[Python-Dev] What do we do about bad slicing and possible crashes (issue 27867) (original) (raw)

Ethan Furman ethan at stoneleaf.us
Mon Aug 29 12🔞00 EDT 2016

On 08/28/2016 09:26 AM, Nick Coghlan wrote:

On 28 August 2016 at 08:25, Terry Reedy <tjreedy at udel.edu> wrote:

Slicing can be made to malfunction and even crash with an 'evil' index method. https://bugs.python.org/issue27867

The crux of the problem is this: PySliceGetIndicesEx receives a slice object and a sequence length. Calling index on the start, stop, and step components can mutate the sequence and invalidate the length. Adjusting the int values of start and stop according to an invalid length (in particular, one that is too long) will result in invalid results or a crash. Possible actions -- very briefly. For more see end of https://bugs.python.org/issue27867?@okmessage=msg 273801 0. Do nothing. 1. Detect length change and raise. I suggest taking this path - it's the lowest impact, and akin to the "dictionary changed size during iteration" runtime error.

+1. Being able to do such strange things with list but not dict would be irritating and a nuisance (although maybe not attractive ;) .

index having side effects is pathological code behaviour, so we really just need to prevent the interpreter crash, rather than trying to make it sense of it.

Agreed.

-- Ethan

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