[cfe-dev] Clang Address Sanitizer + Linker Set + Linker Section Alignment?

[Kostya Serebryany via cfe-dev]

Hi Kyle,

ASAN inserts redzones (padding) between C/C++ globals in order to detect
global buffer overflow.
If you are playing tricks with globals -- all bets are off.
I am not familiar with "linker sets" so can't comment on the specific
problem.
You probably need to make sure that your globals are not instrumented by
ASAN (have them in an assembler file, or in a .c file that is not
asan-ified)

--kcc

On Mon, Oct 5, 2015 at 10:06 AM, Kyle Manna via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi guys,
>
> I started playing around with Clang's address sanitizer on a project that
> uses linker sets to to store an array of structs in a linker section.
>
> The code runs fine on clang, but when I pass '-fsanitize=address' it
> breaks.  The spacing between the structs doubles.  Readelf reports an
> alignment of 32 (vs 8, sizeof(struct) is actually 32 bytes).
>
> Clang's docs warn that "AddressSanitizer is not expected to produce false
> positives"
> (ref: http://clang.llvm.org/docs/AddressSanitizer.html#issue-suppression).
> Am I making a bad assumption regarding the layout of linker sections?
>
> I have a simple GitHub project that reproduces the problem as well as a
> travis-ci build that repeats the same result:
> * https://github.com/kylemanna/cmocka-init
> * https://travis-ci.org/kylemanna/cmocka-init/builds
>
>
> With clang -fsanitize=address:
>
> $ nm -n ./alignment
> ...
> 000000000070ac40 D __start_cmocka_init
> 000000000070ac40 D test_Simple_Init
> 000000000070ac80 D test_Simple_Life
> 000000000070acc0 D test_Test0
> 000000000070ad00 D test_Test1
> 000000000070ad40 D __stop_cmocka_init
>
> $ readelf -S ./alignment
>   [27] cmocka_init       PROGBITS         000000000070ac40  0010ac40
>        0000000000000100  0000000000000000  WA       0     0     32
>
> Note the alignment = 32.
>
>
> Without clang -fsanitize=address:
>
> $ nm -n ./alignment
> ...
> 0000000000609800 D __start_cmocka_init
> 0000000000609800 D test_Simple_Init
> 0000000000609820 D test_Simple_Life
> 0000000000609840 D test_Test0
> 0000000000609860 D test_Test1
> 0000000000609880 D __stop_cmocka_init
>
> $ readelf -S ./alignment
>   [25] cmocka_init       PROGBITS         0000000000609800  00009800
>        0000000000000080  0000000000000000  WA       0     0     8
>
> Also, gcc's address sanitizer doesn't appear to do anything.  It ignores
> intentional issues (i.e. overflow) but also doesn't change the alignment.
>
>
> - Kyle
> ᐧ
>
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