I've observed a lot of false positives on WebKit that resulted from our inability to inline relatively simple functions in C++, specifically constructors and operator new(), due to unimplemented modeling. So i wanted to document, and eventually fix, a variety of language feature modeling we're currently lacking. Note that WebKit, like LLVM, is not a typical C++ codebase; it uses many language constructs that regular projects don't use, and uses very little standard library headers.
Hi Artem!
Hi Artem!
I've observed a lot of false positives on WebKit that resulted
from our inability to inline relatively simple functions in C++,
specifically constructors and operator new(), due to unimplemented
modeling. So i wanted to document, and eventually fix, a variety
of language feature modeling we're currently lacking. Note that
WebKit, like LLVM, is not a typical C++ codebase; it uses many
language constructs that regular projects don't use, and uses very
little standard library headers.Great news!
---
Normally we expect the analyzer to work even without any inlining,
by conservatively invalidating the program state, which eliminates
the clearly false assumptions but sometimes causes infeasible
paths when the invalidation was too aggressive and we start
denoting the same value with a different symbol after invalidation
and assume contradictory stuff on the old and the new symbol. The
false positives resulting from aggressive invalidation are usually
treated as less scary than the ones suppressed by invalidation,
because they can be easily suppressed (with assertions or const
qualifiers etc.) without loss of coverage. However, in C++ there
are a lot of implicit function calls which cause massive
frustration when evaluated conservatively. For example,class C {
bool haveX;
class D d;public:
C(int *x): haveX(x != 0) {
if (haveX)
*x = 1; // null dereference??
}
};In this case, 'haveX' should assume that x is non-null. The code
eagerly splits states into {x == 0} and {x != 0}, which seems
reasonable. However, after 'haveX' have been initialized, the
default constructor for field 'd' kicks in. If only this
constructor is not inlined, or any of its callees are not inlined,
value stored in 'haveX' would be invalidated on both paths. In
particular, the path on which the original 'haveX' is false but
the invalidated 'haveX' is true have now opened up.I observed the very same pattern on Ericsson internal codebases, so sorting this out is definitely a huge win for many projects (not just LLVM and WebKit).
---
Inlining of the constructor itself is disabled in many cases for
many reasons. In particular, we are currently only trying to
inline the constructor when the target this-region is a DeclRegion
(variable, member variable, Objective-C++ instance var), and the
destructor is non-trivial. This cuts away a lot of cases:It is not clear for my why the non-trivial destructor is a requirement here. Do yo have any info on that?
I believe that if the destructor is trivial, it's irrelevant whether the destructor would be evaluated at all during analysis, or it is missing completely due to improper modeling. When the destructor is non-trivial and doesn't get evaluated at all after we inlined a constructor, we'd get all sorts of leak FPs and such.
Btw, one thing that i forgot to mention is that we have r290140 (https://bugs.llvm.org/show_bug.cgi?id=15599) - we currently generate a sink whenever we construct an object into a temporary and its destructor is *no-return*, losing coverage between the constructor and the destructor.
* Constructing into temporaries is disabled when destructor is
non-trivial. At least, we should be able to inline those when the
destructor is present at all, so that it would be evaluated
conservatively. One thing to note here is that our CFG has been
recently fixed, so we're much closer to fixing this properly
nowadays. However, CFG alone is not enough to figure out which
destructor needs to be called; for instance, if a
lifetime-extended temporary is initialized with an operator?:
expression, we'd need path-sensitive information to figure out
which object to destroy.* Temporaries are also special because our pass-by-value is not
always working correctly. In particular, when we inline 'foo(c)',
where variable 'c' is of 'class C', we first copy 'c' into a
temporary region, and then trivial-copy it into the stack
variable-region of the function parameter, while we should be
constructing directly into the parameter region. We cannot
construct directly into the parameter region because the stack
frame has not yet been constructed, because arguments are not yet
computed. More reading on the subject, even if a bit outdated, is
at
http://clang-developers.42468.n3.nabble.com/Inlining-temporary-destructors-in-the-analyzer-td4040851.html
<http://clang-developers.42468.n3.nabble.com/Inlining-temporary-destructors-in-the-analyzer-td4040851.html>
This is a hard problem, but i wasn't noticing many instances of it
yet.I am wondering what the analyzer does with copy elision? Do we model that somehow?
Not much, i think, and not sure if we should. When the standard is not explicitly forcing us to copy (or to avoid copying), i think that out of the two, we'd rather do the thing that takes less code to implement - it doesn't sound like a huge performance win if we elide a copy (though i might be wrong).
* Base object region constructors are disabled when destructor is
non-trivial. This sounds like an accidental omission.Nice catch.
* Construction into ElementRegion is disabled regardless of
destructors. In particular, mass array constructions are disabled.
There is a special AST for this case, which emulates the loop
through the array (or return value of operator new) with a loop
counter variable and all. We have no support for this whole
language construct. Note, however, that ElementRegion is much more
than array element; it is often used for representing casts, and
in particular it appears in return values of a conservatively
evaluated operator new() (i.e. element{SymRegion}) and is likely
to appear in placement-new() return values for the same reason. So
we should discriminate between these two cases.Do you know why casts are represented this way? Is this something that we might want to change in the future? I think one of the reasons why construction into ElementRegion is disabled because arrays are populated lazily by the analyzer. And this does not work well with code that relies on the fact that the number of constructor/destructor invocations is the same as the number of elements in the array.
I wish to come up with a better representation of pointer casts for years, but i still have no better idea. On one hand, i don't see why the cast should at all be a part of the SVal which merely represents the pointer value (as opposed to MemRegion which represents a segment), on the other hand we need to do pointer arithmetic in SValBuilder somehow, and we often have no other type to rely on. Suggestions are very welcome.
For now when we construct arrays, we model the first constructor and invalidate the rest of the array. The first constructor is, of course, not inlined, because, well, it's into an ElementRegion.
Hi Artem!
I've observed a lot of false positives on WebKit that resulted
from our inability to inline relatively simple functions in C++,
specifically constructors and operator new(), due to unimplemented
modeling. So i wanted to document, and eventually fix, a variety
of language feature modeling we're currently lacking. Note that
WebKit, like LLVM, is not a typical C++ codebase; it uses many
language constructs that regular projects don't use, and uses very
little standard library headers.Great news!
---
Normally we expect the analyzer to work even without any inlining,
by conservatively invalidating the program state, which eliminates
the clearly false assumptions but sometimes causes infeasible
paths when the invalidation was too aggressive and we start
denoting the same value with a different symbol after invalidation
and assume contradictory stuff on the old and the new symbol. The
false positives resulting from aggressive invalidation are usually
treated as less scary than the ones suppressed by invalidation,
because they can be easily suppressed (with assertions or const
qualifiers etc.) without loss of coverage. However, in C++ there
are a lot of implicit function calls which cause massive
frustration when evaluated conservatively. For example,class C {
bool haveX;
class D d;public:
C(int *x): haveX(x != 0) {
if (haveX)
*x = 1; // null dereference??
}
};In this case, 'haveX' should assume that x is non-null. The code
eagerly splits states into {x == 0} and {x != 0}, which seems
reasonable. However, after 'haveX' have been initialized, the
default constructor for field 'd' kicks in. If only this
constructor is not inlined, or any of its callees are not inlined,
value stored in 'haveX' would be invalidated on both paths. In
particular, the path on which the original 'haveX' is false but
the invalidated 'haveX' is true have now opened up.I observed the very same pattern on Ericsson internal codebases, so sorting this out is definitely a huge win for many projects (not just LLVM and WebKit).
---
Inlining of the constructor itself is disabled in many cases for
many reasons. In particular, we are currently only trying to
inline the constructor when the target this-region is a DeclRegion
(variable, member variable, Objective-C++ instance var), and the
destructor is non-trivial. This cuts away a lot of cases:It is not clear for my why the non-trivial destructor is a requirement here. Do yo have any info on that?
* Constructing into temporaries is disabled when destructor is
non-trivial. At least, we should be able to inline those when the
destructor is present at all, so that it would be evaluated
conservatively. One thing to note here is that our CFG has been
recently fixed, so we're much closer to fixing this properly
nowadays. However, CFG alone is not enough to figure out which
destructor needs to be called; for instance, if a
lifetime-extended temporary is initialized with an operator?:
expression, we'd need path-sensitive information to figure out
which object to destroy.* Temporaries are also special because our pass-by-value is not
always working correctly. In particular, when we inline 'foo(c)',
where variable 'c' is of 'class C', we first copy 'c' into a
temporary region, and then trivial-copy it into the stack
variable-region of the function parameter, while we should be
constructing directly into the parameter region. We cannot
construct directly into the parameter region because the stack
frame has not yet been constructed, because arguments are not yet
computed. More reading on the subject, even if a bit outdated, is
at
http://clang-developers.42468.n3.nabble.com/Inlining-temporary-destructors-in-the-analyzer-td4040851.html
<http://clang-developers.42468.n3.nabble.com/Inlining-temporary-destructors-in-the-analyzer-td4040851.html>
This is a hard problem, but i wasn't noticing many instances of it
yet.I am wondering what the analyzer does with copy elision? Do we model that somehow?
* Base object region constructors are disabled when destructor is
non-trivial. This sounds like an accidental omission.Nice catch.
Actually not so much. This bailout only fires for CK_Complete constructor kinds, and base-object constructors aren't "complete" in this sense, so they work fine.
This might still affect a complete constructor that follows placement-new into a base-object region (i'd be shocked to see code that actually does it).
Speaking of the code sample from the first e-mail: does anyone has an
explanation why it is the non-inlined constructor being a special
case? If you replace with an arbitrary non-inlined function call
between the haveX initialization and the "if" condition the analysis
seem to work fine. Compare assembly in [1] with the "D d;" line
commented out and active.
Alexey
In this particular example this happens because we may potentially have the constructor of D defined as
D::D() {
bool *haveXptr = ((void *)this) - (offsetof(C, d) - offsetof(C, x));
*haveXptr = !*haveXptr;
}
which is valid code that changes haveX between initialization and assignment.
However, an external function is unlikely to contain the 'this' pointer (pointer to variable 'c'). At least in your code generation example, where 'c' is a local variable and the pointer to it never escapes to the global scope.
Generally, in the analyzer when we don't know if something can happen, we often prefer to assume the worst, much like in codegen. In this particular example i'd be for toning down the invalidation (as in http://lists.llvm.org/pipermail/cfe-dev/2017-November/056103.html), but there may be more reasons for invalidating stuff, some of which sound much more convincing.
Thanks!
I had the "this" pointer leaking to D:
in mind but thought that the
offsetof trick would involve UB at some point. It seems that as long
there's is an actual boolean where D: looks for it this is probably
fine, even though I'm having a hard time with proving it from the
standard wording.
Alexey
I've observed a lot of false positives on WebKit that resulted from our inability to inline relatively simple functions in C++, specifically constructors and operator new(), due to unimplemented modeling. So i wanted to document, and eventually fix, a variety of language feature modeling we're currently lacking. Note that WebKit, like LLVM, is not a typical C++ codebase; it uses many language constructs that regular projects don't use, and uses very little standard library headers.
---
Normally we expect the analyzer to work even without any inlining, by conservatively invalidating the program state, which eliminates the clearly false assumptions but sometimes causes infeasible paths when the invalidation was too aggressive and we start denoting the same value with a different symbol after invalidation and assume contradictory stuff on the old and the new symbol. The false positives resulting from aggressive invalidation are usually treated as less scary than the ones suppressed by invalidation, because they can be easily suppressed (with assertions or const qualifiers etc.) without loss of coverage. However, in C++ there are a lot of implicit function calls which cause massive frustration when evaluated conservatively. For example,
class C {
bool haveX;
class D d;public:
C(int *x): haveX(x != 0) {
if (haveX)
*x = 1; // null dereference??
}
};In this case, 'haveX' should assume that x is non-null. The code eagerly splits states into {x == 0} and {x != 0}, which seems reasonable. However, after 'haveX' have been initialized, the default constructor for field 'd' kicks in. If only this constructor is not inlined, or any of its callees are not inlined, value stored in 'haveX' would be invalidated on both paths. In particular, the path on which the original 'haveX' is false but the invalidated 'haveX' is true have now opened up.
---
Inlining of the constructor itself is disabled in many cases for many reasons. In particular, we are currently only trying to inline the constructor when the target this-region is a DeclRegion (variable, member variable, Objective-C++ instance var), and the destructor is non-trivial. This cuts away a lot of cases:
* Constructing into temporaries is disabled when destructor is non-trivial. At least, we should be able to inline those when the destructor is present at all, so that it would be evaluated conservatively. One thing to note here is that our CFG has been recently fixed, so we're much closer to fixing this properly nowadays. However, CFG alone is not enough to figure out which destructor needs to be called; for instance, if a lifetime-extended temporary is initialized with an operator?: expression, we'd need path-sensitive information to figure out which object to destroy.
* Temporaries are also special because our pass-by-value is not always working correctly. In particular, when we inline 'foo(c)', where variable 'c' is of 'class C', we first copy 'c' into a temporary region, and then trivial-copy it into the stack variable-region of the function parameter, while we should be constructing directly into the parameter region. We cannot construct directly into the parameter region because the stack frame has not yet been constructed, because arguments are not yet computed. More reading on the subject, even if a bit outdated, is at http://clang-developers.42468.n3.nabble.com/Inlining-temporary-destructors-in-the-analyzer-td4040851.html This is a hard problem, but i wasn't noticing many instances of it yet.
* Base object region constructors are disabled when destructor is non-trivial. This sounds like an accidental omission.
* Construction into ElementRegion is disabled regardless of destructors. In particular, mass array constructions are disabled. There is a special AST for this case, which emulates the loop through the array (or return value of operator new) with a loop counter variable and all. We have no support for this whole language construct. Note, however, that ElementRegion is much more than array element; it is often used for representing casts, and in particular it appears in return values of a conservatively evaluated operator new() (i.e. element{SymRegion}) and is likely to appear in placement-new() return values for the same reason. So we should discriminate between these two cases.
* Constructing into return value of operator new() is disabled completely anyway, because there's a modeling problem that causes us to be unable to connect the constructor with the correct this-region. The CFG part of this problem was fixed by Jordan in 2014 by adding the CFGNewAllocator element, so we now actually call the operator and the constructor in the correct order, but we still need to pass the operator new's return value to the constructor. Note how pointless it is to enable it, or even inline a custom operator new, until construction into ElementRegion is fixed.
One more syntax pattern that we don't support was unveiled in https://reviews.llvm.org/D40841 : construction as part of a bigger aggregate initialization would be never inlined, neither it is performed with a correct this-region. This includes cases like
struct A { A(); }; // not an aggregate, has constructor.
struct B { A a; }; // aggregate, no constructor needed.
B b = {}; // A() is not handled correctly here.
Or, since C++17, also
struct B: public A {};
works similarly.