[cfe-dev] Is this a bug or just illegal code
Richard Smith via cfe-dev
cfe-dev at lists.llvm.org
Thu Jun 8 11:16:27 PDT 2017
On 8 June 2017 at 11:13, Hubert Tong via cfe-dev <cfe-dev at lists.llvm.org>
wrote:
> On Thu, Jun 8, 2017 at 2:05 PM, don hinton via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> I'm seeing memory corruption when I use a literal string and std::string
>> in a ternary (?:) expression. If I change ss to ss.c_str() on line 5, the
>> corruption goes away. [1] is an AST diff, which might help explain it.
>>
>> Although clang doesn't give me an error or warning, is this code legal,
>> or is this a bug?
>>
>> thanks...
>> don
>>
>>
>> $ grep -n "." x.cpp
>> 1:#include "llvm/ADT/StringRef.h"
>> 3:int main() {
>> 4: const std::string ss;
>> 5: llvm::StringRef Ref = true ? "noexcept" : ss;
>>
> This creates a std::string temporary, leaving the StringRef pointing to an
> object whose lifetime ends at the end of the full expression.
> Maybe the StringRef interface can be adjusted to cause this to fail at
> compile time.
>
It's a nice idea, but IIRC last time I tried this it didn't work out due to
code like this relying on an implicit std::string -> StringRef conversion:
std::string idiomatically_return_a_string();
void idiomatically_take_a_string(StringRef);
void f() {
idiomatically_take_a_string(idiomatically_return_a_string());
}
6: std::string s = Ref;
>> 7: return 0;
>> 8:}
>>
>> $ ../../4.0.0/build/Release/bin/clang++ x.cpp -O1 -g -fsanitize=address
>> -fno-omit-frame-pointer -std=c++11 -I ../../llvm/include/ -I include
>>
>> $ ./a.out
>> =================================================================
>> ==14769==ERROR: AddressSanitizer: heap-use-after-free on address
>> 0x604000000068 at pc 0x0000004651b2 bp 0x7ffcc5b3a7d0 sp 0x7ffcc5b39f80
>> READ of size 8 at 0x604000000068 thread T0
>> #0 0x4651b1 in __interceptor_memcpy.part.36
>> /home/d80049854/projects/clang/4.0.0/llvm/projects/compilerr
>> t/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:655
>> #1 0x7f00fe4d10ed (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xc40ed)
>> #2 0x7f00fe4d298a in std::basic_string<char, std::char_traits<char>,
>> std::allocator<char> >::basic_string(char const*, unsigned long,
>> std::allocator<char> const&) (/usr/lib/x86_64-linux-gnu/lib
>> stdc++.so.6+0xc598a)
>> #3 0x50d3bc in llvm::StringRef::str() const
>> /home/d80049854/projects/clang/build/Debug/../../llvm/includ
>> e/llvm/ADT/StringRef.h:230:14
>> #4 0x50d2cd in llvm::StringRef::operator std::string() const
>> /home/d80049854/projects/clang/build/Debug/../../llvm/includ
>> e/llvm/ADT/StringRef.h:257:14
>> #5 0x50d0aa in main /home/d80049854/projects/clang
>> /build/Debug/x.cpp:6:19
>> #6 0x7f00fd522f44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so
>> .6+0x21f44)
>> #7 0x41a10b in _start (/home/d80049854/projects/clan
>> g/build/Debug/a.out+0x41a10b)
>>
>> 0x604000000068 is located 24 bytes inside of 33-byte region
>> [0x604000000050,0x604000000071)
>> freed by thread T0 here:
>> #0 0x5098b0 in operator delete(void*) /home/d80049854/projects/clang
>> /4.0.0/llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:126
>> #1 0x7f00fe4d11cd in std::basic_string<char, std::char_traits<char>,
>> std::allocator<char> >::~basic_string() (/usr/lib/x86_64-linux-gnu/lib
>> stdc++.so.6+0xc41cd)
>> #2 0x7f00fd522f44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so
>> .6+0x21f44)
>>
>> previously allocated by thread T0 here:
>> #0 0x508b70 in operator new(unsigned long)
>> /home/d80049854/projects/clang/4.0.0/llvm/projects/compiler-
>> rt/lib/asan/asan_new_delete.cc:82
>> #1 0x7f00fe4d0f68 in std::string::_Rep::_S_create(unsigned long,
>> unsigned long, std::allocator<char> const&) (/usr/lib/x86_64-linux-gnu/lib
>> stdc++.so.6+0xc3f68)
>>
>> SUMMARY: AddressSanitizer: heap-use-after-free
>> /home/d80049854/projects/clang/4.0.0/llvm/projects/compiler-
>> rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:655 in
>> __interceptor_memcpy.part.36
>> Shadow bytes around the buggy address:
>> 0x0c087fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> 0x0c087fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> 0x0c087fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> 0x0c087fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> 0x0c087fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> =>0x0c087fff8000: fa fa 00 00 00 00 00 01 fa fa fd fd fd[fd]fd fa
>> 0x0c087fff8010: fa fa 00 00 00 00 01 fa fa fa fa fa fa fa fa fa
>> 0x0c087fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>> 0x0c087fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>> 0x0c087fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>> 0x0c087fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>> Shadow byte legend (one shadow byte represents 8 application bytes):
>> Addressable: 00
>> Partially addressable: 01 02 03 04 05 06 07
>> Heap left redzone: fa
>> Freed heap region: fd
>> Stack left redzone: f1
>> Stack mid redzone: f2
>> Stack right redzone: f3
>> Stack after return: f5
>> Stack use after scope: f8
>> Global redzone: f9
>> Global init order: f6
>> Poisoned by user: f7
>> Container overflow: fc
>> Array cookie: ac
>> Intra object redzone: bb
>> ASan internal: fe
>> Left alloca redzone: ca
>> Right alloca redzone: cb
>> ==14769==ABORTING
>>
>> 1. AST diff:
>> < uses ss
>> > uses ss.c_str()
>>
>> 49566c49566
>> < | | |-CXXConstructorDecl 0xXXXXXXX <line:3010:7, col:XX> col:XX used
>> basic_string 'void (const char *, const class std::allocator<char> &)'
>> ---
>> > | | |-CXXConstructorDecl 0xXXXXXXX <line:3010:7, col:XX> col:XX
>> basic_string 'void (const char *, const class std::allocator<char> &)'
>> 49568,49573c49568
>> < | | | `-ParmVarDecl 0xXXXXXXX <col:XX, col:XX> col:XX __a 'const class
>> std::allocator<char> &' cinit
>> < | | | `-ExprWithCleanups 0xXXXXXXX <col:XX, col:XX> 'const class
>> std::allocator<char>':'const class std::allocator<char>' lvalue
>> < | | | `-MaterializeTemporaryExpr 0xXXXXXXX <col:XX, col:XX> 'const
>> class std::allocator<char>':'const class std::allocator<char>' lvalue
>> < | | | `-ImplicitCastExpr 0xXXXXXXX <col:XX, col:XX> 'const class
>> std::allocator<char>':'const class std::allocator<char>' <NoOp>
>> < | | | `-CXXBindTemporaryExpr 0xXXXXXXX <col:XX, col:XX> 'class
>> std::allocator<char>':'class std::allocator<char>' (CXXTemporary 0xXXXXXXX)
>> < | | | `-CXXTemporaryObjectExpr 0xXXXXXXX <col:XX, col:XX>
>> 'class std::allocator<char>':'class std::allocator<char>' 'void (void)
>> throw()'
>> ---
>> > | | | `-ParmVarDecl 0xXXXXXXX <col:XX, col:XX> col:XX __a 'const class
>> std::allocator<char> &'
>> 108945c108940
>> < | | |-CXXConstructorDecl 0xXXXXXXX <../../llvm/include/llvm/Support/Compiler.h:194:38,
>> ../../llvm/include/llvm/ADT/StringRef.h:96:49> line:95:18 used StringRef
>> 'void (const std::string &)'
>> ---
>> > | | |-CXXConstructorDecl 0xXXXXXXX <../../llvm/include/llvm/Support/Compiler.h:194:38,
>> ../../llvm/include/llvm/ADT/StringRef.h:96:49> line:95:18 StringRef
>> 'void (const std::string &)'
>> 111747,111765c111742,111749
>> < | `-CXXConstructExpr 0xXXXXXXX <col:XX, col:XX>
>> 'llvm::StringRef':'class llvm::StringRef' 'void (const std::string &)'
>> < | `-MaterializeTemporaryExpr 0xXXXXXXX <col:XX, col:XX>
>> 'const std::string':'const class std::basic_string<char>' lvalue
>> < | `-ConditionalOperator 0xXXXXXXX <col:XX, col:XX>
>> 'const std::string':'const class std::basic_string<char>'
>> < | |-CXXBoolLiteralExpr 0xXXXXXXX <col:XX> '_Bool'
>> true
>> < | |-CXXBindTemporaryExpr 0xXXXXXXX <col:XX> 'const
>> std::string':'const class std::basic_string<char>' (CXXTemporary 0xXXXXXXX)
>> < | | `-CXXConstructExpr 0xXXXXXXX <col:XX> 'const
>> std::string':'const class std::basic_string<char>' 'void (const class
>> std::basic_string<char> &)' elidable
>> < | | `-MaterializeTemporaryExpr 0xXXXXXXX <col:XX>
>> 'const std::string':'const class std::basic_string<char>' lvalue
>> < | | `-CXXBindTemporaryExpr 0xXXXXXXX <col:XX>
>> 'const std::string':'const class std::basic_string<char>' (CXXTemporary
>> 0xXXXXXXX)
>> < | | `-CXXConstructExpr 0xXXXXXXX <col:XX>
>> 'const std::string':'const class std::basic_string<char>' 'void (class
>> std::basic_string<char> &&) noexcept' elidable
>> < | | `-MaterializeTemporaryExpr 0xXXXXXXX
>> <col:XX> 'std::string':'class std::basic_string<char>' xvalue
>> < | | `-CXXBindTemporaryExpr 0xXXXXXXX
>> <col:XX> 'std::string':'class std::basic_string<char>' (CXXTemporary
>> 0xXXXXXXX)
>> < | | `-ImplicitCastExpr 0xXXXXXXX
>> <col:XX> 'std::string':'class std::basic_string<char>'
>> <ConstructorConversion>
>> < | | `-CXXConstructExpr 0xXXXXXXX
>> <col:XX> 'std::string':'class std::basic_string<char>' 'void (const char *,
>> const class std::allocator<char> &)'
>> < | | |-ImplicitCastExpr 0xXXXXXXX
>> <col:XX> 'const char *' <ArrayToPointerDecay>
>> < | | | `-StringLiteral 0xXXXXXXX
>> <col:XX> 'const char [9]' lvalue "noexcept"
>> < | | `-CXXDefaultArgExpr 0xXXXXXXX
>> <<invalid sloc>> 'const class std::allocator<char>':'const class
>> std::allocator<char>' lvalue
>> < | `-CXXBindTemporaryExpr 0xXXXXXXX <col:XX> 'const
>> std::string':'const class std::basic_string<char>' (CXXTemporary 0xXXXXXXX)
>> < | `-CXXConstructExpr 0xXXXXXXX <col:XX> 'const
>> std::string':'const class std::basic_string<char>' 'void (const class
>> std::basic_string<char> &)'
>> < | `-DeclRefExpr 0xXXXXXXX <col:XX> 'const
>> std::string':'const class std::basic_string<char>' lvalue Var 0xXXXXXXX
>> 'ss' 'const std::string':'const class std::basic_string<char>'
>> ---
>> > | `-CXXConstructExpr 0xXXXXXXX <col:XX, col:XX>
>> 'llvm::StringRef':'class llvm::StringRef' 'void (const char *)'
>> > | `-ConditionalOperator 0xXXXXXXX <col:XX, col:XX>
>> 'const char *'
>> > | |-CXXBoolLiteralExpr 0xXXXXXXX <col:XX> '_Bool' true
>> > | |-ImplicitCastExpr 0xXXXXXXX <col:XX> 'const char
>> *' <ArrayToPointerDecay>
>> > | | `-StringLiteral 0xXXXXXXX <col:XX> 'const char
>> [9]' lvalue "noexcept"
>> > | `-CXXMemberCallExpr 0xXXXXXXX <col:XX, col:XX>
>> 'const char *'
>> > | `-MemberExpr 0xXXXXXXX <col:XX, col:XX> '<bound
>> member function type>' .c_str 0xXXXXXXX
>> > | `-DeclRefExpr 0xXXXXXXX <col:XX> 'const
>> std::string':'const class std::basic_string<char>' lvalue Var 0xXXXXXXX
>> 'ss' 'const std::string':'const class std::basic_string<char>'
>>
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