Bug#813133: grub2: Backspace does not work when typing username at grub authentication

[Szőgyényi Gábor]

Source: grub2
Severity: normal
Tags: upstream

Dear Maintainer,

There is a bug with the character parsing when typing username at the grub
authentication screen. The backspace does not delete character from the screen,
but delets from the "username" variable.

How to reproduce:
1. Set superuser password in grub.cfg:
set superusers="user"
password user 12345

2. save&reboot

3. At boot menu pressing "c" or choose one menuitem, grub will ask for the
username/password.

4. Type "aaaaa"+\b\b\b\b\b+"user" , to the username field. Means five "a" then
five backslash then "user" (Screenshot attached. ) You will see, the backslash
does not delete.

5. Press \n, and type the password: 12345.

6. Grub will accept the username / password.



-- System Information:
Debian Release: 8.3
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 3.16.0-4-amd64 (SMP w/2 CPU cores)
Locale: LANG=hu_HU.UTF-8, LC_CTYPE=hu_HU.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
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