Bug#1135862:

[Graeme Vetterlein]

I suspect there is a single bug here, but the behaviour differs. This 
and another IPv6 related new bug held me up for several weeks, so I've 
spent a lot of time looking at this.


I initially also thought it was related to treatment of comments . 
However after quite a lot of testing and code inspection, it appears the 
root cause is


     sysctl(8) uses write(2)

     systemd variant uses some form of stream IO (e.f prinntf) **


** so when a write(2) of a 10 byte buffer returns "1" (byte sent) , 
systemd keeps calling , until all 10 bytes are consumed. sysctl(8) just 
stops at 1 byte

*** that said I've seen write(2) also send many bytes ... so , a bit 
more complicated.




The interaction is then how the kernel level code accepts the IO for 
this particular /proc variable , it accepts just 1 byte (and seems to 
only differentiate "1" or not "1" ..so "6" acts like "0")

for other /proc settings the kernel side obviously behaves differently , 
since for example:


cat /proc/sys/kernel/version
#111-Ubuntu SMP PREEMPT_DYNAMIC Sat Apr 11 23:16:02 UTC 2026


Lots of bytes, including a '#'  :-)


(BTW, from a different system, just a for instance)



The line in sysctl.c is:


   } else {
             if (0 < fprintf(fp, "%s\n", value))
                 rc = EXIT_SUCCESS;


Which looks like regular printf()  but that "fp" comes, not from 
fopen(3) but from:


        if ((fp = fprocopen(path, "w")) == NULL) {

     ...


Then, via a long tortuous route , ends up at close() doing: (this is 
from memory now, it's a while since I traced it)


     if (cookie->final) {
         len = write(cookie->fd, cookie->buf, cookie->offset);


There is a whole bunch of special cases "delimiter characters" etc  and 
a liberal scattering of gotos :-)



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