document how to provide useful information in case of segfault

doc/FAQ.md

@@ -40,3 +40,64 @@ to:
 netstat -lpt
 ```
 
+I get a segmentation fault!
+===========================
+
+Well, it's not yours (fault): a segfault is always a bug in
+the programme. Usually standard use cases are well tested,
+so it may be related to something unusual in your
+configuration, or even something wrong, but it should still
+never result in a segfault.
+
+Thankfully, when they are deterministic, segfaults are
+usually fairly easy to fix if you're willing to run a few
+diagnostics to help the developer.
+
+First, make sure you have debug symbols:
+```
+$ file sslh-select
+sslh-select: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=a758ac75ff11f1ace577705b4d6627e301940b59, with debug_info, not stripped
+```
+
+Note `with debug_info, not stripped` at the end. If you
+don't have that, your distribution stripped the binary: you
+will need to get the source code and compile it yourself
+(that way, you will also get the latest version).
+
+Install `valgrind` and run `sslh` under it:
+
+```
+valgrind --leak-check=full ./sslh-fork -v 2 -f -F yourconfig.cfg
+```
+
+Report the full output to the mailing list or github.
+Valgrind is very powerful and gives precise hints of what is
+wrong and why. For example on `sslh` issue
+(#273)[https://github.com/yrutschle/sslh/issues/273]:
+
+```
+sudo valgrind --leak-check=full ./sslh-fork -v 2 -f -F /etc/sslh.cfg
+==20037== Memcheck, a memory error detector
+==20037== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
+==20037== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
+==20037== Command: ./sslh-fork -v 2 -f -F /etc/sslh.cfg
+==20037==
+sslh-fork v1.21b-1-g2c93a01-dirty started
+--20037-- WARNING: unhandled arm-linux syscall: 403
+--20037-- You may be able to write your own handler.
+--20037-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
+--20037-- Nevertheless we consider this a bug.  Please report
+--20037-- it at http://valgrind.org/support/bug_reports.html.
+==20040== Conditional jump or move depends on uninitialised value(s)
+==20040==    at 0x112A3C: parse_tls_header (tls.c:162)
+==20040==    by 0x111CEF: is_tls_protocol (probe.c:214)
+==20040==    by 0x11239F: probe_client_protocol (probe.c:366)
+==20040==    by 0x10A8F7: start_shoveler (sslh-fork.c:98)
+==20040==    by 0x10AE9B: main_loop (sslh-fork.c:200)
+==20040==    by 0x1114FB: main (sslh-main.c:322)
+==20040==
+```
+
+Here we see that something wrong is happening at `tls.c`
+line 162, and it's linked to an uninitialised value.
+