bug#35996: User account password got locked when booting old generation

["Ludovic Courtès" <ludo@gnu.org>]

"pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de> skribis:

> On Sun, Jun 02, 2019 at 11:38:36AM +0200, Ludovic Courtès wrote:

[...]

>> Actually, another thing that could happen is that Guix reads an
>> incomplete /etc/shadow because some other program is writing to it.
>> 
>> In that case, suppose Guix reads a partial /etc/shadow where user
>> “florian” is missing.  It would then create a new /etc/shadow where the
>> password for “florian” is uninitialized (or set to the initial value
>> that appears in config.scm.)
>> 
>> Could it be what happened to you?  You’d have to be running ‘passwd’ or
>> ‘usermod’ or whatever at exactly the same time as ‘guix system
>> reconfigure’ (and you’d have to be “lucky”).
>>
>
> No, I did not change my password in a very long time.
>
> Is there no proper cross-application locking mechanism for
> /etc/passwd?  elogind uses
>
> struct flock flock = {
>   .l_type = F_WRLCK,
>   .l_whence = SEEK_SET,
>   .l_start = 0,
>   .l_len = 0,
> };
> […]
> fd = open(path, O_WRONLY|O_CREAT|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, 0600);
> […]
> r = fcntl(fd, F_SETLKW, &flock;
>
> Should Guix adopt something similar for shadow/passwd/… database
> reads?

We could do that yes, that I’d lean towards using the same thing as libc
and Shadow.  The whole scenario just sounds very unlikely though.

Thanks,
Ludo’.