[Bug c/79816] New: -Wformat-security should warn about missing or excess precision/width in %s specifiers

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79816

            Bug ID: 79816
           Summary: -Wformat-security should warn about missing or excess
                    precision/width in %s specifiers
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.von.s at posteo dot de
  Target Milestone: ---

Created attachment 40869
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40869&action=edit
Sample source code

gcc with -Wformat-security should warn if a printf-/scanf-style function is
passed a character array with a %s specifier that has missing or excess
precision/field width. For scanf-style functions, gcc should also warn if a %s
specifier is given without field width.

If the character array happens not to be null-terminated, a missing precision
specifier in printf() will trigger an out-of-bounds read, leading to an
information leak or a crash. As for scanf(), I don't think I need to explain;
it's another version of the gets() pitfall.

This feature would be good at catching mistakes like the one corrected here:
<https://github.com/dosfstools/dosfstools/commit/09769e678cb4c0235e3c395b6418ee329f41ec02>.

The attached file provides a summary of proposed behaviour.