Memalign interceptor reporting false positives

[vportal]

Hello,

I have compiled the program below with ASAN (gcc -fsanitize=address test.c -o test -ggdb)

#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <malloc.h>

#define PAGE_SIZE 4096

typedef uint64_t uint64;
typedef uint32_t uint32;
typedef uint16_t uint16;
typedef uint8_t uint8;

typedef struct {
     uint8 field1      :5,
           field2      :3;
     uint8             :6,
           field3      :1,
           field4      :1;
     uint8 field5      :3,
           field6      :3,
           field7      :2;
     uint8 field8      :4,
           field9      :1,
           field10     :1,
           field11     :1,
           field12     :1;
     uint8 field13;
} TestObj;

int main()
{
    void *buf = memalign(PAGE_SIZE, 3);
    if(buf)
    {
        TestObj *obj = buf;
        obj->field5=0x0;
    }
    free(buf);

    return 0;
}

There is no out of bound access when obj->field5 is accessed, but ASAN is reporting this access as a heap overflow.

==400==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x625000001000 at pc 0x5ba8f6558266 bp 0x7fff671dae80 sp 0x7fff671dae70
WRITE of size 4 at 0x625000001000 thread T0
    #0 0x5ba8f6558265 in main /home/vportal/ahci/ahci2.c:36
    #1 0x7a718d232082 in __libc_start_main ../csu/libc-start.c:308
    #2 0x5ba8f655810d in _start (/home/vportal/ahci/ahci2+0x110d)

0x625000001003 is located 0 bytes to the right of 3-byte region [0x625000001000,0x625000001003)
allocated by thread T0 here:
    #0 0x7a718d50e005 in __interceptor_memalign ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:169
    #1 0x5ba8f65581e3 in main /home/vportal/ahci/ahci2.c:32
    #2 0x7a718d232082 in __libc_start_main ../csu/libc-start.c:308

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/vportal/ahci/ahci2.c:36 in main
Shadow bytes around the buggy address:
  0x0c4a7fff81b0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff81c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff81d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff81e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff81f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c4a7fff8200:[03]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff8210: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff8220: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff8230: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff8240: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c4a7fff8250: 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
  Shadow gap:              cc
==400==ABORTING

Regards,
Victor