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Teach contain_leaked_vars that assignment SubscriptingRefs are leaky.
array_get_element and array_get_slice qualify as leakproof, since
they will silently return NULL for bogus subscripts. But
array_set_element and array_set_slice throw errors for such cases,
making them clearly not leakproof. contain_leaked_vars was evidently
written with only the former case in mind, as it gave the wrong answer
for assignment SubscriptingRefs (nee ArrayRefs).
This would be a live security bug, were it not that assignment
SubscriptingRefs can only occur in INSERT and UPDATE target lists,
while we only care about leakproofness for qual expressions; so the
wrong answer can't occur in practice. Still, that's a rather shaky
answer for a security-related question; and maybe in future somebody
will want to ask about leakproofness of a tlist. So it seems wise to
fix and even back-patch this correction.
(We would need some change here anyway for the upcoming
generic-subscripting patch, since extensions might make different
tradeoffs about whether to throw errors. Commit 558d77f20
attempted
to lay groundwork for that by asking check_functions_in_node whether a
SubscriptingRef contains leaky functions; but that idea fails now that
the implementation methods of a SubscriptingRef are not SQL-visible
functions that could be marked leakproof or not.)
Back-patch to 9.6. While 9.5 has the same issue, the code's a bit
different. It seems quite unlikely that we'd introduce any actual bug
in the short time 9.5 has left to live, so the work/risk/reward balance
isn't attractive for changing 9.5.
Discussion: https://postgr.es/m/3143742.1607368115@sss.pgh.pa.us
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@ -1411,7 +1411,6 @@ contain_leaked_vars_walker(Node *node, void *context)
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case T_ScalarArrayOpExpr:
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case T_CoerceViaIO:
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case T_ArrayCoerceExpr:
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case T_SubscriptingRef:
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/*
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* If node contains a leaky function call, and there's any Var
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@ -1423,6 +1422,23 @@ contain_leaked_vars_walker(Node *node, void *context)
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return true;
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break;
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case T_SubscriptingRef:
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{
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SubscriptingRef *sbsref = (SubscriptingRef *) node;
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/*
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* subscripting assignment is leaky, but subscripted fetches
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* are not
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*/
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if (sbsref->refassgnexpr != NULL)
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{
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/* Node is leaky, so reject if it contains Vars */
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if (contain_var_clause(node))
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return true;
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}
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}
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break;
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case T_RowCompareExpr:
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{
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/*
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