less: fix very obscure memory corruption bug

diff --git a/miscutils/less.c b/miscutils/less.c
index d6b71b9..58c1ec2 100644 (file)
--- a/miscutils/less.c
+++ b/miscutils/less.c
@@ -204,7 +204,27 @@ static void fill_match_lines(unsigned pos);
 #define fill_match_lines(pos) ((void)0)
 #endif
 
-
+/* Devilishly complex routine.
+ *
+ * Has to deal with EOF and EPIPE on input,
+ * with line wrapping, with last line not ending in '\n'
+ * (possibly not ending YET!), with backspace and tabs.
+ *
+ * Variables used:
+ * flines[] - array of lines already read. Linewrap may cause
+ *      one source file line to occupy several flines[n].
+ * flines[max_fline] - last line, possibly incomplete.
+ * terminated - 1 if flines[max_fline] is 'terminated'
+ *      (if there was '\n' [which isn't stored itself, we just remember
+ *      that it was seen])
+ * max_lineno - last line's number, this one doesn't increment
+ *      on line wrap, only on "real" new lines.
+ * readbuf[0..readeof-1] - small preliminary buffer.
+ * readbuf[readpos] - next character to add to current line.
+ * linepos - screen line position of next char to be read
+ *      (takes into account tabs and backspaces)
+ * eof_error - < 0 error, == 0 EOF, > 0 not EOF/error
+ */
 static void read_lines(void)
 {
 #define readbuf bb_common_bufsiz1
@@ -225,6 +245,7 @@ static void read_lines(void)
                        cp += 8;
                strcpy(current_line, cp);
                p += strlen(current_line);
+               /* linepos is still valid from previous read_lines() */
        } else {
                linepos = 0;
        }
@@ -275,17 +296,27 @@ static void read_lines(void)
                        if (c == '\x8' && linepos && p[-1] != '\t') {
                                readpos++; /* eat it */
                                linepos--;
+                       /* was buggy (p could end up <= current_line)... */
                                *--p = '\0';
                                continue;
                        }
-                       if (c == '\t')
-                               linepos += (linepos^7) & 7;
-                       linepos++;
-                       if (linepos >= w)
-                               break;
+                       {
+                               size_t new_linepos = linepos + 1;
+                               if (c == '\t') {
+                                       new_linepos += 7;
+                                       new_linepos &= (~7);
+                               }
+                               if (new_linepos >= w)
+                                       break;
+                               linepos = new_linepos;
+                       }
                        /* ok, we will eat this char */
                        readpos++;
-                       if (c == '\n') { terminated = 1; break; }
+                       if (c == '\n') {
+                               terminated = 1;
+                               linepos = 0;
+                               break;
+                       }
                        /* NUL is substituted by '\n'! */
                        if (c == '\0') c = '\n';
                        *p++ = c;