[cfe-dev] [RFC] Extending and improving Clang's undefined behavior checking

[Richard Smith]

Hi,

There are three different (and mostly orthogonal, design-wise) areas where
I would like to make improvements to Clang's undefined behavior checking:

1) Completeness of checks. There are integer undefined behaviors which
-ftrapv and -fcatch-undefined-behavior don't catch, and there is almost no
checking available for any other undefined behaviors outside of those and
the ones caught by {Address,Thread,Memory} Sanitizer.

I would like to add support for checking the following additional undefined
behaviors, which Clang does not currently appear to be able to check for:

 - Using a null pointer or empty glvalue (a dereferenced null pointer) as
the object expression in a class member access expression or member
function call.
 - Using an object of polymorphic class type where either the static and
dynamic types are incompatible or the object's lifetime has not started or
has ended, in a delete-expression, class member access expression, member
function call, derived-to-virtual-base conversion, dynamic_cast, or typeid,
or using the value of a reference bound to such a glvalue.
 - Base-to-derived cast on wrong polymorphic type
 - Binding a reference to an inappropriately-aligned address or to an empty
lvalue
 - VLA size evaluates to a nonpositive value
 - Reaching the end of a function with a non-void return type, in C++ (in
C, this is only UB if the caller uses the return value)
 - Overflow in conversion from floating point to smaller floating point or
to integral type
 - Overflow in conversion from integral type to floating point (int128 to
float, or when converting to __fp16)
 - Converting an out-of-range integer to an enumerated type
 - Floating-point arithmetic overflow
 - Pointer arithmetic which leaves the bounds which can be determined by
llvm.objectsize
 - Pointer subtraction between pointers which can be determined to be in
different objects
 - Signed << where the LHS is negative, or where the result doesn't fit in
the result type (C99, C11) or doesn't fit in the corresponding unsigned
type (C++11)
 - Assignment where the LHS and RHS overlap but are not equal

Regehr et al's IOC has code to handle a few of these checks already, which
should be straightforward to apply to Clang trunk (assuming the IOC guys
are still happy with that?). Low overhead would be an explicit goal of all
of these checks.


2) Command-line interface. We currently have the following options to
enable various flavors of undefined behavior checks:
  -ftrapv
  -fcatch-undefined-behavior
  -faddress-sanitizer
  -fthread-sanitizer

I would like for us to have a single argument which enables all undefined
behavior checking which can be performed cheaply and with no changes to
address space layout or ABI; I propose we extend -fcatch-undefined-behavior
to be that argument. IOC adds the -ftrapv checks to
-fcatch-undefined-behavior (unless we're in -fwrapv mode), so we can easily
pick up that part of this change.

IOC also adds:
  -fcatch-undefined-ansic-behavior
  -fcatch-undefined-c99-behavior
  -fcatch-undefined-cxx0x-behavior
  -fcatch-undefined-cxx98-behavior
  -fcatch-undefined-nonarith-behavior

I think we should support this kind of configuration through a mechanism
similar to warning flags: -fcatch-undefined-behavior=c++11
-fno-catch-undefined-behavior=null-reference, for instance.

Also, I think we should consider renaming this switch (and/or possibly the
-f*-sanitizer switches) to provide a consistent set of names, but I don't
have a concrete proposal for that.


3) Handling and reporting of undefined behavior, once caught. The
sanitizers produce decent information (which is both useful and detailed),
but it could be prettier. The other checks just emit @llvm.trap(), which is
far from ideal.

IOC is in some ways quite ambitious here, and provides options to not only
diagnose a problem, but also to call a function which can produce a value
for the operation and recover. I'm not proposing adding such a feature to
Clang at this time (I'll leave that to the IOC folks).

I would like to propose adding a runtime library for the
-fcatch-undefined-behavior checks, and emit calls to relevant diagnostic
functions instead of calls to @llvm.trap. I'm planning on having one
library entry point per flavor of check, named __ubsan_report_<check_name>,
which would be passed relevant operand values, and a pointer to a structure
containing information about the context (file, line and column number,
operand types as strings, and possibly a code snippet to display with the
diagnostic).

As a strawman proposal for runtime error output, we could aim to mirror the
formatting style of compile-time diagnostics (since we've already got one
way of beautifully presenting diagnostic information, why invent another?).
For instance:

$ clang++ bad.cc -o bad -fcatch-undefined-behavior
$ ./bad
*test.cc:4:12: runtime error: left shift count 33 >= width of type 'int'
(32 bits)*
  return i << n;
           ^  ~
              33
*test.cc:7:30: note:* in call to 'f' here
Illegal instruction
$

It would seems sensible to provide a common library for producing such
diagnostic output and also use it in the sanitizers. Again, strawman output:

*/usr/include/c++/4.6.x-google/bits/stl_algobase.h:192:10: runtime
error:global buffer overflow reading address 0x2b29a5e899f4
*
      if (__b < __a)
          ^
*<memory>:0x2b29a5e899f4: note:* contents of nearby memory:
  ... e8 a5 29 2b 00 00  01 00 00 00 gg gg gg gg  gg gg gg gg gg gg gg gg
 gg gg ...
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~
  ...ace)::GetBucket(int)::threshold read
*<memory>:0x2b29a5e899e0: note:* read address is 0 bytes after global
variable '(anonymous namespace)::GetBucket(int)::threshold'
  ... gg gg gg gg  08 00 00 00 01 00 00 00  f4 99 e8 a5 29 2b 00 00  01 00
00 00 ...

 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   (anonymous namespace)::GetBucket(int)::threshold
*foo/bar.cc:863: note:* in call to 'std::min<int>(const int &, const int
&)' here
*foo/bar.cc:894: note:* in call to '(anonymous namespace)::GetBucket(int)'
here
[...]
*gunit/gunit_main.cc:59: note:* in call to 'testing::UnitTest::Run()' here
*<__libc_start_main>:0x2b29cb224d5d: note:* in call to 'main' here


This proposal intends to incorporate some, but not all, of IOC into
mainline Clang. The parts which I'm not currently intending to address as
part of this work are:
 * -fuse-intrinsic. This allows the choice of whether to use LLVM's
overflow intrinsics. I intend to use such intrinsics unconditionally.
 * -fuse-random-value. This allows recovery from detected undefined
behavior by providing a value for the operation, which is not part of my
proposal.
 * -fhandler-null. This selects the usage of an alternate runtime library.
 * -fchecks-num. This provides logging output when checks are inserted, and
would not be valuable to my target audience.
 * -fcatch-non-ubc-type. This is designed to catch unsigned integer
overflow, and some other cases which don't have undefined behavior, and so
is not directly connected to this work.

Does this sound good?
Richard
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