Got it!
(gdb) continue
Continuing.
Program received signal SIG34, Real-time event 34.
0x00007f890b358384 in __libc_read (fd=fd@entry=17, buf=buf@entry=0x7fff6affa4f8, nbytes=nbytes@entry=1)
at ../sysdeps/unix/sysv/linux/read.c:27
27 in ../sysdeps/unix/sysv/linux/read.c
(gdb) backtrace
#0 0x00007f890b358384 in __libc_read (fd=fd@entry=17, buf=buf@entry=0x7fff6affa4f8, nbytes=nbytes@entry=1)
at ../sysdeps/unix/sysv/linux/read.c:27
#1 0x0000565452c65fde in read (__nbytes=1, __buf=0x7fff6affa4f8, __fd=17) at /usr/include/x86_64-linux-gnu/bits/unistd.h:44
#2 read_write (fd=fd@entry=17, packet=packet@entry=0x7fff6affa4f8 "", size=size@entry=1, rw=rw@entry=1) at src/dnsmasq/util.c:696
#3 0x0000565452c3e482 in tcp_request (confd=confd@entry=17, now=now@entry=1589985445, local_addr=local_addr@entry=0x7fff6affa5f0,
netmask=..., netmask@entry=..., auth_dns=auth_dns@entry=0) at src/dnsmasq/forward.c:1911
#4 0x0000565452c57824 in check_dns_listeners (now=now@entry=1589985445) at src/dnsmasq/dnsmasq.c:1961
#5 0x0000565452c597bc in main_dnsmasq (argc=<optimized out>, argv=<optimized out>) at src/dnsmasq/dnsmasq.c:1203
#6 0x0000565452c158ac in main (argc=<optimized out>, argv=<optimized out>) at src/main.c:93
(gdb)
This happened when I added test record to whitelist.
I tested it because I had an issue today on this instance where I could not add anything to whitelist due to readonly database, so I ran pihole -g -r to fix it.
I already did it here in another instance of Pi-hole:
Actually, I also tried
sudo setfacl -Rbn /etc/pihole/
sudo setfacl -Rbn /etc/pihole/gravity.db
as discussed in V5.0 Docker, Whitelist domain, writing to readonly database - #41 by Freemann , but it did not have any effect. User www-data could not write to gravity.db
So, did I cause that crash doing what I should not have or is it an issue you can look into?