[llvm-dev] [RFC] Design of a TBAA sanitizer

Tue Apr 11 01:46:42 PDT 2017

Hal,

To clarify, my example meant to illustrate that for memory references to
structures' fields you have to keep a user-defined type, even for one byte
accesses. C++ allows references to "initial member sequence" using pointers
to structures of different types. And yes, there are programs in the wild
that rely on this (don't remember details... this was from previous life).

Another thing to consider -- what about memset / memcpy / etc that
inherently rely on type punning? If not inlined, they don't present
problems for an optimizer -- probably shouldn't be considered as aliasing
rules violation?

Yours,
Andrey
===
Compiler Architect
NXP

On Tue, Apr 11, 2017 at 12:05 AM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 04/10/2017 09:55 AM, Andrey Bokhanko wrote:
>
> Hi Hal,
>
> I wonder how your solution will handle the following?
>
> struct {
>   int s1_f1;
>   float s1_f2;
>   int s1_f3;
>   float s1_f4;
> } S1;
>
> struct {
>   int s2_f1;
>   float s2_f2;
>   int *s2_f3; // to add some interest, suppose that sizeof(int) ==
> sizeof(int *)
>   float s2_f4;
> } S2;
>
> S1 *s1; S2 *s2;
> ...
> s2 = (S1*)s1;
> s2->s2_f1 = 0; // allowed
> s2->s2_f2 = 0; // allowed
> s2->s2_f3 = 0; // not allowed
> s2->s2_f4 = 0; // not allowed
>
> Also, when you plan to set types for allocated memory?
>
>
> The most-general thing seems to be to set the types along with a store. As
> a result, the proposed scheme would not find a fault with the code above,
> but would complain if anyone actually later read S1.s1_f3.
>
> If we want to catch these kinds of problems directly we'd need to have the
> compiler insert code when the type is constructed to mark the types, and
> then we'd need to check those types around stores. This also sounds like a
> useful enhancement (although somewhat more complicated to implement).
>
> What types will be set for memory allocated by a malloc call?
>
>
> Memory would be untyped (or of unknown type) when allocated.
>
> Thanks again,
> Hal
>
>
>
> Yours,
> Andrey
>
>
> On Tue, Apr 4, 2017 at 10:13 PM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi everyone,
>>
>> At EuroLLVM, Chandler and I chatted about the design for a potential TBAA
>> sanitizer. Here's my attempt to summarize:
>>
>> C/C++ have type-based aliasing rules, and LLVM's optimizer can exploit
>> these given TBAA metadata added by Clang. Roughly, a pointer of given type
>> cannot be used to access an object of a different type (with, of course,
>> certain exceptions). Unfortunately, there's a lot of code in the wild that
>> violates these rules (e.g. for type punning), and such code often must be
>> built with -fno-strict-aliasing. Performance is often sacrificed as a
>> result. Part of the problem is the difficulty of finding TBAA violations. A
>> sanitizer would help.
>>
>> A design goal of a TBAA sanitizer is to limit the shadow-memory overhead
>> of the implementation. ASan, for example, uses 1 bit per byte. Here we're
>> hoping to keep the overhead down to 2 bits per byte for the TBAA
>> sanitizing. We might be able to do this, while handling all common types on
>> the fast path, if we use both alignment and type information. When
>> accessing data of B bytes, 2*B bits of shadow memory can be used. Thus,
>> we'll get 2 bits for a one-byte type, 4 bits for a two-byte type, etc.
>> Moreover, we need appropriate holes in the encoding space so that no type
>> has a shadow encoding that overlaps with an aligned part of a larger type's
>> encoding.
>> For example, we need to detect:
>>
>>   double f = ...; return *(int*) &f; // We should catch this.
>>
>> We might use the following encoding. The idea is that the common case,
>> for which we need a reasonable fast path, is that type types are exactly
>> equal. For this case, we want a simple comparison of the shadow type
>> encodings to be sufficient to validate the access. For cases where the
>> encodings don't match (and isn't zero to indicate an unknown type), or for
>> which there is no direct encoding for the access type, a slow path must be
>> used. All of this assumes that we're validating the the pointer alignment
>> first, and then checking the type encodings.
>>
>> 1 Byte:
>> 00 = 0  = unknown type
>> 01 = 1 = hole
>> 10 = 2 = hole
>> 11 = 3  = all one-byte types (slow path, see note later on this)
>>
>> 2 Bytes:
>> 0000 = 0 = unknown type
>> 0101 = 5 = short
>> 0110 = 6 = hole (A)
>> 0111 = 7 = wchar_t (under some ABIs)
>> 1001 = 9 = hole (B)
>> 1010 = 10 = hole (C)
>> 1011 = 11 = char16_t
>> 1111 = 15 = all other types (slow path)
>>
>> It is important here to have wchar_t have a direct encoding here because
>> wchar_t is two bytes on Windows, and moreover, wchar_t is very commonly
>> used on Windows. The partial encoding overlap of wchar_t (i.e. 0111) with
>> the 11 one-byte-type encoding works because 11 always indicates a slow-path
>> check.
>>
>> 4 Bytes:
>> 0000 0000 = 0 = unknown type
>> A A = int
>> A B = float
>> B A  = pointer (under some ABIs)
>> B B = long (under some ABIs)
>> A 1111 = wchar_t (under some ABIs)
>> B 1111 = char32_t
>> A C = hole (D)
>> C A = hole (E)
>> B C = hole (F)
>> C B = hole (G)
>> C C = hole (H)
>> 1111 1111 = 255 = all other types (slow path)
>>
>> 8 Bytes:
>> 0000 0000 0000 0000 = 0 = unknown type
>> D D = double
>> D E = long (under some ABIs)
>> E D = long long (under some ABIs)
>> E E = long double (under some ABIs)
>> D F = pointer (under some ABIs)
>> F D = hole (I)
>> E F = hole (J)
>> F E = hole
>> F F = hole
>> ...
>> 1111 1111 1111 1111 = 65535 = all other types
>>
>> 16 Bytes:
>> 0 = unknown type
>> | | = __int128_t
>> I J = long long (under some ABIs)
>> J I = long double (under some ABIs)
>> J J = hole
>> ...
>> -1 = all other types
>>
>> For pointers, this scheme would consider all pointers to be the same
>> (regardless of pointee type). Doing otherwise would mostly requiring
>> putting pointer-type checking on the slow path (i.e. access via a pointer
>> pointer), and that could add considerable overhead. We might, however,
>> split out function pointers from other pointers. We could provide a
>> compile-time option to control the granularity of pointer-type checks.
>>
>> Builtin vector types for which the vector element type has a direct
>> encoding also naturally have a direct encoding (the concatenation of the
>> encoding for the element type).
>>
>> Obviously the fact that we have no fast-path encodings for one-byte types
>> could be problematic. Note however that:
>>
>>  1. If a larger type is being used to access a smaller type (plus more),
>> the encodings won't match, so we always end up on the slow path.
>>
>>  2. If the access type is a one-byte type, we would want to validate
>> quickly. However, most common one-byte types are universally aliasing (i.e.
>> not subject to TBAA violations). Specifically, for C/C++, pointers to char,
>> unsigned char, signed char (C only), and std::byte, can be used to access
>> any part of any type. That leaves signed char (C++ only), bool/_Bool, and
>> enums with a [signed/unsigned] char base type (C++ only, std::byte
>> exempted) as pointee types we might wish to validate. We'd always need to
>> fall back to the slow path to validate these. We could provide a
>> compile-time option to disable such one-byte access checking if necessary.
>>
>> How would the slow path work? First, the code needs to find the beginning
>> of the allocation. It can do this by scanning backwards in the ASan shadow
>> memory. Once located, we'll read a pointer to a type-description structure
>> from that "red zone" location. For dynamic allocations, ASan's allocator
>> will ensure such a space for the pointer exists. For static allocations and
>> globals, the compiler will ensure it exists. The compiler will make sure
>> that all constructors locate this field and fill it in. Destructors can
>> clear it. If two of these type-description-structure pointers are equal,
>> then we can conclude that the types are equal. If not, then we need to
>> interpret the structure. The pointer itself might be to an interval map (to
>> deal with arrays, placement new, etc. - we can use the low bit of the
>> pointer to differentiate between an actual type-description structure and
>> an interval map), and the leaves of the interval map point to actual
>> type-description structures. The type-description structure is an array of
>> (offset, type) pairs, where the type field is also a
>> type-description-structure pointer. The type-description structures
>> themselves are comdat globals emitted in each relevant translation unit,
>> where the comdat key is formed using the mangled type name (and size,
>> etc.), and pointers to these symbols are then used to identify the types.
>>
>> Thoughts?
>>
>> Thanks again,
>> Hal
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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