[Bug 591100] Re: autofs5 eats the cpu if you have large groups

[Martin Pitt]

SRU ack, please upload.

** Also affects: autofs5 (Ubuntu Lucid)
   Importance: Undecided
       Status: New

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autofs5 eats the cpu if you have large groups
https://bugs.launchpad.net/bugs/591100
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Status in “autofs5” package in Ubuntu: Fix Released
Status in “autofs5” source package in Lucid: New

Bug description:
Binary package hint: autofs5

The issue is in lib/mounts.c,  set_tsd_user_vars - editing out the boring bits, it looks like this:

grplen = sysconf(_SC_GETGR_R_SIZE_MAX);
       while (1) {
               char *tmp = realloc(gr_tmp, tmplen+1);
               status = getgrgid_r(gid, pgr, gr_tmp, tmplen, ppgr);
               if (status != ERANGE)
                       break;
               tmplen += grplen;
       }


It's trying to get the members of the users primary group, but doesn't know how big a buffer to allocate, so it keeps trying until the buffer is big enough, incrementing it each time.  The increment is only 1024 bytes at a time, however, so it takes several hundred iterations to get a big enough buffer.

This shouldn't be relying on_SC_GETGR_R_SIZE_MAX  to give a reasonable increment.  See http://www.mail-archive.com/debian-glibc@lists.debian.org/msg40443.html for some discussion about whether the value of SC_GETGR_R_SIZE_MAX should really be that low, but it seems that debian decided it should be, and the man page was wrong.

I've verified that bumping the increment value by 1000x fixes the issue, and stat'ing non-existent homedirs is now instantaneous.