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Filter out all symbols starting with a double underbar.  In particular,
this filters out all retguard symbols, which are no fun to guess. One
recognizes them easily but can get yourself hanged by a single digit!

An earlier version filtering only __retguard symbols was

ok deraadt, jsing

guenther agreed that filtering all double underbar symbols makes sense.
He also suggested to filter out symbols containing several consecutive
digits, but how much fun is guessing libcrypto symbols without all the
X509 goodness?

/*	$OpenBSD: ksyms.c,v 1.13 2021/12/24 23:01:56 tb Exp $	*/

/*
 * Copyright (c) 2008 Miodrag Vallat.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <ctype.h>
#include <curses.h>
#include <elf.h>
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include "hangman.h"

static int ksyms_elf_parse(void);

void
sym_getword(void)
{
	uint tries;
	off_t pos;
	int buflen;
	char symbuf[1 + BUFSIZ], *sym, *end;
	size_t symlen;

	for (tries = 0; tries < MAXBADWORDS; tries++) {
		pos = arc4random_uniform(symsize);
		if (lseek(symfd, pos + symoffs, SEEK_SET) == -1)
			continue;
		buflen = read(symfd, symbuf, BUFSIZ);
		if (buflen == -1)
			continue;

		/*
		 * The buffer is hopefully large enough to hold at least
		 * a complete symbol, i.e. two occurences of NUL, or
		 * one occurence of NUL and the buffer containing the end
		 * of the string table. We make sure the buffer will be
		 * NUL terminated in all cases.
		 */
		if (buflen + pos >= symsize)
			buflen = symsize - pos;
		*(end = symbuf + buflen) = '\0';

		for (sym = symbuf; *sym != '\0'; sym++)
			;
		if (sym == end)
			continue;

		symlen = strlen(++sym);
		if (symlen < MINLEN || symlen > MAXLEN)
			continue;

		/* ignore symbols containing dots or dollar signs */
		if (strchr(sym, '.') != NULL || strchr(sym, '$') != NULL)
			continue;
		/* ignore all double unberbar symbols */
		if (strncmp(sym, "__", 2) == 0)
			continue;

		break;
	}

	if (tries >= MAXBADWORDS) {
		mvcur(0, COLS - 1, LINES -1, 0);
		endwin();
		errx(1, "can't seem a suitable symbol in %s",
		    Dict_name);
	}

	strlcpy(Word, sym, sizeof Word);
	strlcpy(Known, sym, sizeof Known);
	for (sym = Known; *sym != '\0'; sym++) {
		if (*sym == '-')
			*sym = '_';	/* try not to confuse player */
		if (isalnum((unsigned char)*sym))
			*sym = '-';
	}
}

int
sym_setup(void)
{
	if ((symfd = open(Dict_name, O_RDONLY)) == -1)
		return -1;

	if (ksyms_elf_parse() == 0)
		return 0;

	close(symfd);
	errno = ENOEXEC;
	return -1;
}

int
ksyms_elf_parse(void)
{
	Elf_Ehdr eh;
	Elf_Shdr sh;
	uint s;

	if (lseek(symfd, 0, SEEK_SET) == -1)
		return -1;

	if (read(symfd, &eh, sizeof eh) != sizeof eh)
		return -1;

	if (!IS_ELF(eh))
		return -1;

	if (lseek(symfd, eh.e_shoff, SEEK_SET) == -1)
		return -1;

	symoffs = 0;
	symsize = 0;

	for (s = 0; s < eh.e_shnum; s++) {
		if (read(symfd, &sh, sizeof sh) != sizeof sh)
			return -1;

		/*
		 * There should be two string table sections, one with the
		 * name of the sections themselves, and one with the symbol
		 * names. Just pick the largest one.
		 */
		if (sh.sh_type == SHT_STRTAB) {
			if (symsize > (off_t)sh.sh_size)
				continue;

			symoffs = (off_t)sh.sh_offset;
			symsize = (off_t)sh.sh_size;
		}
	}

	if (symsize == 0)
		return -1;

	return 0;
}