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From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Fri, 24 Aug 2018 23:26:10 -0600
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Subject: Re: [PATCH 1012/1012] Support for unicode interface names: only '%',
 ':' and '/' must be avoided
To: jorgeanton@gmail.com
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Indeed it looks like we could be slightly more permissive. From net/core/dev.c:

/**
*      dev_valid_name - check if name is okay for network device
*      @name: name string
*
*      Network device names need to be valid file names to
*      to allow sysfs to work.  We also disallow any kind of
*      whitespace.
*/
bool dev_valid_name(const char *name)
{
       if (*name == '\0')
               return false;
       if (strnlen(name, IFNAMSIZ) == IFNAMSIZ)
               return false;
       if (!strcmp(name, ".") || !strcmp(name, ".."))
               return false;

       while (*name) {
               if (*name == '/' || *name == ':' || isspace(*name))
                       return false;
               name++;
       }
       return true;
}

And earlier in the callstack, %d is checked for, which indeed adds %
to the blacklist, as your commit indicates.

But should wg-quick enable insane dev names?