kala-tamin Why does DTrace give me invalid-address errors sometimes but not always?


Why does DTrace give me invalid-address errors sometimes but not always?



My program:

typedef struct objc_class { 	struct objc_class *isa; 	struct objc_class *super_class; 	char *name; 	long version; 	long info; 	long instance_size; 	void *ivars; 	void *methodLists; 	void *cache; 	void *protocols; } *Class; struct objc_object { 	Class isa; };  /* Code to extract the class name from arg0 based on a snippet by Bill Bumgarner: http://friday.com/bbum/2008/01/26/objective-c-printing-class-name-from-dtrace/ */  objc$target:NSObject:-init:entry { 	printf("time: %llu\n", timestamp); 	printf("arg0: %p\n", arg0); 	obj = (struct objc_object *)copyin(arg0, sizeof(struct objc_object)); 	printf("obj: %p\n", obj); 	printf("obj->isa: %p\n", obj->isa); 	isa = (Class)copyin((user_addr_t)obj->isa, sizeof(struct objc_class)); 	printf("isa: %p\n", obj->isa); 	classname = copyinstr((user_addr_t)(isa->name)); 	printf("classname: %s\n", classname); } 

Some output:

dtrace: script 'test.d' matched 1 probe dtrace: error on enabled probe ID 1 (ID 61630: objc5936:NSObject:-init:entry): invalid address (0x90206b98) in action #8 at DIF offset 28 dtrace: error on enabled probe ID 1 (ID 61630: objc5936:NSObject:-init:entry): invalid address (0x90206b98) in action #8 at DIF offset 28 dtrace: error on enabled probe ID 1 (ID 61630: objc5936:NSObject:-init:entry): invalid address (0x90206b98) in action #8 at DIF offset 28 CPU     ID                    FUNCTION:NAME   0  61630                      -init:entry time: 28391086668386 arg0: 1291ae10 obj: 6f0a1158 obj->isa: a023f360 isa: a023f360 classname: NSBitmapImageRep    1  61630                      -init:entry time: 28391586872297 arg0: 12943560 obj: 6f4a1158 obj->isa: 2fca0 isa: 2fca0 classname: GrowlApplicationTicket    1  61630                      -init:entry time: 28391586897807 arg0: 152060 obj: 6f4a1280 obj->isa: 2fe20 isa: 2fe20 classname: GrowlNotificationTicket    2  61630                      -init:entry time: 28391079142905 arg0: 129482d0 obj: 700a1128 obj->isa: a0014140 isa: a0014140 classname: NSDistributedObjectsStatistics    2  61630                      -init:entry time: 28391079252640 arg0: 147840 obj: 700a1250 obj->isa: a0014780 isa: a0014780 classname: NSDistantObjectTableEntry 

Why the errors? It seems to be the class name (that's the only %s, and I don't get any errors if I remove it), but why does it think some classes' names are invalid pointers?

Is there any way to get the error messages to actually tell me which line of my DTrace program caused a problem?

Is there a way to call object_getClassName instead of doing this structure-inspection dance?

For what it's worth, the program I'm tracing works fine—it's not crashing, so I don't believe that the classes really are broken.


NSTimeInterval to readable NSNumber

1:

Need a background process. Thread?
Colin is pretty close to correct.. Copy/Paste Not Working in Modal Window See:. Cocoa NSArray/NSSet: -makeObjectsPerformSelector: vs. fast enumeration http://www.friday.com/bbum/2008/01/03/objective-c-using-dtrace-to-trace-messages-to-nil/. What kind of odd behavior can occur during a memory leak on the iPhone? More likely than not, you need to set the DYLD_SHARED_REGION environment variable to avoid. Instruments: checking for memory leaks inquiry dtrace only really works against mapped memory this is actually resident in physical memory.. What are the main differences between Cocoa and CocoaTouch? You must figure out what is missing by using the vmmap command line tool.. Propagated delete in code or database? Do a vmmap PID on your application after the above failure messages are generated. Looking at the output, see what region the addresses like 0x90206b98 fall into. Given this address, it is likely in a non-writeable shared chunk of memory this probably isn't resident and, thus, dtrace can't read from it..

2:

This error happens when copyin / copyinstr is used on a page that's not faulted in yet. A common workaround is to let the function use the data in question, and then copyin[str] in a :::return clause. For example:.
syscall::open:entry {     self->filename = arg0;  /* Hang on to the file name pointer.  */ }  syscall::open:return /self->filename/ {     @files[copyinstr(self->filename)] = count();     self->filename = 0; }  END {     trunc(@files, 5); } 

3:

I haven't entirely tracked this down myself. It's possible this DTrace is endeavor to resolve any Objective-C symbols. Although DTrace is a dynamic tracing facility it doesn't mesh well with Objective-C dynamically loading things at runtime. When Objective-C does load new classes,etc DTrace has to resolve this and it takes a little time, especially when your app is just starting up. Even if it does receive things loaded, and your objc app is still loading new classes onto the objc runtime its possible DTrace could receive screwed up and print methods in the wrong rule (if you care around seeing the correct rule methods are being executed in), print incorrect timing results,etc..

4:

This is my best guess based on the information provided.. DTrace was purposefully designed in such a way as to make DTrace scripts as deterministic as possible. This is why there are no if statements, loops, subroutines (another than the pseudo-subroutines provided by DTrace itself), etc. This is for the reason this the code in your DTrace script is running in kernel mode, not user-land as part of the process(es) being traced. In general, the information DTrace has access to is "read-only" (like most generalizations, this is not strictly true), being able to twiddle bits in programs, or the kernel, with any thing as powerful as DTrace must cause things to go very, very wrong, very very quickly.. Dollars to donuts, the problem you're having is for the reason this the page this the pointer points to is not mapped in to core by the VM system. DTrace must only examine information for memory this is in core- it can't double-fault to receive the VM system to load in the page.. You must probably guidance alleviate the problem if you've got an idea of what the classes "should" be and forcing the pages to be mapped in to core by doing a bunch of dummy NSLog() statements this reference the needed classes at any convenient point early in your programs start up..


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