[PATCH v3] stdio-common: Reject insufficient character data in scanf [BZ #12701]

[Florian Weimer]

* Maciej W. Rozycki:

> Reject invalid formatted scanf character data with the 'c' conversion 
> where there is not enough input available to satisfy the field width 
> requested.  It is required by ISO C that this conversion matches a 
> sequence of characters of exactly the number specified by the field 
> width and it is also already documented as such in our own manual:
>
> "It reads precisely the next N characters, and fails if it cannot get 
> that many."
>
> Currently a matching success is instead incorrectly produced where the 
> EOF condition is encountered before the required number of characters 
> has been retrieved, and the characters actually obtained are stored in 
> the buffer provided.
>
> Add test cases accordingly and remove placeholders from 'c' conversion 
> input data for the existing scanf tests.
>
> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

This reportedly broke the dmstats tool in lvm2:

  glibc: sscanf parsing of %XXXc is not being returned as read
  <https://bugzilla.redhat.com/show_bug.cgi?id=2392416>

I don't know how common it is to use %NNNc where %NNNs was meant.  It
may not be possible to release with this change.

Thanks,
Florian