selftests/bpf: make veristat's verifier log parsing faster and more robust

Make sure veristat doesn't spend ridiculous amount of time parsing
verifier stats from verifier log, especially for very large logs or
truncated logs (e.g., when verifier returns -ENOSPC due to too small
buffer). For this, parse lines from the end of the log and make sure we
parse only up to 100 last lines, where stats should be, if at all.
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-3-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/veristat.c

@@ -419,19 +419,30 @@ static void free_verif_stats(struct verif_stats *stats, size_t stat_cnt)
 
 static char verif_log_buf[64 * 1024];
 
-static int parse_verif_log(const char *buf, size_t buf_sz, struct verif_stats *s)
+#define MAX_PARSED_LOG_LINES 100
+
+static int parse_verif_log(char * const buf, size_t buf_sz, struct verif_stats *s)
 {
-	const char *next;
-	int pos;
+	const char *cur;
+	int pos, lines;
+
+	buf[buf_sz - 1] = '\0';
 
-	for (pos = 0; buf[0]; buf = next) {
-		if (buf[0] == '\n')
-			buf++;
-		next = strchrnul(&buf[pos], '\n');
+	for (pos = strlen(buf) - 1, lines = 0; pos >= 0 && lines < MAX_PARSED_LOG_LINES; lines++) {
+		/* find previous endline or otherwise take the start of log buf */
+		for (cur = &buf[pos]; cur > buf && cur[0] != '\n'; cur--, pos--) {
+		}
+		/* next time start from end of previous line (or pos goes to <0) */
+		pos--;
+		/* if we found endline, point right after endline symbol;
+		 * otherwise, stay at the beginning of log buf
+		 */
+		if (cur[0] == '\n')
+			cur++;
 
-		if (1 == sscanf(buf, "verification time %ld usec\n", &s->stats[DURATION]))
+		if (1 == sscanf(cur, "verification time %ld usec\n", &s->stats[DURATION]))
 			continue;
-		if (6 == sscanf(buf, "processed %ld insns (limit %*d) max_states_per_insn %ld total_states %ld peak_states %ld mark_read %ld",
+		if (6 == sscanf(cur, "processed %ld insns (limit %*d) max_states_per_insn %ld total_states %ld peak_states %ld mark_read %ld",
 				&s->stats[TOTAL_INSNS],
 				&s->stats[MAX_STATES_PER_INSN],
 				&s->stats[TOTAL_STATES],