[UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour
Summary:
Quote from http://eel.is/c++draft/expr.add#4:
```
4 When an expression J that has integral type is added to or subtracted
from an expression P of pointer type, the result has the type of P.
(4.1) If P evaluates to a null pointer value and J evaluates to 0,
the result is a null pointer value.
(4.2) Otherwise, if P points to an array element i of an array object x with n
elements ([dcl.array]), the expressions P + J and J + P
(where J has the value j) point to the (possibly-hypothetical) array
element i+j of x if 0≤i+j≤n and the expression P - J points to the
(possibly-hypothetical) array element i−j of x if 0≤i−j≤n.
(4.3) Otherwise, the behavior is undefined.
```
Therefore, as per the standard, applying non-zero offset to `nullptr`
(or making non-`nullptr` a `nullptr`, by subtracting pointer's integral value
from the pointer itself) is undefined behavior. (*if* `nullptr` is not defined,
i.e. e.g. `-fno-delete-null-pointer-checks` was *not* specified.)
To make things more fun, in C (6.5.6p8), applying *any* offset to null pointer
is undefined, although Clang front-end pessimizes the code by not lowering
that info, so this UB is "harmless".
Since rL369789 (D66608 `[InstCombine] icmp eq/ne (gep inbounds P, Idx..), null -> icmp eq/ne P, null`)
LLVM middle-end uses those guarantees for transformations.
If the source contains such UB's, said code may now be miscompiled.
Such miscompilations were already observed:
* https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190826/687838.html
* https://github.com/google/filament/pull/1566
Surprisingly, UBSan does not catch those issues
... until now. This diff teaches UBSan about these UB's.
`getelementpointer inbounds` is a pretty frequent instruction,
so this does have a measurable impact on performance;
I've addressed most of the obvious missing folds (and thus decreased the performance impact by ~5%),
and then re-performed some performance measurements using my [[ https://github.com/darktable-org/rawspeed | RawSpeed ]] benchmark:
(all measurements done with LLVM ToT, the sanitizer never fired.)
* no sanitization vs. existing check: average `+21.62%` slowdown
* existing check vs. check after this patch: average `22.04%` slowdown
* no sanitization vs. this patch: average `48.42%` slowdown
Reviewers: vsk, filcab, rsmith, aaron.ballman, vitalybuka, rjmccall, #sanitizers
Reviewed By: rsmith
Subscribers: kristof.beyls, nickdesaulniers, nikic, ychen, dtzWill, xbolva00, dberris, arphaman, rupprecht, reames, regehr, llvm-commits, cfe-commits
Tags: #clang, #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D67122
llvm-svn: 374293
2019-10-10 17:25:02 +08:00
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// RUN: %clang_cc1 -x c -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefixes=CHECK,CHECK-NOSANITIZE
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// RUN: %clang_cc1 -x c -fsanitize=pointer-overflow -fno-sanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLNOTOK,CHECK-SANITIZE-NULLNOTOK-C,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-NORECOVER,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c -fsanitize=pointer-overflow -fsanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLNOTOK,CHECK-SANITIZE-NULLNOTOK-C,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-RECOVER
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// RUN: %clang_cc1 -x c -fsanitize=pointer-overflow -fsanitize-trap=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLNOTOK,CHECK-SANITIZE-NULLNOTOK-C,CHECK-SANITIZE-TRAP,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c -fno-delete-null-pointer-checks -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefixes=CHECK,CHECK-NOSANITIZE
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// RUN: %clang_cc1 -x c -fno-delete-null-pointer-checks -fsanitize=pointer-overflow -fno-sanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLOK,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-NORECOVER,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c -fno-delete-null-pointer-checks -fsanitize=pointer-overflow -fsanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLOK,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-RECOVER
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// RUN: %clang_cc1 -x c -fno-delete-null-pointer-checks -fsanitize=pointer-overflow -fsanitize-trap=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLOK,CHECK-SANITIZE-TRAP,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c++ -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefixes=CHECK,CHECK-NOSANITIZE
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// RUN: %clang_cc1 -x c++ -fsanitize=pointer-overflow -fno-sanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLNOTOK,CHECK-SANITIZE-NULLNOTOK-CPP,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-NORECOVER,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c++ -fsanitize=pointer-overflow -fsanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLNOTOK,CHECK-SANITIZE-NULLNOTOK-CPP,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-RECOVER
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// RUN: %clang_cc1 -x c++ -fsanitize=pointer-overflow -fsanitize-trap=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLNOTOK,CHECK-SANITIZE-NULLNOTOK-CPP,CHECK-SANITIZE-TRAP,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c++ -fno-delete-null-pointer-checks -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s --check-prefixes=CHECK,CHECK-NOSANITIZE
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// RUN: %clang_cc1 -x c++ -fno-delete-null-pointer-checks -fsanitize=pointer-overflow -fno-sanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLOK,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-NORECOVER,CHECK-SANITIZE-UNREACHABLE
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// RUN: %clang_cc1 -x c++ -fno-delete-null-pointer-checks -fsanitize=pointer-overflow -fsanitize-recover=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLOK,CHECK-SANITIZE-ANYRECOVER,CHECK-SANITIZE-RECOVER
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// RUN: %clang_cc1 -x c++ -fno-delete-null-pointer-checks -fsanitize=pointer-overflow -fsanitize-trap=pointer-overflow -emit-llvm %s -o - -triple x86_64-linux-gnu | FileCheck %s -implicit-check-not="call void @__ubsan_handle_pointer_overflow" --check-prefixes=CHECK,CHECK-SANITIZE,CHECK-SANITIZE-NULLOK,CHECK-SANITIZE-TRAP,CHECK-SANITIZE-UNREACHABLE
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// CHECK-SANITIZE-ANYRECOVER-DAG: @[[LINE_100:.*]] = {{.*}}, i32 100, i32 15 } }
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#ifdef __cplusplus
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extern "C" {
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#endif
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char *add_unsigned(char *base, unsigned long offset) {
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2020-02-04 02:09:39 +08:00
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// CHECK: define i8* @add_unsigned(i8* %[[BASE:.*]], i64 %[[OFFSET:.*]])
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[UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour
Summary:
Quote from http://eel.is/c++draft/expr.add#4:
```
4 When an expression J that has integral type is added to or subtracted
from an expression P of pointer type, the result has the type of P.
(4.1) If P evaluates to a null pointer value and J evaluates to 0,
the result is a null pointer value.
(4.2) Otherwise, if P points to an array element i of an array object x with n
elements ([dcl.array]), the expressions P + J and J + P
(where J has the value j) point to the (possibly-hypothetical) array
element i+j of x if 0≤i+j≤n and the expression P - J points to the
(possibly-hypothetical) array element i−j of x if 0≤i−j≤n.
(4.3) Otherwise, the behavior is undefined.
```
Therefore, as per the standard, applying non-zero offset to `nullptr`
(or making non-`nullptr` a `nullptr`, by subtracting pointer's integral value
from the pointer itself) is undefined behavior. (*if* `nullptr` is not defined,
i.e. e.g. `-fno-delete-null-pointer-checks` was *not* specified.)
To make things more fun, in C (6.5.6p8), applying *any* offset to null pointer
is undefined, although Clang front-end pessimizes the code by not lowering
that info, so this UB is "harmless".
Since rL369789 (D66608 `[InstCombine] icmp eq/ne (gep inbounds P, Idx..), null -> icmp eq/ne P, null`)
LLVM middle-end uses those guarantees for transformations.
If the source contains such UB's, said code may now be miscompiled.
Such miscompilations were already observed:
* https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190826/687838.html
* https://github.com/google/filament/pull/1566
Surprisingly, UBSan does not catch those issues
... until now. This diff teaches UBSan about these UB's.
`getelementpointer inbounds` is a pretty frequent instruction,
so this does have a measurable impact on performance;
I've addressed most of the obvious missing folds (and thus decreased the performance impact by ~5%),
and then re-performed some performance measurements using my [[ https://github.com/darktable-org/rawspeed | RawSpeed ]] benchmark:
(all measurements done with LLVM ToT, the sanitizer never fired.)
* no sanitization vs. existing check: average `+21.62%` slowdown
* existing check vs. check after this patch: average `22.04%` slowdown
* no sanitization vs. this patch: average `48.42%` slowdown
Reviewers: vsk, filcab, rsmith, aaron.ballman, vitalybuka, rjmccall, #sanitizers
Reviewed By: rsmith
Subscribers: kristof.beyls, nickdesaulniers, nikic, ychen, dtzWill, xbolva00, dberris, arphaman, rupprecht, reames, regehr, llvm-commits, cfe-commits
Tags: #clang, #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D67122
llvm-svn: 374293
2019-10-10 17:25:02 +08:00
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// CHECK-NEXT: [[ENTRY:.*]]:
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// CHECK-NEXT: %[[BASE_ADDR:.*]] = alloca i8*, align 8
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// CHECK-NEXT: %[[OFFSET_ADDR:.*]] = alloca i64, align 8
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// CHECK-NEXT: store i8* %[[BASE]], i8** %[[BASE_ADDR]], align 8
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// CHECK-NEXT: store i64 %[[OFFSET]], i64* %[[OFFSET_ADDR]], align 8
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// CHECK-NEXT: %[[BASE_RELOADED:.*]] = load i8*, i8** %[[BASE_ADDR]], align 8
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// CHECK-NEXT: %[[OFFSET_RELOADED:.*]] = load i64, i64* %[[OFFSET_ADDR]], align 8
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// CHECK-NEXT: %[[ADD_PTR:.*]] = getelementptr inbounds i8, i8* %[[BASE_RELOADED]], i64 %[[OFFSET_RELOADED]]
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// CHECK-SANITIZE-NEXT: %[[COMPUTED_OFFSET_AGGREGATE:.*]] = call { i64, i1 } @llvm.smul.with.overflow.i64(i64 1, i64 %[[OFFSET_RELOADED]]), !nosanitize
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// CHECK-SANITIZE-NEXT: %[[COMPUTED_OFFSET_OVERFLOWED:.*]] = extractvalue { i64, i1 } %[[COMPUTED_OFFSET_AGGREGATE]], 1, !nosanitize
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// CHECK-SANITIZE-NEXT: %[[COMPUTED_OFFSET:.*]] = extractvalue { i64, i1 } %[[COMPUTED_OFFSET_AGGREGATE]], 0, !nosanitize
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// CHECK-SANITIZE-NEXT: %[[BASE_RELOADED_INT:.*]] = ptrtoint i8* %[[BASE_RELOADED]] to i64, !nosanitize
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// CHECK-SANITIZE-NEXT: %[[COMPUTED_GEP:.*]] = add i64 %[[BASE_RELOADED_INT]], %[[COMPUTED_OFFSET]], !nosanitize
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// CHECK-SANITIZE-NULLNOTOK-NEXT: %[[BASE_IS_NOT_NULLPTR:.*]] = icmp ne i8* %[[BASE_RELOADED]], null, !nosanitize
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// CHECK-SANITIZE-NULLNOTOK-NEXT: %[[COMPUTED_GEP_IS_NOT_NULL:.*]] = icmp ne i64 %[[COMPUTED_GEP]], 0, !nosanitize
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// CHECK-SANITIZE-NULLNOTOK-C-NEXT: %[[BOTH_POINTERS_ARE_NULL_OR_BOTH_ARE_NONNULL:.*]] = and i1 %[[BASE_IS_NOT_NULLPTR]], %[[COMPUTED_GEP_IS_NOT_NULL]], !nosanitize
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// CHECK-SANITIZE-NULLNOTOK-CPP-NEXT: %[[BOTH_POINTERS_ARE_NULL_OR_BOTH_ARE_NONNULL:.*]] = icmp eq i1 %[[BASE_IS_NOT_NULLPTR]], %[[COMPUTED_GEP_IS_NOT_NULL]], !nosanitize
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// CHECK-SANITIZE-NEXT: %[[COMPUTED_OFFSET_DID_NOT_OVERFLOW:.*]] = xor i1 %[[COMPUTED_OFFSET_OVERFLOWED]], true, !nosanitize
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// CHECK-SANITIZE-NEXT: %[[COMPUTED_GEP_IS_UGE_BASE:.*]] = icmp uge i64 %[[COMPUTED_GEP]], %[[BASE_RELOADED_INT]], !nosanitize
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// CHECK-SANITIZE-NEXT: %[[GEP_DID_NOT_OVERFLOW:.*]] = and i1 %[[COMPUTED_GEP_IS_UGE_BASE]], %[[COMPUTED_OFFSET_DID_NOT_OVERFLOW]], !nosanitize
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// CHECK-SANITIZE-NULLNOTOK-NEXT: %[[GEP_IS_OKAY:.*]] = and i1 %[[BOTH_POINTERS_ARE_NULL_OR_BOTH_ARE_NONNULL]], %[[GEP_DID_NOT_OVERFLOW]], !nosanitize
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// CHECK-SANITIZE-NULLNOTOK-NEXT: br i1 %[[GEP_IS_OKAY]],
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// CHECK-SANITIZE-NULLOK-NEXT: br i1 %[[GEP_DID_NOT_OVERFLOW]],
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// CHECK-SANITIZE-SAME: label %[[CONT:.*]], label %[[HANDLER_POINTER_OVERFLOW:[^,]+]],{{.*}} !nosanitize
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// CHECK-SANITIZE: [[HANDLER_POINTER_OVERFLOW]]:
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// CHECK-SANITIZE-NORECOVER-NEXT: call void @__ubsan_handle_pointer_overflow_abort(i8* bitcast ({ {{{.*}}} }* @[[LINE_100]] to i8*), i64 %[[BASE_RELOADED_INT]], i64 %[[COMPUTED_GEP]])
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// CHECK-SANITIZE-RECOVER-NEXT: call void @__ubsan_handle_pointer_overflow(i8* bitcast ({ {{{.*}}} }* @[[LINE_100]] to i8*), i64 %[[BASE_RELOADED_INT]], i64 %[[COMPUTED_GEP]])
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// CHECK-SANITIZE-TRAP-NEXT: call void @llvm.trap(){{.*}}, !nosanitize
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// CHECK-SANITIZE-UNREACHABLE-NEXT: unreachable, !nosanitize
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// CHECK-SANITIZE: [[CONT]]:
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// CHECK-NEXT: ret i8* %[[ADD_PTR]]
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#line 100
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return base + offset;
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}
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#ifdef __cplusplus
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}
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#endif
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