bug#27270: display-raw-bytes-as-hex generates ambiguous output for Emacs strings

[Paul Eggert <eggert@cs.ucla.edu>]

On 4/24/22 04:24, Lars Ingebrigtsen wrote:

> The likelihood of anybody actually encountering this issue is ... small.

Sure, if strings are random. But strings from opponents aren't random.

I'll readily grant that it's a much smaller exposure than SQL injection. 
Still, like SQL injection it's an exposure and should be fixed.


> You want to quote all %c as if they were raw bytes?  Or only following a
> raw byte?

Closer to the latter, but even less than the latter. I am being 
conservative and am proposing that Emacs do what it does now unless the 
resulting output would be misinterpreted on input. So I wouldn't change 
how all characters are quoted; only how characters are quoted when the 
result would be interpreted incorrectly.


> what about (format "%cf" #x9e)

Since that returns a multibyte string, I suggest "\u009ef" which is 
multibyte. For its unibyte counterpart (encode-coding-string (format 
"%cf" #x9e) 'iso-latin-1) I suggest the syntax "\x9e\ f" which is 
unibyte. (These are not the only possibilities; for example, the former 
could be "\u009e\ f" if you think that's clearer.)

This string syntax is already supported by Emacs, so this wouldn't 
change the Lisp reader.


> it creates
> very confusing displayed strings.

These examples are not *that* confusing. And although they may not be 
beautiful, correct strings are less confusing than incorrect strings.