Clang Address Sanitizer + Linker Set + Linker Section Alignment?

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: 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:

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

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)