diff -up vim80/src/structs.h.tag-overflow vim80/src/structs.h
--- vim80/src/structs.h.tag-overflow	2018-04-24 17:10:42.000000000 +0200
+++ vim80/src/structs.h	2026-02-25 15:20:08.027012072 +0100
@@ -3423,3 +3423,7 @@ typedef struct {
     int		save_opcount;
     tasave_T	tabuf;
 } save_state_T;
+
+// Return the length of a string literal
+#define STRLEN_LITERAL(s) (sizeof(s) - 1)
+
diff -up vim80/src/tag.c.tag-overflow vim80/src/tag.c
--- vim80/src/tag.c.tag-overflow	2018-03-05 15:55:42.000000000 +0100
+++ vim80/src/tag.c	2026-02-25 15:21:18.623604260 +0100
@@ -2677,7 +2677,7 @@ get_tagfname(
 	    if (tnp->tn_hf_idx > tag_fnames.ga_len || *p_hf == NUL)
 		return FAIL;
 	    ++tnp->tn_hf_idx;
-	    STRCPY(buf, p_hf);
+	    vim_strncpy(buf, p_hf, MAXPATHL - 1);
 	    STRCPY(gettail(buf), "tags");
 	}
 	else
diff -up vim80/src/testdir/test_help.vim.tag-overflow vim80/src/testdir/test_help.vim
--- vim80/src/testdir/test_help.vim.tag-overflow	2017-12-11 10:20:46.000000000 +0100
+++ vim80/src/testdir/test_help.vim	2026-02-25 15:21:53.687898395 +0100
@@ -49,3 +49,12 @@ func Test_help_local_additions()
   call delete('Xruntime', 'rf')
   let &rtp = rtp_save
 endfunc
+
+" This caused a buffer overflow
+func Test_helpfile_overflow()
+  let _helpfile = &helpfile
+  let &helpfile = repeat('A', 5000)
+  help
+  helpclose
+  let &helpfile = _helpfile
+endfunc