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[PATCH] devel: fix segfault by unset 'assoc[${x[0]}]'

From: Koichi Murase
Subject: [PATCH] devel: fix segfault by unset 'assoc[${x[0]}]'
Date: Tue, 5 Oct 2021 20:05:19 +0900

Bash Version:
  devel branch (441078402919f6f0dd677cad18d55c7a89d294fc),
  5.1.8(2)-maint (x86_64-pc-linux-gnu)


  In the devel branch, « unset 'assoc[${x[0]}]' » causes a
  segmentation fault, where `assoc' is the name of an associative
  array.  This does not happen with Bash 5.1.

  In Bash 4.4--5.1, the same unset command causes the following
  bad-substitution error:

    bash-4.4: ${x[0: bad substitution

  In Bash 4.0--5.3, the unset command causes the following error:

    bash-4.0: [${x[0]}]: bad array subscript


  The following command causes a segmentation fault.

  $ bash-dev -c "declare -A a; unset 'a[\${b[0]}]'"

  The stack trace reads

  #0  0x00007ffff7de4e35 in raise () from /lib64/libc.so.6
  #1  0x00007ffff7dcf895 in abort () from /lib64/libc.so.6
  #2  0x0000000000452fbf in programming_error ()
  #3  0x00000000004fd4d7 in internal_free.isra ()
  #4  0x0000000000474c10 in expand_word_internal.isra ()
  #5  0x00000000004774e0 in expand_subscript_string ()
  #6  0x000000000048890a in unbind_array_element ()
  #7  0x00000000004b0ed5 in unset_builtin ()
  #8  0x000000000043ad4b in execute_builtin.isra ()
  #9  0x000000000043f69f in execute_command_internal ()
  #10 0x0000000000442ac3 in execute_connection ()
  #11 0x000000000043dfd9 in execute_command_internal ()
  #12 0x00000000004a7191 in parse_and_execute ()
  #13 0x0000000000423f8b in run_one_command ()
  #14 0x0000000000422c0a in main ()


  The segmentation fault is caused in `expand_word_internal (WORD_DESC
  *word, ...)' (subst.c:10325) which releases the memory block
  `word->word' when it fails to expand the word.  The problem is that
  `expand_subscript_string (char *string, ...)' (subst.c:10184) tries
  to directly pass the pointer `string' to `word->word' for the call
  of `expand_word_internal (WORD_DESC *word, ...)'. `word->word' needs
  to be a pointer which may be released on the expansion error.

  * In the first patch `0001-....patch', the argument string is copied
    using `savestring ()', and the copy is passed to
    `expand_word_internal' through `td.word'.  Finally, the copy is
    deleted by `free (td.word)'.  When an expansion error occurs,
    `NULL' is assigned to `fd.word' by `expand_word_internal', so
    `free (td.word)'---i.e., `free (NULL)'---does nothing.

  The segmentation fault is fixed by the above patch, but there
  still remains the same error as bash 4.4.

    bash-patch1: ${x[0: bad substitution

  This is caused by an inconsistency between `valid_array_reference
  (name,flags)' (arrayfunc.c:1187) and `unbind_array_element
  (var,sub,flags)' (arrayfunc.c:1033) in the extraction of
  associative-array subscripts.  Note that `valid_array_reference' is
  called from `unset_builtin' (builtins/set.def:834) to check if the
  unset name has the form of an array element.  Also,
  `unbind_array_element' is called from `unset_builtin' to perform the
  actual unset.  In `valid_array_reference', the length of the
  associative-array subscripts are determined as

      else if (isassoc)
        len = skipsubscript (t, 0, flags&VA_NOEXPAND);  /* VA_NOEXPAND
must be 1 */

  whereas in `unbind_array_element', the length is determined as

      if (var && assoc_p (var) && (flags&VA_ONEWORD))
        len = strlen (sub) - 1;
        len = skipsubscript (sub, 0, (flags&VA_NOEXPAND) || (var &&
assoc_p(var)));  /* XXX */

  `skipsubscript' does not consider the nesting of ${} and $() when
  bit 1 is set to the third argument.  In the former code, nesting is
  not considered only when VA_NOEXPAND is specified.  However, in the
  latter code, nesting is never considered for associative arrays
  (even when VA_NOEXPAND is not specified).  I believe the former code
  should be the expected one.

  * In the second patch `0002-....patch', the subscript extraction in
    `unbind_array_element' is adjusted to match with that of


Attachment: 0001-fix-unset-segfault-on-assoc-subscripts-with-paramexp.patch
Description: Binary data

Attachment: 0002-allow-nesting-and-quoting-in-assoc-subscripts-when-a.patch
Description: Binary data

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