llvm dev - Apr 2017 - [RFC] Design of a TBAA sanitizer

Hi,

On April 11, 2017 at 11:55:12 AM, Kostya Serebryany via llvm-dev
(llvm-dev at lists.llvm.org) wrote:
> Evgeniy and I recently discussed something similar for detecting bad casts
> (code named: TypeSanitizer).
> The approach with the shadow memory looked attractive at the first glance,
> but then we've drowned in details.
>
> Specifically for TBAA, I had another idea, not involving shadow memory.
> Consider LLVM queries MayAlias(P1, P2) and the result is false, i.e. we
> think that P1 and P2 point to disjoint memory regions.
> Then we insert a run-time check that intervals [P1,sizeof(*P1)) and
> [P2,sizeof(*P2)) don't intersect.
>
> For functions with a reasonable number of pointer pairs where MayAlias(P1,
> P2)==false we could insert checks for all such pairs.
I'm not very clear on how this will fit into TBAA -- I don't think
TBAA decides aliasing relation between pointers, but it decides
aliasing relation between accesses (!tbaa metadata only exists on
accesses).

This means, at least in LLVM IR, you have to account for cases like:

if (A)
  *(int *)ptr = 20;
if (B)
  float f = *(float *)ptr;

where ptr does not have a type (according to TBAA), but you want to
crash the program if both A and B are true.

-- Sanjoy
> For larger functions -- only for those pairs where the optimizer actually
> queried MayAlias(P1, P2).
>
> --kcc
>
>
> On Tue, Apr 11, 2017 at 3:49 AM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> >
> > On 04/11/2017 03:46 AM, Andrey Bokhanko wrote:
> >
> > 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?
> >
> >
> > Good point. You (likely) wouldn't want to compile your memcpy /
memset /
> > etc. implementations with the TBAA sanitizer enabled (or we'd need
to
> > provide an attribute to disable it for certain functions). If they
only use
> > char* themselves, then it's fine, but if not, that's
implementation magic.
> > However, memset, etc. does need to be able to 'unset' the type
of memory
> > and memcpy needs to be able to copy types (at least for PODs). The
> > sanitizer would need to hook them for that purpose.
> >
> > -Hal
> >
> >
> >
> > Yours,
> > Andrey
> > ==> > Compiler Architect
> > NXP
> >
> >
> > On Tue, Apr 11, 2017 at 12:05 AM, Hal Finkel 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
> >>
> >>
> >
> > --
> > 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
> >
> >
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

On Tue, Apr 11, 2017 at 1:25 PM, Sanjoy Das <sanjoy at
playingwithpointers.com>
wrote:
> Hi,
>
> On April 11, 2017 at 11:55:12 AM, Kostya Serebryany via llvm-dev
> (llvm-dev at lists.llvm.org) wrote:
> > Evgeniy and I recently discussed something similar for detecting bad
> casts
> > (code named: TypeSanitizer).
> > The approach with the shadow memory looked attractive at the first
> glance,
> > but then we've drowned in details.
> >
> > Specifically for TBAA, I had another idea, not involving shadow
memory.
> > Consider LLVM queries MayAlias(P1, P2) and the result is false, i.e.
we
> > think that P1 and P2 point to disjoint memory regions.
> > Then we insert a run-time check that intervals [P1,sizeof(*P1)) and
> > [P2,sizeof(*P2)) don't intersect.
> >
> > For functions with a reasonable number of pointer pairs where
> MayAlias(P1,
> > P2)==false we could insert checks for all such pairs.
>
> I'm not very clear on how this will fit into TBAA -- I don't think
> TBAA decides aliasing relation between pointers, but it decides
> aliasing relation between accesses (!tbaa metadata only exists on
> accesses).
>
of course, but accesses are done via pointers, and if TBAA queries
MayAlias(AccessViaP1, AccessViaP2)
there should (??) be a point in the IR where both P1 and P2 exist together
and can be compared.

>
> This means, at least in LLVM IR, you have to account for cases like:
>
> if (A)
>   *(int *)ptr = 20;
> if (B)
>   float f = *(float *)ptr;
>
> where ptr does not have a type (according to TBAA), but you want to
> crash the program if both A and B are true.
>
> -- Sanjoy
>
> > For larger functions -- only for those pairs where the optimizer
actually
> > queried MayAlias(P1, P2).
> >
> > --kcc
> >
> >
> > On Tue, Apr 11, 2017 at 3:49 AM, Hal Finkel via llvm-dev <
> > llvm-dev at lists.llvm.org> wrote:
> >
> > >
> > > On 04/11/2017 03:46 AM, Andrey Bokhanko wrote:
> > >
> > > 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?
> > >
> > >
> > > Good point. You (likely) wouldn't want to compile your memcpy
/ memset
> /
> > > etc. implementations with the TBAA sanitizer enabled (or we'd
need to
> > > provide an attribute to disable it for certain functions). If
they
> only use
> > > char* themselves, then it's fine, but if not, that's
implementation
> magic.
> > > However, memset, etc. does need to be able to 'unset' the
type of
> memory
> > > and memcpy needs to be able to copy types (at least for PODs).
The
> > > sanitizer would need to hook them for that purpose.
> > >
> > > -Hal
> > >
> > >
> > >
> > > Yours,
> > > Andrey
> > > ==> > > Compiler Architect
> > > NXP
> > >
> > >
> > > On Tue, Apr 11, 2017 at 12:05 AM, Hal Finkel 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
> > >>
> > >>
> > >
> > > --
> > > 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
> > >
> > >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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Hi Kostya,

On April 11, 2017 at 1:30:10 PM, Kostya Serebryany (kcc at google.com) wrote:
> of course, but accesses are done via pointers, and if TBAA queries
> MayAlias(AccessViaP1, AccessViaP2)
> there should (??) be a point in the IR where both P1 and P2 exist together
> and can be compared.
That may not be possible (though I'm second guessing what exactly you
have in mind so maybe I'm missing something here):

ptr0 = malloc();
*(int*)ptr0 = 20;  // access0
free(ptr0);
ptr1 = calloc();   // bitwise equal to ptr0 by chance
float f = *(float *)ptr1;  // access1

The program above is fine (no TBAA violations), but at location
access1 ptr1 and ptr0 overlap despite being NoAlias.

-- Sanjoy
>
>
> >
> > This means, at least in LLVM IR, you have to account for cases like:
> >
> > if (A)
> > *(int *)ptr = 20;
> > if (B)
> > float f = *(float *)ptr;
> >
> > where ptr does not have a type (according to TBAA), but you want to
> > crash the program if both A and B are true.
> >
> > -- Sanjoy
> >
> > > For larger functions -- only for those pairs where the optimizer
actually
> > > queried MayAlias(P1, P2).
> > >
> > > --kcc
> > >
> > >
> > > On Tue, Apr 11, 2017 at 3:49 AM, Hal Finkel via llvm-dev <
> > > llvm-dev at lists.llvm.org> wrote:
> > >
> > > >
> > > > On 04/11/2017 03:46 AM, Andrey Bokhanko wrote:
> > > >
> > > > 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?
> > > >
> > > >
> > > > Good point. You (likely) wouldn't want to compile your
memcpy / memset
> > /
> > > > etc. implementations with the TBAA sanitizer enabled (or
we'd need to
> > > > provide an attribute to disable it for certain functions).
If they
> > only use
> > > > char* themselves, then it's fine, but if not, that's
implementation
> > magic.
> > > > However, memset, etc. does need to be able to
'unset' the type of
> > memory
> > > > and memcpy needs to be able to copy types (at least for
PODs). The
> > > > sanitizer would need to hook them for that purpose.
> > > >
> > > > -Hal
> > > >
> > > >
> > > >
> > > > Yours,
> > > > Andrey
> > > > ==> > > > Compiler Architect
> > > > NXP
> > > >
> > > >
> > > > On Tue, Apr 11, 2017 at 12:05 AM, Hal Finkel 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
> > > >>
> > > >>
> > > >
> > > > --
> > > > 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
> > > >
> > > >
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > llvm-dev at lists.llvm.org
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > >
> >
>