Valgrind best options to trade
Earlier this week, Jesse Ruderman reported a problem found with the Valgrind tool Memcheck:. It turns out that the Memcheck was right, and the problem was real. So why did Mats have trouble understanding what Valgrind was trying to tell him? Memcheck tracks every single value bit present in a running program, and tracks whether it is defined or undefined.
Furthermore, undefinedness gets propagated through all operations. For example, if you add an undefined value to a defined value, the result is undefined. This paper describes how it works in detail. For those of you who have used Purify, this bit-precise definedness tracking is one of the main things that distinguishes Memcheck from Purify.
Furthermore, the technique can be to track valgrind best options to trade binary property, and has been reused in tools tracking which bits are tainted and which bits are secret.
This is most common with heap allocations, which in practice often valgrind best options to trade in all the allocated memory being zeroed. How does the infection propagate?
Well, once you have a possibly incorrect value, there are many ways for things valgrind best options to trade go wrong:. In other words, incorrect values can lead to all sorts of problems. So where in this chain does Memcheck complain? Ideally it would be as soon as the defect is executed. You might think that Memcheck should complain if an undefined value is used in any way. But that too would lead to false positives — it turns that normal programs copy around lots of undefined values, because of padding within structures.
You might think that Memcheck should complain if an undefined value is used in an arithmetic operation. What Memcheck does is complain when the use of an undefined value is very likely to lead to defects:.
The end result of this is that the place where Memcheck complains can be quite some distance from the defect, and also quite some distance from any failures that manifest as a result. This is why Mats had trouble with this warning. But do not despair…. When Memcheck complains about a dangerous use of an undefined value, you need to work backwards to find the defect — the missing initialisation. In easy cases, you can do this in your head, just by looking at the code.
But for over seven years that was the best way to do it. What it does is, for every undefined value, it tracks where the undefinedness originated.
The origin is usually a stack or heap allocation. This is useful because the missing initialisation — the defect — is very often supposed to occur immediately after the allocation. So origin-tracking usually takes you immediately to the actual defect. By default, —track-origins is not turned on. This is because it slows Memcheck down a lot, because of all the extra origin information that must be propagated.
But if you have undefined value warnings, you should turn it on immediately! Valgrind best options to trade this information, Mats was able to track down the defect: We can very easily generate warnings about uninitialized fields in constructors using our static analysis toolchain.
Feel free to file a bug. SVN, a dynamic binary instrumentation framework. Eliminating undefined values with Valgrind, the easy way Posted on February 27, by Nicholas Nethercote 5 Comments. Earlier this week, Jesse Ruderman reported a problem found with the Valgrind tool Memcheck: Tracking undefined values Memcheck tracks valgrind best options to trade single value bit present in a running program, and tracks whether it is defined or undefined. Well, once you have a possibly incorrect value, there are many ways for things to go wrong: If an incorrect value is used as an input to an operation that writes a result, such as an assignment, arithmetic operation or system call, the likely result another incorrect value a further infection.
If an incorrect value is used as an input to an operation with user-visible side-effects e. Or it could cause inappropriate memory to be accessed, leading to a crash a failure. If an incorrect value is used in a conditional branch, or as a jump target, that can lead to an incorrect program counter another infectionwhich can lead to failures such as incorrect behaviour, or crashes if the jump destination is sufficiently bogus. Back to Memcheck So where in this chain valgrind best options to trade Memcheck complain?
What Memcheck does is complain when the use of an undefined value is very likely to lead to defects: In the condition of a conditional branch or test, or the target address of a branch, valgrind best options to trade it is likely to lead valgrind best options to trade incorrect control flow, which will lead to incorrect behaviour. The complaint above was an example of this. As an argument to a system call, because it is likely to cause an incorrect externally-visible side-effect.
As the address in a load or store, because it could cause a crash. But do not despair… So how do I get better information? This entry was posted in CorrectnessValgrind.
Jesse Ruderman February 28, at 7: It would be nice if the error message distinguished jumps valgrind best options to trade moves. Nik March 1, at 3: Jesse, —track-origins is advertised at the end if you have any undefinedness warnings: Snark March 1, at 8: Coraline by Automattic Proudly powered by WordPress.