[ast-developers] Memory leak while creating subshells after setting PATH

[Siteshwar Vashisht]

Hello,

There is a bug in path_alias() function that may cause a memory leak while creating subshells.

cat subshell-leak.sh

#!/bin/ksh93

while true
do
PATH=.
for DIR in /usr/bin /usr/sbin \
/bin /usr/local/bin
do
if [ -d ${DIR} ]
then
PATH=${PATH}:${DIR}
true
fi
done

time=$(date +%T)

prev=${page}
page=$(cat /proc/$$/stat|awk '{print $24}')

done

Running above script with valgrind shows this report :

==12108== HEAP SUMMARY:
==12108== in use at exit: 291,164 bytes in 270 blocks
==12108== total heap usage: 5,171 allocs, 4,901 frees, 18,409,728 bytes allocated
==12108==
==12108== LEAK SUMMARY:
==12108== definitely lost: 7,663 bytes in 79 blocks
==12108== indirectly lost: 0 bytes in 0 blocks
==12108== possibly lost: 5,392 bytes in 3 blocks
==12108== still reachable: 278,109 bytes in 188 blocks
==12108== suppressed: 0 bytes in 0 blocks
==12108== Rerun with --leak-check=full to see details of leaked memory
==12108==
==12108== For counts of detected and suppressed errors, rerun with: -v
==12108== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)


ksh maintains value of $PATH as a linked list. When a subshell is forked, ksh duplicates $PATH by increasing refcount for every element in linked list by calling path_dup() and path_alias() functions. However when the subshell exits this refcount is not decreased. Next time when $PATH is reset to a new value, ksh calls path_delete() function to delete linked list which stored older path. But path_delete() function does not free elements whose refcount is greater than 1 and leaves a memory leak in the shell.

Attached patch fixes this issue.