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C++: Simplify use of guard conditions in cpp/missing-check-scanf #15997

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Mar 21, 2024
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C++: Simplify use of guard conditions in cpp/missing-check-scanf #15997

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31 changes: 10 additions & 21 deletions cpp/ql/src/Critical/MissingCheckScanf.ql
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ import semmle.code.cpp.commons.Scanf
import semmle.code.cpp.controlflow.Guards
import semmle.code.cpp.dataflow.new.DataFlow::DataFlow
import semmle.code.cpp.ir.IR
import semmle.code.cpp.ir.ValueNumbering
import semmle.code.cpp.valuenumbering.GlobalValueNumbering
import ScanfChecks
import ScanfToUseFlow::PathGraph

Expand Down Expand Up @@ -155,31 +155,20 @@ predicate hasNonGuardedAccess(
flowPath(source, call, index, sink, e) and
minGuard = getMinimumGuardConstant(call, index)
|
not exists(int value |
e.getBasicBlock() = blockGuardedBy(value, "==", call) and minGuard <= value
not exists(GuardCondition guard |
// call == k and k >= minGuard so call >= minGuard
guard
.ensuresEq(globalValueNumber(call).getAnExpr(), any(int k | minGuard <= k),
e.getBasicBlock(), true)
or
e.getBasicBlock() = blockGuardedBy(value, "<", call) and minGuard - 1 <= value
or
e.getBasicBlock() = blockGuardedBy(value, "<=", call) and minGuard <= value
// call >= k and k >= minGuard so call >= minGuard
guard
.ensuresLt(globalValueNumber(call).getAnExpr(), any(int k | minGuard <= k),
e.getBasicBlock(), false)
)
)
}

/** Returns a block guarded by the assertion of `value op call` */
BasicBlock blockGuardedBy(int value, string op, ScanfFunctionCall call) {
exists(GuardCondition g, Expr left, Expr right |
right = g.getAChild() and
value = left.getValue().toInt() and
localExprFlow(call, right)
|
g.ensuresEq(left, right, 0, result, true) and op = "=="
or
g.ensuresLt(left, right, 0, result, true) and op = "<"
or
g.ensuresLt(left, right, 1, result, true) and op = "<="
)
}

from
ScanfToUseFlow::PathNode source, ScanfToUseFlow::PathNode sink, ScanfFunctionCall call, Expr e,
int minGuard
Expand Down