[PATCH] Bash 5.0+: Fix a problem that interactive sessions close with `eval {'

[Koichi Murase]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -march=native -O3
uname output: Linux chatoyancy 5.6.13-100.fc30.x86_64 #1 SMP Fri May
15 00:36:06 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 0
Release Status: release

Description:

  When a command string with an incomplete construct is supplied to
  the `eval' builtin in interactive sessions, the process will be
  terminated.  This does not happen before Bash 5.0.  This does not
  happen in non-interactive modes.

Repeat-By:

  The entire interactive session ends with `eval {'.

  bash-5.0$ eval {
  bash-5.0: syntax error: unexpected end of file
  exit

  This does not happen in bash-4.4 and before.  The session will not
  close, and one can enter and execute the next command.

  bash-4.4$ eval {
  bash-4.4: syntax error: unexpected end of file
  bash-4.4$ echo $?
  1
  bash-4.4$

  This also happens when `eval {' is executed in a function.  The
  session closes.

  bash-5.0$ function fun { eval {; }
  bash-5.0$ fun
  bash-5.0: syntax error: unexpected end of file
  exit

  This does not happen when `eval {' is executed in the
  non-interactive mode.  The commands following `eval {' will be
  executed as normal.

  bash-5.0$ cat test18.sh
  echo Begin eval
  eval {
  echo End eval
  bash-5.0$ bash-5.0 test18.sh
  Begin eval
  test18.sh: eval: line 2: syntax error: unexpected end of file
  End eval
  bash-5.0$ . test18.sh
  Begin eval
  test18.sh: eval: line 2: syntax error: unexpected end of file
  End eval

Fix:

* This behavior has been introduced in `parse.y' in commit 8a10051
  (commit bash-20170511 snapshot) to solve the infinite loop found by
  fuzzing reported at

    https://lists.gnu.org/archive/html/bug-bash/2017-05/msg00076.html.

  I extracted the related section of the commit change:

  > From 8a100514480a55ad73966e516e38778509f6ace6 Mon Sep 17 00:00:00 2001
  > From: Chet Ramey <chet.ramey@case.edu>
  > Date: Thu, 11 May 2017 14:45:50 -0400
  > Subject: [PATCH] commit bash-20170511 snapshot
  >
  > diff --git a/CWRU/CWRU.chlog b/CWRU/CWRU.chlog
  > index 7892ce94..8ea60a39 100644
  > --- a/CWRU/CWRU.chlog
  > +++ b/CWRU/CWRU.chlog
  > @@ -13840,3 +13840,18 @@ builtins/read.def
  >    a CTLESC the last time through the loop (skip_ctlesc == 0),
  >    especially if i == 0. Another fuzzing bug from Eduardo Bustamante
  >    <dualbus@gmail.com>
  > +
  > +     5/9
  > +     ---
  > +parse.y
  > + - GRAMMAR: add 'error yacc_EOF' production to handle a syntax error
  > +   that's immediately followed by an EOF after resynchronization.
  > +   Fixes another fuzzing bug
  > diff --git a/parse.y b/parse.y
  > index 7ca4a64e..dcc628e7 100644
  > --- a/parse.y
  > +++ b/parse.y
  > @@ -411,7 +411,14 @@ inputunit: simple_list simple_list_terminator
  >        YYABORT;
  >      }
  >  }
  > + | error yacc_EOF
  > + {
  > +   /* EOF after an error.  Do ignoreeof or not.  Really
  > +      only interesting in non-interactive shells */
  > +   global_command = (COMMAND *)NULL;
  > +   handle_eof_input_unit ();
  > +   YYACCEPT;
  > + }
  >  | yacc_EOF
  >  {
  >    /* Case of EOF seen by itself.  Do ignoreeof or
  > --
  > 2.21.3

  The problem here is that `handle_eof_input_unit' terminates the
  entire shell when it is in the interactive mode even when the shell
  is processing the string passed to `eval' command.

  The special treatment of EOF for exiting shell should be processed
  only when the shell processes the top-level commands (i.e., not the
  command strings executed through `eval', etc.)

  I attach a patch for the devel branch (see
  0001-Fix-a-bug-that-syntax-errors-in-eval-causes-the-inte.patch).  I
  moved the position of the line `handle_eof_input_unit ();' so that
  it is only executed when `parse_and_execute_level == 0'.

  [ Note: The current code in `parse.y' has additional fixes after
  2017-05-09, so the code looks slightly different from that in the
  commit 2017-05-09 [for details, see Appendix A below], but the `eval
  {' problem is still present. ]

--
Koichi

----------------------------------------------------------------------

Appendix A:

  One of the fix has been made to solve another problem introduced by
  2017-05-09 fix: Even in non-interactive modes, the exit status of
  `eval {' becomes 0 althogh it prints the error messages of syntax
  errors.  This second problem has been already reported by Martijn at

    https://lists.gnu.org/archive/html/bug-bash/2017-06/msg00236.html

  The fix to this second problem has been made in the commit d7d836dfc
  (commit bash-snap-20170620 snapshot).  I extract the corresponding
  section:

  > From d7d836dfc55b937f463f601ba5117d6442053089 Mon Sep 17 00:00:00 2001
  > From: Chet Ramey <chet.ramey@case.edu>
  > Date: Tue, 20 Jun 2017 10:38:13 -0400
  > Subject: [PATCH] commit bash-snap-20170620 snapshot
  >
  > diff --git a/CWRU/CWRU.chlog b/CWRU/CWRU.chlog
  > index 31724be0..7dd09a3f 100644
  > --- a/CWRU/CWRU.chlog
  > +++ b/CWRU/CWRU.chlog
  > @@ -14102,3 +14102,11 @@ parse.y
  >  lib/readline/display.c
  >  - update_line: when wrapping multibyte characters, make sure we deal
  >    with WCWIDTH returning -1. Fixes a fuzzing bug
  > +
  > +    6/17
  > +    ----
  > +parse.y
  > + - augment `error yacc_EOF' production to call YYABORT in non-interactive
  > +   shells or calls to parse_and_execute (eval, command substitution,
  > +   etc.) Fixes bug reported by Martijn Dekker <martijn@inlv.org>
  > +
  > diff --git a/parse.y b/parse.y
  > index d8d8e885..92da96b9 100644
  > --- a/parse.y
  > +++ b/parse.y
  > @@ -416,9 +416,16 @@ inputunit: simple_list simple_list_terminator
  >    /* EOF after an error.  Do ignoreeof or not.  Really
  >       only interesting in non-interactive shells */
  >    global_command = (COMMAND *)NULL;
  > +   last_command_exit_value = 1;
  >    handle_eof_input_unit ();
  > -   YYACCEPT;
  > +   if (interactive && parse_and_execute_level == 0)
  > +     {
  > +       YYACCEPT;
  > +     }
  > +   else
  > +     {
  > +       YYABORT;
  > +     }
  >  }
  >  | yacc_EOF
  >  {
  > --
  > 2.21.3

  Further lines are added in the commit ab8ded9c3 (commit
  bash-20181214 snapshot), but this change is not relevant for now, so
  I skip the details of the commit.

----------------------------------------------------------------------

0001-Fix-a-bug-that-syntax-errors-in-eval-causes-the-inte.patch
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