hush: fix a bug in FEATURE_SH_STANDALONE=y config. Closes 9186

Run this in a "sh SCRIPT":

sha256sum /dev/null
echo END

sha256sum is a NOEXEC applet. It runs in a forked child. Then child exit()s.
By this time, entire script is read, and buffered in a FILE object
from fopen("SCRIPT"). But fgetc() did not consume entire input.
exit() lseeks back by -9 bytes, from <eof> to 'e' in 'echo'.
(this may be libc-specific).
This change of fd position *is shared with the parent*!

Now parent can read more, and it thinks there is another "echo END".
End result: two "echo END"s are run.

Fix this by _exit()ing instead.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 20 insertions, 3 deletions

diff --git a/shell/hush.c b/shell/hush.c
index ab192e2cd..be5c98a20 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -1580,11 +1580,11 @@ static void hush_exit(int exitcode)
         }
 #endif
 
-#if ENABLE_HUSH_JOB
         fflush_all();
+#if ENABLE_HUSH_JOB
         sigexit(- (exitcode & 0xff));
 #else
-        exit(exitcode);
+        _exit(exitcode);
 #endif
 }
 
@@ -6466,7 +6466,23 @@ static void dump_cmd_in_x_mode(char **argv)
  * Never returns.
  * Don't exit() here.  If you don't exec, use _exit instead.
  * The at_exit handlers apparently confuse the calling process,
- * in particular stdin handling.  Not sure why? -- because of vfork! (vda) */
+ * in particular stdin handling. Not sure why? -- because of vfork! (vda)
+ * Also, it was observed that on exit(), fgetc'ed buffered data
+ * gets "unwound" by some libcs, via lseek(fd, -NUM, SEEK_CUR).
+ * With the net effect that even after fork(), not vfork(),
+ * exit() in NOEXECed applet in "sh SCRIPT":
+ *      noexec_applet_here
+ *      echo END_OF_SCRIPT
+ * lseeks fd in input FILE object from EOF to "e" in "echo END_OF_SCRIPT".
+ * This makes "echo END_OF_SCRIPT" executed twice. exexit() is the fix.
+ */
+#if ENABLE_FEATURE_SH_STANDALONE
+static void exexit(void)
+{
+        fflush_all();
+        _exit(xfunc_error_retval);
+}
+#endif
 static void pseudo_exec_argv(nommu_save_t *nommu_save,
                 char **argv, int assignment_cnt,
                 char **argv_expanded) NORETURN;
@@ -6547,6 +6563,7 @@ static NOINLINE void pseudo_exec_argv(nommu_save_t *nommu_save,
 # if BB_MMU /* see above why on NOMMU it is not allowed */
                         if (APPLET_IS_NOEXEC(a)) {
                                 debug_printf_exec("running applet '%s'\n", argv[0]);
+                                die_func = exexit;
                                 run_applet_no_and_exit(a, argv);
                         }
 # endif