the main() function (with occasional use of static keyword). Is there any reason for that? Should I follow a similar practice? Should I avoid using local variables inside the main?
I have a gloabal variable which is updated within the clock interrupt handle. I am using the same variable inside another function as a loop condition. Don't I need to access this variable using some form of atomic read operation? How can I know that a clock interrupt does not change its value in the middle of the function execution? Should I need to cancel clock interrupt everytime I need to use this variable inside a function? (However, this seems extremely ineffective to me as I use it as loop condition. I believe there should be better ways of doing it).
Keil automatically inserts a startup code which is written in assembly (i.e. startup_stm32f4xx.s). This startup code has the following import statements:
IMPORT SystemInit
IMPORT __main
.In "C", it makes sense. However, in C++ both main and system_init have different names (e.g. _int_main__void). How can this startup code can still work in C++ even without using "extern "C" " (I tried and it worked). How can the c++ linker (armcc --cpp) can associate these statements with the correct functions?
you can use local or global variables, using local in embedded systems has a risk of your stack colliding with your data. with globals you dont have that problem. but this is true no matter where you are, embedded microcontroller, desktop, etc.
I would make a copy of the global in the foreground task that uses it.
unsigned int myglobal;
void fun ( void )
{
unsigned int myg;
myg=myglobal;
and then only use myg for the rest of the function. Basically you are taking a snapshot and using the snapshot. You would want to do the same thing if you are reading a register, if you want to do multiple things based on a sample of something take one sample of it and make decisions on that one sample, otherwise the item can change between samples. If you are using one global to communicate back and forth to the interrupt handler, well I would use two variables one foreground to interrupt, the other interrupt to foreground. yes, there are times where you need to carefully manage a shared resource like that, normally it has to do with times where you need to do more than one thing, for example if you had several items that all need to change as a group before the handler can see them change then you need to disable the interrupt handler until all the items have changed. here again there is nothing special about embedded microcontrollers this is all basic stuff you would see on a desktop system with a full blown operating system.
Keil knows what they are doing if they support C++ then from a system level they have this worked out. I dont use Keil I use gcc and llvm for microcontrollers like this one.
Edit:
Here is an example of what I am talking about
https://github.com/dwelch67/stm32vld/tree/master/stm32f4d/blinker05
stm32 using timer based interrupts, the interrupt handler modifies a variable shared with the foreground task. The foreground task takes a single snapshot of the shared variable (per loop) and if need be uses the snapshot more than once in the loop rather than the shared variable which can change. This is C not C++ I understand that, and I am using gcc and llvm not Keil. (note llvm has known problems optimizing tight while loops, very old bug, dont know why they have no interest in fixing it, llvm works for this example).
Question 1: Local variables
The sample code provided by ST is not particularly efficient or elegant. It gets the job done, but sometimes there are no good reasons for the things they do.
In general, you use always want your variables to have the smallest scope possible. If you only use a variable in one function, define it inside that function. Add the "static" keyword to local variables if and only if you need them to retain their value after the function is done.
In some embedded environments, like the PIC18 architecture with the C18 compiler, local variables are much more expensive (more program space, slower execution time) than global. On the Cortex M3, that is not true, so you should feel free to use local variables. Check the assembly listing and see for yourself.
Question 2: Sharing variables between interrupts and the main loop
People have written entire chapters explaining the answers to this group of questions. Whenever you share a variable between the main loop and an interrupt, you should definitely use the volatile keywords on it. Variables of 32 or fewer bits can be accessed atomically (unless they are misaligned).
If you need to access a larger variable, or two variables at the same time from the main loop, then you will have to disable the clock interrupt while you are accessing the variables. If your interrupt does not require precise timing, this will not be a problem. When you re-enable the interrupt, it will automatically fire if it needs to.
Question 3: main function in C++
I'm not sure. You can use arm-none-eabi-nm (or whatever nm is called in your toolchain) on your object file to see what symbol name the C++ compiler assigns to main(). I would bet that C++ compilers refrain from mangling the main function for this exact reason, but I'm not sure.
STM's sample code is not an exemplar of good coding practice, it is merely intended to exemplify use of their standard peripheral library (assuming those are the examples you are talking about). In some cases it may be that variables are declared external to main() because they are accessed from an interrupt context (shared memory). There is also perhaps a possibility that it was done that way merely to allow the variables to be watched in the debugger from any context; but that is not a reason to copy the technique. My opinion of STM's example code is that it is generally pretty poor even as example code, let alone from a software engineering point of view.
In this case your clock interrupt variable is atomic so long as it is 32bit or less so long as you are not using read-modify-write semantics with multiple writers. You can safely have one writer, and multiple readers regardless. This is true for this particular platform, but not necessarily universally; the answer may be different for 8 or 16 bit systems, or for multi-core systems for example. The variable should be declared volatile in any case.
I am using C++ on STM32 with Keil, and there is no problem. I am not sure why you think that the C++ entry points are different, they are not here (Keil ARM-MDK v4.22a). The start-up code calls SystemInit() which initialises the PLL and memory timing for example, then calls __main() which performs global static initialisation then calls C++ constructors for global static objects before calling main(). If in doubt, step through the code in the debugger. It is important to note that __main() is not the main() function you write for your application, it is a wrapper with different behaviour for C and C++, but which ultimately calls your main() function.
How does a virtual machine generate native machine code on the fly and execute it?
Assuming you can figure out what are the native machine op-codes you want to emit, how do you go about actually running it?
Is it something as hacky as mapping the mnemonic instructions to binary codes, stuffing it into an char* pointer and casting it as a function and executing?
Or would you generate a temporary shared library (.dll or .so or whatever) and load it into memory using standard functions like LoadLibrary ?
You can just make the program counter point to the code you want to execute. Remember that data can be data or code. On x86 the program counter is the EIP register. The IP part of EIP stands for instruction pointer. The JMP instruction is called to jump to an address. After the jump EIP will contain this address.
Is it something as hacky as mapping the mnemonic instructions to binary codes, stuffing it into an char* pointer and casting it as a function and executing?
Yes. This is one way of doing it. The resulting code would be cast to a pointer to function in C.
Is it something as hacky as mapping the mnemonic instructions to binary codes, stuffing it into an char* pointer and casting it as a function and executing?
Yes, if you were doing it in C or C++ (or something similar), that's exactly what you'd do.
It appears hacky, but that's actually an artifact of the language design. Remember, the actual algorithm you want to use is very simple: determine what instructions you want to use, load them into a buffer in memory, and jump to the beginning of that buffer.
If you really try to do this, though, make sure you get the calling convention right when you return to your C program. I think if I wanted to generate code I'd look for a library to take care of that aspect for me. Nanojit's been in the news recently; you could look at that.
Yup. You just build up a char* and execute it. However, you need to note a couple details. The char* must be in an executable section of memory and must have proper alignment.
In addition to nanojit you can also check out LLVM which is another library that's capable of compiling various program representations down to a function pointer. It's interface is clean and the generated code tends to be efficient.
As far as i know it compiles everything in memory because it has to run some heuristics to to optimize the code (i.e.: inlining over time) but you can have a look at the Shared Source Common Language Infrastructure 2.0 rotor release. The whole codebase is identical to .NET except for the Jitter and the GC.
As well as Rotor 2.0 - you could also take a look at the HotSpot virtual machine in the OpenJDK.
About generating a DLL: the additional required I/O for that, plus linking, plus the complexity of generating the DLL format, would make that much more complicate, and above all they'd kill performance; additionally, in the end you still call a function pointer to the loaded code, so...
Also, JIT compilation can happen one method at a time, and if you want to do that you'd generate lots of small DLLs.
About the "executable section" requirement, calling mprotect() on POSIX systems can fix the permissions (there's a similar API on Win32). You need to do that for a big memory segment instead that once per method since it'd be too slow otherwise.
On plain x86 you wouldn't notice the problem, on x86 with PAE or 64bit AMD64/Intel 64 bit machines you'd get a segfault.
Is it something as hacky as mapping
the mnemonic instructions to binary
codes, stuffing it into an char*
pointer and casting it as a function
and executing?
Yes, that works.
To do this in windows you must set PAGE_EXECUTE_READWRITE to the allocated block:
void (*MyFunc)() = (void (*)()) VirtualAlloc(NULL, sizeofblock, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
//Now fill up the block with executable code and issue-
MyFunc();