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Hardening of readonly variables (#239)

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Ksh currently restricts readonly scalar variables from having their
values directly changed via a value assignment. However, since ksh
allows variable attributes to be altered, the variable's value can
be indirectly altered. For instance, if TMOUT=900 (for a 15 minute
idle timeout) was set to readonly, all that is needed to alter the
value of TMOUT from 900 to 0 is to issue 'typeset -R1 TMOUT',
perhaps followed by a 'typeset -i TMOUT' to turn off the shell's
timeout value.

In addition, there are problems with arrays. The following is
incorrectly allowed:

        typeset -a arr=((a b c) 1)
        readonly arr
        arr[0][1]=d

        arr=(alphas=(a b c);name=x)
        readonly arr.alphas
        arr.alphas[1]=([b]=5)

        arr=(alphas=(a b c);name=x)
        readonly arr.alphas
        arr.alphas[1]=(b)

        typeset -C arr=(typeset -r -a alphas=(a b c);name=x)
        arr.alphas[1]=()

src/cmd/ksh93/bltins/typeset.c: setall():
- Relocate readonly attribute check higher up the code and widen
  its application to issue an error message if the pre-existing
  name-pair has the readonly bit flag set.
- To avoid compatibility problems, don't check for readonly if
  NV_RDONLY is the only attribute set (ignoring NV_NOFREE). This
  allows 'readonly foo; readonly foo' to keep working.

src/cmd/ksh93/sh/array.c: nv_endsubscript():
- Apply a readonly flag check when an array subscript or append
  assignment occurs, but allow type variables (typeset -T) as they
  utilize '-r' for 'required' sub-variables.

src/cmd/ksh93/tests/readonly.sh:
- New file. Create readonly tests that validate the warning message
  and validate that the readonly variable did not change.

src/cmd/ksh93/sh/streval.c:
- Bump MAXLEVEL from 9 to 1024 as a workaround for arithmetic
  expansion, avoiding a spurious error about too much recursion
  when the readonly.sh tests are run. This change is backported
  from ksh 93v-.
  TODO: debug a spurious increase in arithmetic recursion level
  variable when readonly.sh tests with 'typeset -i' are run.
  That is a different bug for a different commit.

Co-authored-by: Martijn Dekker <martijn@inlv.org>
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hyenias 2021-04-05 01:43:19 -04:00 • committed by GitHub
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@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh
Any uppercase BUG_* names are modernish shell bug IDs.
2021-04-04:
- Harden readonly variables. Readonly variables or arrays no longer allow
attribute changes which would otherwise allow their value to be altered.
Expanded support for readonly variables within multidimensional arrays.
2021-04-03:
- Fixed a bug that caused the uname builtin's -d option to change the output