Re: Bug#1141670: util-linux: very slow code and undefined behaviour in handle_interrupt

[Chris Hofstaedtler <zeha@debian.org>]

Hi util-linux maintainers,

the following was reported to Debian, regarding the hardlink 
utility. It appears to be an upstream issue. Maybe someone can look 
at it. CC:ing наб who seems to have touched the relevant function 
last.

Thanks,
Chris

* Marc Lehmann <debian-reportbug@plan9.de> [260708 07:23]:
>Version: 2.41-5
>
>Dear Maintainer,
>
>while strace'ing to find out why hardlink is so slow, IO found it does this sequence between every fstatat call:
>
>   gettid()                                = 36482
>   getpid()                                = 36482
>   tgkill(36482, 36482, 0)                 = 0
>
>this seems to be a pretty weird thing to do in a tighht scanning
>loop. Looking at the code, handle_interrupt is responsible:
>
>   /**
>    * handle_interrupt - Handle a signal
>    */
>   static void handle_interrupt(void)
>   {
>           switch (last_signal) {
>           case SIGUSR1:
>                   print_stats();
>                   putchar('\n');
>                   break;
>           default:
>                   signal(last_signal, SIG_DFL);
>                   raise(last_signal);
>                   break;
>           }
>           last_signal = 0;
>   }
>
>this is already a weird function to begin with, but crucially, it calls signal(0,... and raise(0).
>
>it seems to me the former is likely undefined behaviour and the latter is
>just unnecessartily slow.
>
>most likely, handle_interrupt simply should not be called after every fstatat, or at all, without last_signal bering set beforehand.
>