[compiler-rt] r284901 - [sanitizers] support strict_string_checks for strncmp
Kostya Serebryany via llvm-commits
llvm-commits at lists.llvm.org
Fri Nov 11 17:24:05 PST 2016
On Fri, Nov 11, 2016 at 5:21 PM, Kostya Serebryany <kcc at google.com> wrote:
> btw, Nick, do you have a test where this fails?
>
> On Tue, Nov 8, 2016 at 5:51 PM, Nick Lewycky <nlewycky at google.com> wrote:
>
>> On 8 November 2016 at 17:20, Kostya Serebryany <kcc at google.com> wrote:
>>
>>>
>>>
>>>
>>>
>>> On Tue, Nov 8, 2016 at 2:30 PM, Nick Lewycky <nlewycky at google.com>
>>> wrote:
>>>
>>>> On 21 October 2016 at 16:52, Kostya Serebryany via llvm-commits <
>>>> llvm-commits at lists.llvm.org> wrote:
>>>>
>>>>> Author: kcc
>>>>> Date: Fri Oct 21 18:52:26 2016
>>>>> New Revision: 284901
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=284901&view=rev
>>>>> Log:
>>>>> [sanitizers] support strict_string_checks for strncmp
>>>>>
>>>>> Added:
>>>>> compiler-rt/trunk/test/asan/TestCases/strncmp_strict.c
>>>>> Modified:
>>>>> compiler-rt/trunk/lib/sanitizer_common/sanitizer_common_inte
>>>>> rceptors.inc
>>>>>
>>>>> Modified: compiler-rt/trunk/lib/sanitizer_common/sanitizer_common_inte
>>>>> rceptors.inc
>>>>> URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/sa
>>>>> nitizer_common/sanitizer_common_interceptors.inc?rev=284901&
>>>>> r1=284900&r2=284901&view=diff
>>>>> ============================================================
>>>>> ==================
>>>>> --- compiler-rt/trunk/lib/sanitizer_common/sanitizer_common_interceptors.inc
>>>>> (original)
>>>>> +++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_common_interceptors.inc
>>>>> Fri Oct 21 18:52:26 2016
>>>>> @@ -304,8 +304,8 @@ INTERCEPTOR(int, strncmp, const char *s1
>>>>> c2 = (unsigned char)s2[i];
>>>>> if (c1 != c2 || c1 == '\0') break;
>>>>> }
>>>>> - COMMON_INTERCEPTOR_READ_RANGE(ctx, s1, Min(i + 1, size));
>>>>> - COMMON_INTERCEPTOR_READ_RANGE(ctx, s2, Min(i + 1, size));
>>>>> + COMMON_INTERCEPTOR_READ_STRING(ctx, s1, Min(i + 1, size));
>>>>> + COMMON_INTERCEPTOR_READ_STRING(ctx, s2, Min(i + 1, size));
>>>>>
>>>>
>>>> I think this change is wrong, but please double check my analysis. I
>>>> think arguments to strncmp don't need to be NUL terminated like arguments
>>>> to strcmp do.
>>>>
>>>> C99 7.1.1 defines: "A string is a contiguous sequence of characters
>>>> terminated by and including the first null character." Then strcmp is
>>>> defined as a function which "compares the strings" in 7.21.4.2 while
>>>> strncmp is defined to operate over two "possibly null-terminated array"s in
>>>> 7.21.4.4. The bug manifests in COMMON_INTERCEPTOR_READ_STRING calling
>>>> REAL(strlen) on these arguments which aren't guaranteed to be NUL
>>>> terminated.
>>>>
>>>> What do you think?
>>>>
>>>
>>> Mmmm. Are these parts equivalent in C99 and C++ standards?
>>> At least here http://www.cplusplus.com/reference/cstring/strncmp/ I
>>> read:
>>> str1C string to be compared.str2C string to be compared.
>>>
>> I think they're simplifying. But cppreference.com also disagrees with
>> me: http://en.cppreference.com/w/cpp/string/byte/strncmp . They have
>> separate entries for strncmp in C and for C++ and they differ, for C++ it
>> says they compare two null-terminated strings, while in C it says they
>> compare two possibly null-terminated strings.
>>
>> The C++ standard doesn't define either strcmp or strncmp. I think the
>> normative text is in 21.8 [c.strings]:
>> "1 Tables 74, 75, 76, 77, 78, and 79 describe headers <cctype>,
>> <cwctype>, <cstring>, <cwchar>, <cstdlib>
>> (character conversions), and <cuchar>, respectively."
>>
>> The tables list all the various functions, table 76 includes both strcmp
>> and strncmp.
>>
>> "2 The contents of these headers shall be the same as the Standard C
>> Library headers <ctype.h>, <wctype.h>,
>> <string.h>, <wchar.h>, and <stdlib.h> and the C Unicode TR header
>> <uchar.h>, respectively, with the
>> following modifications:"
>>
>> The modifications that follow affect types char16_t, char32_t and wchar_t
>> and functions strchr, strpbrk, strrchr, strstr, memchr, wcschr, wcspbrk,
>> wcsrchr, wcsstr, wmemchr, strerror, strtok, as well as anything that takes
>> an mbstate_t*.
>>
>> I can't see anything else that would make the semantics different between
>> C and C++.
>>
>> I probably still need to replace this with something like
>>> COMMON_INTERCEPTOR_READ_RANGE((ctx), (s), \
>>> common_flags()->strict_string_checks ? size : (Min(i + 1, size)) )
>>> to avoid calling strlen
>>>
>>
>> I don't think that's quite right either. strncmp will read up to the
>> first NUL at which point it stops, or it reads up to size. That makes it:
>>
>> unsigned char c1 = 0, c2 = 0;
>> uptr i;
>> for (i = 0; i < size; i++) {
>> c1 = (unsigned char)s1[i];
>> c2 = (unsigned char)s2[i];
>> if (c1 != c2 || c1 == '\0') {
>> ++i; // include trailing NUL
>> break;
>> }
>> }
>> assert(i <= size);
>> COMMON_INTERCEPTOR_READ_RANGE((ctx), (s1), i)
>> COMMON_INTERCEPTOR_READ_RANGE((ctx), (s2), i)
>>
>> and there's no need to check for strict string handling?
>>
>> Nick
>>
>> Nick
>>>>
>>>> int result = CharCmpX(c1, c2);
>>>>> CALL_WEAK_INTERCEPTOR_HOOK(__sanitizer_weak_hook_strncmp,
>>>>> GET_CALLER_PC(), s1,
>>>>> s2, size, result);
>>>>>
>>>>> Added: compiler-rt/trunk/test/asan/TestCases/strncmp_strict.c
>>>>> URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/test/a
>>>>> san/TestCases/strncmp_strict.c?rev=284901&view=auto
>>>>> ============================================================
>>>>> ==================
>>>>> --- compiler-rt/trunk/test/asan/TestCases/strncmp_strict.c (added)
>>>>> +++ compiler-rt/trunk/test/asan/TestCases/strncmp_strict.c Fri Oct 21
>>>>> 18:52:26 2016
>>>>> @@ -0,0 +1,26 @@
>>>>> +// Test strict_string_checks option in strncmp function
>>>>> +// RUN: %clang_asan %s -o %t && %run %t 2>&1
>>>>> +// RUN: %env_asan_opts=strict_string_checks=false %run %t 2>&1
>>>>> +// RUN: %env_asan_opts=strict_string_checks=true not %run %t 2>&1 |
>>>>> FileCheck %s
>>>>> +
>>>>> +#include <assert.h>
>>>>> +#include <stdlib.h>
>>>>> +#include <string.h>
>>>>> +
>>>>> +int main(int argc, char **argv) {
>>>>> + size_t size = 100;
>>>>> + char fill = 'o';
>>>>> + char *s1 = (char*)malloc(size);
>>>>> + memset(s1, fill, size);
>>>>> + char *s2 = (char*)malloc(size);
>>>>> + memset(s2, fill, size);
>>>>> + s1[size - 1] = 'z';
>>>>> + s2[size - 1] = 'x';
>>>>> + int r = strncmp(s1, s2, size + 1);
>>>>> + // CHECK: {{.*ERROR: AddressSanitizer: heap-buffer-overflow on
>>>>> address}}
>>>>> + // CHECK: READ of size 101
>>>>> + assert(r == 1);
>>>>> + free(s1);
>>>>> + free(s2);
>>>>> + return 0;
>>>>> +}
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at lists.llvm.org
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>>>>>
>>>>
>>>>
>>>>
>>>
>>
>
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