Ethereum buffer overrun

The fix shipped with solc 0. The text was updated successfully, but these errors were encountered:. It may be something that I could expose as part of the error recovery API Sorry, something went wrong. Multiple dynamic types in the event signature data seem to complicate this a little. For example with. Have tried disabling the buffer overrun check in Reader peekBytes and mis-formatted event data seems to decode ok for a range of inputs including v.

Do you think a "tolerant" flag for abi. I will have to look into this more, but I think this might indicate the data is absolutely corrupted and irrecoverable in a generic sense. Since there is no way to detect the bug, since one bytes may render the output congruent to 4 mod 32 and another bytes render the output to be congruent to 28 mod 32, the final result is now congruent to 0 mod But now comes the bigger issue. If a contract used external and public functions with the same event, they could be mixed.

Also, if an external caller a public with an emit, does it bug out or work? I guess that matters less, since an external can call and external using call so usually to another contract. Which means the output could literally have multiple valid but different interpretations.

With no way to tell which is correct. The least worst option I can think of, would be adding another type, buggy-external- bytes. Just to clarify my understanding. This means that even when the ethers AbiCoder accepted buggy events, it only worked if the bytes was the very last left-to-right, depth-first evaluation item in external method, right?

My impression is that the consensus in the companion Web3 issue is that if there was a way to "sweep this under the rug" by allowing non-strict decoding of events via a flag that disabled the buffer check , people would be ok with it. The decoding seems to work in the identified cases without the overflow check and in general, event data is being formatted correctly by Solidity. After quite a bit of experimentation and playing around with possible scenarios, I have come to the conclusion that it seems safe to support events and only events using a "loose" Reader.

Any extraneous bytes read from an adjacent non-word aligned ABI word will be discarded, and the offsets are always loaded from a base offset within each nest of the Coder, which in itself is popped off one the way back up the call stack implicitly, since the base reader retains its location in the calling frame. I have added support in 5. All other dynamic types in ABIv1 have word-aligned sizes, so this is not an issue. In Solidity 0. If someone out there has a strong need for support for ABIv2 nested structures in legacy Solidity external functions, please let me know, but I suspect no one was using this feature, and a lot more effort will be required to prove it is safe to process those with a loose Reader.

Please try this out and let me know if it works for you. I've tried it against the various examples opened up in all the issues I could find related to non-word-aligned bytes and string event data. Thanks ricmoo! We Don't have Times to develope manage setting only we Wan setup automaticlly moving forward. If last behind. Got this exact same error Error: deferred error during ABI decoding triggered accessing property on reading an event.

Event data type is a string, which is an abi. Also tried not packing, got the same error. Combined with weak type-safety, lack of range checking and manual memory management, such flaws often provide a starting point for building a full remote-code execution exploit.

What is unusual is that the Ethereum virtual machine EVM design and the Solidity language made a series of bad decisions that resurrected a vulnerability class from low-level systems programming in the context of a wildly different environment intended for decentralized smart-contracts. Suppose we are asked to sum two unsigned integers X and Y, and compare them to some other value Z.

If arithmetic in C operated in the Platonic realm of natural numbers, the above code snippet would always run correctly. Instead they are limited to a fixed range, defined by the size of the integral type. This is dependent on the compilation model, which itself is tightly coupled to the hardware architecture code is being targeted at.

Integral types can also be signed, further reducing the representable magnitude since roughly half the possible values are now allocated to negative numbers. Equally important, it can not represent any value outside that range. In other words, the operation overflows the result type. So what happens when a C program is asked to run a calculation resulting in overflow? Intuitively the options are:. It will come as no surprise to those familiar with the minimalist philosophy of C that the language opts for a combination of 3 and 4.

Overflow for unsigned types is perfectly legal and truncates the result. Specifically only the least-significant bits of the result are taken. Meanwhile overflow on signed integral types is undefined. But pragmatically speaking, for most compilers and hardware architectures the result ends up being very similar to unsigned behavior: values are truncated to squeeze into the expected type.

Looked another way, C programs are not working in the realm of natural numbers with their infinite possibilities. If this property of C looks arbitrary, going one more level down to look at how processors handle arithmetic at the hardware level may vindicate the designers. C is close to the metal, so to speak. Its constructs often map into capabilities of an underlying hardware.

Consider the common Intel x86 architecture. This architecture sports bit registers and features instructions for operating on them. Since registers are also limited in the range of numbers they can express, executing this instruction poses the same dilemma: what happens if EAX and EBX hold values that added together would exceed the maximum integer representable in bits?

Nor is that a quirk limited to Intel: most hardware follows this pattern. Given that context, it is hard to fault C for following the same model: it allows using efficient CPU instructions to perform arithmetic without having to double-check or adjust the results spit out by the processor.

And while C is rarely accused of being a high-level language, many other languages that followed including those targeting virtual machines such as Java, have also taken the path of least resistance by going along with the hardware. Languages like Python and Scheme where integers have arbitrary precision by default are the exception, not the rule. That said, there are some crucial differences in how arithmetic works at a low-level which are missing from the abstractions offered in high-level languages:.

By contrast checking for overflow in a language such as C or Java is far more kludgy because these low-level signals are missing. There is no facility provided by the language itself for dealing with overflows in an efficient manner. Options are:. Many libraries exist for doing arithmetic safely with such overflows checks included, but they trade-off safety for kludgy syntax and readability. However in the absence of direct compiler support or very diligent use of inline assembly, these user-defined substitutes will be very inefficient compared to native types.

Where the language falls short, compiler authors can sometimes step in. For example GCC has an option called trapv that will raise a trap when operations on signed integers overflow. A research paper from explores the idea of instrumenting existing binaries to fail on unsigned overflow, confronting exactly that problem of having to identify code that deliberately relies on truncation.

Solving the problem for signed integers may sound good enough except for the fine-print in C conversion rules: in any operation involving a mixture of signed and unsigned operands, the signed input gets typecast to unsigned and arithmetic performed on unsigned values.

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