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C++: Enforce more uniqueness in Buffer.qll #21219

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Merged
MathiasVP merged 5 commits into from
Jan 27, 2026
+39 −35
Merged

C++: Enforce more uniqueness in Buffer.qll #21219

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91752e5
C++: Enforce uniqueness in 'getVariableSize'.
MathiasVP Jan 26, 2026
13a5249
C++: Enforce uniqueness in the other branch of 'getSize'.
MathiasVP Jan 26, 2026
aed0e68
C++: Remove uniqueness since it is enforced earlier now.
MathiasVP Jan 26, 2026
544015d
C++: Add change note.
MathiasVP Jan 26, 2026
980c4cf
Update cpp/ql/lib/semmle/code/cpp/commons/Buffer.qll
MathiasVP Jan 26, 2026
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4 changes: 4 additions & 0 deletions cpp/ql/lib/change-notes/2026-01-26-buffer-overflow-fps.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* The `Buffer.qll` library will no longer report incorrect buffer sizes on certain malformed databases. As a result, the queries `cpp/static-buffer-overflow`, `cpp/overflow-buffer`, `cpp/badly-bounded-write`, `cpp/overrunning-write`, `cpp/overrunning-write-with-float`, and `cpp/very-likely-overrunning-write` will report fewer false positives on such databases.
70 changes: 35 additions & 35 deletions cpp/ql/lib/semmle/code/cpp/commons/Buffer.qll
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Original file line number Diff line number Diff line change
Expand Up @@ -62,11 +62,13 @@ private Class getRootType(FieldAccess fa) {
* unspecified type of `v` is a `ReferenceType`.
*/
private int getVariableSize(Variable v) {
exists(Type t |
t = v.getUnspecifiedType() and
not t instanceof ReferenceType and
result = t.getSize()
)
result =
unique(Type t |
t = v.getUnspecifiedType() and
not t instanceof ReferenceType
|
t.getSize()
)
}

/**
Expand All @@ -79,30 +81,32 @@ private int getSize(VariableAccess va) {
not v instanceof Field and
result = getVariableSize(v)
or
exists(Class c, int trueSize |
// Otherwise, we find the "outermost" object and compute the size
// as the difference between the size of the type of the "outermost
// object" and the offset of the field relative to that type.
// For example, consider the following structs:
// ```
// struct S {
// uint32_t x;
// uint32_t y;
// };
// struct S2 {
// S s;
// uint32_t z;
// };
// ```
// Given an object `S2 s2` the size of the buffer `&s2.s.y`
// is the size of the base object type (i.e., `S2`) minutes the offset
// of `y` relative to the type `S2` (i.e., `4`). So the size of the
// buffer is `12 - 4 = 8`.
c = getRootType(va) and
// we calculate the size based on the last field, to avoid including any padding after it
trueSize = max(Field f | | f.getOffsetInClass(c) + getVariableSize(f)) and
result = trueSize - v.(Field).getOffsetInClass(c)
)
result =
unique(Class c, int trueSize |
// Otherwise, we find the "outermost" object and compute the size
// as the difference between the size of the type of the "outermost
// object" and the offset of the field relative to that type.
// For example, consider the following structs:
// ```
// struct S {
// uint32_t x;
// uint32_t y;
// };
// struct S2 {
// S s;
// uint32_t z;
// };
// ```
// Given an object `S2 s2` the size of the buffer `&s2.s.y`
// is the size of the base object type (i.e., `S2`) minus the offset
// of `y` relative to the type `S2` (i.e., `4`). So the size of the
// buffer is `12 - 4 = 8`.
c = getRootType(va) and
// we calculate the size based on the last field, to avoid including any padding after it
trueSize = max(Field f | | f.getOffsetInClass(c) + getVariableSize(f))
|
trueSize - v.(Field).getOffsetInClass(c)
)
)
}

Expand All @@ -116,12 +120,8 @@ private int isSource(Expr bufferExpr, Element why) {
exists(Variable bufferVar | bufferVar = bufferExpr.(VariableAccess).getTarget() |
// buffer is a fixed size array
exists(bufferVar.getUnspecifiedType().(ArrayType).getSize()) and
result =
unique(int size | // more generous than .getSize() itself, when the array is a class field or similar.
size = getSize(bufferExpr)
|
size
) and
// more generous than .getSize() itself, when the array is a class field or similar.
result = getSize(bufferExpr) and
why = bufferVar and
not memberMayBeVarSize(_, bufferVar) and
not exists(BuiltInOperationBuiltInOffsetOf offsetof | offsetof.getAChild*() = bufferExpr) and
Expand Down