elftools: StringTableSection: Check table size before returning string

[vapier]

Some ELF files have strings pointing to an offset outside the string table dimension, let's throw an exception in that case.

Bug-Url: https://crbug.com/788925
Reviewed-on: https://chromium-review.googlesource.com/792553

[eliben]

Adding a unit test could be nice here

Otherwise LGTM

@sevaa any objections?

[sevaa]

The second assert is unnecessary - parse_cstring_from_stream will not read past the end of the stream (this ain't C, people). If there is indeed an unterminated string at the end of the section, parse_cstring_from_stream will return None and the expression in the assert will throw an exception that you are calling len() on a None object.

Remove the second check, it's pointless and wrong.

[sevaa]

@vapier Are you still interested in this change?

[sevaa]

Now that I look at it, should we really spend time on providing descriptive exceptions for various flavors of corrupt data? As things stand right now, pyelftools will exception on a file with a string offset leading to nowhere (decode called on NoneType will be the exception); with this patch, the exception will be more descriptive. On one hand, that's a user friendly thing to do; on the other hand, given the myriad ways an extremely interlinked data structure can be corrupt, should we endeavor to catalogue them all?