RFC: *scanf vs. overflow

[Richard W.M. Jones]

The context to this is that nbdkit uses sscanf to parse simple file
formats in various places, eg:

https://github.com/libguestfs/nbdkit/blob/b23f4f53cf71326f1dba481f64f7f182c20fa3dc/plugins/data/format.c#L171-L172
https://github.com/libguestfs/nbdkit/blob/b23f4f53cf71326f1dba481f64f7f182c20fa3dc/filters/ddrescue/ddrescue.c#L98

We can only do this safely where we can prove that overflow does not
matter.  In other cases we've had to change sscanf uses to strto* etc
which is much more difficult to use correctly.  Just look at how much
code is required to wrap strto* functions to use them safely:

https://github.com/libguestfs/nbdkit/blob/b23f4f53cf71326f1dba481f64f7f182c20fa3dc/server/public.c#L113-L296

Rich.

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Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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