fix(xxd): buffer overread #40236

Problem
Buffer overflow if lines exceed the expected buffer size.

Solution:
Use snprintf instead of strcpy.

src/xxd/xxd.c

@@ -573,7 +573,7 @@ static void xxdline(FILE *fp, char *l, char *colors, int nz)
   static signed char zero_seen = 0;
 
   if (!nz && zero_seen == 1) {
-    strcpy(z, l);
+    snprintf(z, sizeof(z), "%s", l);
     if (colors) {
       memcpy(z_colors, colors, strlen(z));
     }

test/functional/editor/xxd_spec.lua

@@ -2,6 +2,7 @@ local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
 
 local eq = t.eq
+local eval = n.eval
 local clear = n.clear
 local fn = n.fn
 local testprg = n.testprg
@@ -17,4 +18,11 @@ describe('xxd', function()
     local decoded = fn.system({ testprg('xxd'), '-r' }, encoded)
     eq(input, decoded)
   end)
+
+  it('handles long lines in revert mode', function()
+    t.skip(t.is_arch('s390x'), 'FIXME: xxd not built correctly on s390x with QEMU?')
+    local long_line = ('4'):rep(512) .. '\n'
+    fn.system({ testprg('xxd'), '-r' }, long_line)
+    eq(0, eval('v:shell_error'))
+  end)
 end)