ptx: avoid infloop due to zero-length matches with -S regex

* src/ptx.c (find_occurs_in_text): Die with an appropriate error
diagnostic when the given regular expression returns a match of
length 0.
* tests/misc/ptx.pl (S-infloop): Add a test.
* NEWS (Bug fixes): Mention the fix.

Fixes https://bugs.gnu.org/28417 which was detected using
Symbolic Execution techniques developed in the course of the
SYMBIOSYS research project at COMSYS, RWTH Aachen University.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/ptx.c b/src/ptx.c
index 2aababff6..b7aa1079b 100644
--- a/src/ptx.c
+++ b/src/ptx.c
@@ -818,6 +818,11 @@ find_occurs_in_text (int file_index)
           case -1:
             break;
 
+          case 0:
+            die (EXIT_FAILURE, 0,
+                 _("error: regular expression has a match of length zero: %s"),
+                 quote (context_regex.string));
+
           default:
             next_context_start = cursor + context_regs.end[0];
             break;