[Bug preprocessor/66318] Error messages contain raw file name; malicious #line directives can do bad things

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66318

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
You're right that the text of diagnostics is outside the scope of the C
standard.  But as you note, even diagnostics are relied on by tools to
manipulate the referenced files, and changing the file names in the compiler
output would break such tools.

For example, in Emacs Compilation Mode
(http://www.gnu.org/software/emacs/manual/html_node/emacs/Compilation-Mode.html)
if diagnostics in a file named "beep\a.c" were introduced by the string
"beep<BEL>.c:" instead, Emacs would no longer be able to open the file.

On the other hand, "beep\a.c" seems like a corner case not worth worrying
about, and since most multibyte encodings (with the exception of ISO 10646)
seem to take care to avoid using control characters for the second and
subsequent bytes, maybe the impact of such filtering wouldn't be as bad as
might at first seem, especially if an option were provided to disable the
filtering.  David Wheeler presents some good arguments in favor of restricting
the set of valid file name characters in his paper on Fixing Unix/Linux/POSIX
Filenames:
http://www.dwheeler.com/essays/fixing-unix-linux-filenames.html