[Bug 234494] dns/maradns: memory exhaustion with default config (with workaround)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234494

            Bug ID: 234494
           Summary: dns/maradns: memory exhaustion with default config
                    (with workaround)
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: je3kmz at FreeBSD.org
          Reporter: nino80 at gmail.com
             Flags: maintainer-feedback?(je3kmz at FreeBSD.org)
          Assignee: je3kmz at FreeBSD.org

12.0-RELEASE FreeBSD 12.0-RELEASE r341666 GENERIC  i386 w/ 512MB RAM

After upgrading the system to 12.0-RELEASE and updating MaraDNS to the latest
package (2.0.15), MaraDNS started consistently crashing with the following
error in the log:

Dec 28 12:58:31 srv /usr/local/sbin/maradns: Aieeeeee, can not allocate memory!

Traced the error to the following code section in js_alloc() in JsStrOS.c:

    data = (void *)malloc(unit_count * unit_size);
    ...
    if(data == NULL) {
        /* Securty: In a situtation where we can not allocate memory,
           the subsequent behavior of the program is undefined.  Hence,
           the best thing to do is exit then and there */
        printf("Aieeeeee, can not allocate memory!");
        exit(64);
        return (void *)0;
        }

Results in the following trace:

    mmap(0x0,20480,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANON,-1,0x0) ERR#12
'Cannot allocate memory'
    write(1,"Aieeeeee, can not allocate memor"...,34) = 34 (0x22)
    exit(0x40)

MaraDNS appears to limit memory usage itself to about 2.5MB by default:

    setrlimit(RLIMIT_VMEM,{ cur=2646016,max=2646016 }) = 0 (0x0)

I have checked the arguments to malloc (unit_count, unit_size) and the overall
memory allocation and there doesn't appear to be a bug or a leak. While MaraDNS
is not memory-hungry, it seems that small heap size and some heap fragmentation
is enough to exhaust the available memory. 

Workaround is to add max_mem parameter to mararc config. I have set it to 8MB
(in bytes) and that helped to avoid crash so far. Will keep monitoring and
update the PR if further crashes are experienced.

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