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Fix: running external command influences $RANDOM sequence

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The pseudorandom generator generates a reproducible sequnece of
values after seeding it with a known value. But:

    $ (RANDOM=1; echo $RANDOM; echo $RANDOM)
    2100
    18270
    $ (RANDOM=1; echo $RANDOM; ls >/dev/null; echo $RANDOM)
    2100
    30107

Since RANDOM was seeded with a specific value, the two command
lines should output the same pair of numbers. Running 'ls' in the
middle should make no difference.

The cause is a nv_getval(RANDNOD) call in xec.c, case TFORK, that
is run for all TFORK cases, in the parent process -- including
background jobs and external commands. What should happen instead
is that $RANDOM is reseeded in the child process.

This bug is in every version of ksh93 since before 1995.

There is also an opportunity for optimisation. As of 396b388e, the
RANDOM seed may be invalidated by setting rand_last to -2,
triggering a reseed the next time a $RANDOM value is obtained. This
was done to optimise the virtual subshell mechanism. But that can
also be used to eliminate unconditional reseeding elsewhere. So as
of this commit, RANDOM is never (re)seeded until it's used.

src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/sh/subshell.c:
- Add RAND_SEED_INVALIDATED macro, a single source of truth for the
  value that triggers a reseeding in sh_save_rand_seed().
- Add convenient sh_invalidate_rand_seed() function macro.

src/cmd/ksh93/sh/init.c,
src/cmd/ksh93/sh/xec.c:
- Optimisation: invalidate seed instead of reseeding directly.
- sh_exec(): case TFORK: Delete the nv_getval(RANDNOD) call. Add a
  sh_invalidate_rand_seed() to the child part. This fixes the bug.
This commit is contained in:
Martijn Dekker 2022-07-21 00:22:30 +02:00
parent a43bb4f1bd
commit 948fab26aa
7 changed files with 25 additions and 12 deletions
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2
src/cmd/ksh93/include/version.h
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@ -23,7 +23,7 @@
#define SH_RELEASE_FORK "93u+m" /* only change if you develop a new ksh93 fork */
#define SH_RELEASE_SVER "1.0.0-beta.2" /* semantic version number: https://semver.org */
#define SH_RELEASE_DATE "2022-07-14" /* must be in this format for $((.sh.version)) */
#define SH_RELEASE_DATE "2022-07-21" /* must be in this format for $((.sh.version)) */
#define SH_RELEASE_CPYR "(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
/* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */