Bug 662085

Created attachment 467992 [details]
Test case

ElectricFence malloc() wrapper did not return properly aligned memory pointer.
Aligned memory is a requirement of malloc() and must match the system ABI:

From Open Group Base Specifications Issue 7 / IEEE Std 1003.1-2008 :

"The pointer returned if the allocation succeeds shall be suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object in the space allocated (until the space is explicitly freed or reallocated)."


The attached test case could be run with and without ElectricFence.
The testcase allocte a few block of a given size, report the size, the address and the alignement of each block.

Without ElectricFence, on a x86_64 system where base alignment is 8 bytes, but 16 bytes is used (requirement for SSE):

$ ./malloc-alignment
   1: 0x1703010:   16 0x1703030:   16 0x1703050:   16 0x1703070:   16
   2: 0x1703010:   16 0x1703030:   16 0x1703050:   16 0x1703070:   16
   4: 0x1703010:   16 0x1703030:   16 0x1703050:   16 0x1703070:   16
   7: 0x1703010:   16 0x1703030:   16 0x1703050:   16 0x1703070:   16

With ElectricFence, still on x86_64 and EF_PROTECT_BELOW=1 which force malloc() to return a pointer to a memory page:

$ EF_PROTECT_BELOW=1 ef ./malloc-alignment

  Electric Fence 2.2.2 Copyright (C) 1987-1999 Bruce Perens <bruce>
   1: 0x7ffa306aa000: 4096 0x7ffa306ac000: 4096 0x7ffa306ae000: 4096 0x7ffa306b0000: 4096
   2: 0x7ffa306aa000: 4096 0x7ffa306ac000: 4096 0x7ffa306ae000: 4096 0x7ffa306b0000: 4096
   4: 0x7ffa306aa000: 4096 0x7ffa306ac000: 4096 0x7ffa306ae000: 4096 0x7ffa306b0000: 4096
   7: 0x7ffa306aa000: 4096 0x7ffa306ac000: 4096 0x7ffa306ae000: 4096 0x7ffa306b0000: 4096

With ElectricFence and EF_ALIGNMENT=16 which force an alignment of 16 bytes:

$ EF_ALIGNMENT=16 ef ./malloc-alignment

  Electric Fence 2.2.2 Copyright (C) 1987-1999 Bruce Perens <bruce>
   1: 0x7f5289642ff0:   16 0x7f5289644ff0:   16 0x7f5289646ff0:   16 0x7f5289648ff0:   16
   2: 0x7f5289642ff0:   16 0x7f5289644ff0:   16 0x7f5289646ff0:   16 0x7f5289648ff0:   16
   4: 0x7f5289642ff0:   16 0x7f5289644ff0:   16 0x7f5289646ff0:   16 0x7f5289648ff0:   16
   7: 0x7f5289642ff0:   16 0x7f5289644ff0:   16 0x7f5289646ff0:   16 0x7f5289648ff0:   16

And at last, with ElectricFence, still with default parameters

$ ef ./malloc-alignment

  Electric Fence 2.2.2 Copyright (C) 1987-1999 Bruce Perens <bruce>
   1: 0x7fb4334cdffc:    4 0x7fb4334cfffc:    4 0x7fb4334d1ffc:    4 0x7fb4334d3ffc:    4
   2: 0x7fb4334cdffc:    4 0x7fb4334cfffc:    4 0x7fb4334d1ffc:    4 0x7fb4334d3ffc:    4
   4: 0x7fb4334cdffc:    4 0x7fb4334cfffc:    4 0x7fb4334d1ffc:    4 0x7fb4334d3ffc:    4
   7: 0x7fb4334cdff8:    8 0x7fb4334cfff8:    8 0x7fb4334d1ff8:    8 0x7fb4334d3ff8:    8

With default parameters, ElectricFence returns memory address aligned on 4 bytes at minimum. This should be OK for such small allocations, but the testcase shows also bigger allocations:

  33: 0x7f4cc606dfdc:    4 0x7f4cc606ffdc:    4 0x7f4cc6071fdc:    4 0x7f4cc6073fdc:    4
  65: 0x7f4cc606dfbc:    4 0x7f4cc606ffbc:    4 0x7f4cc6071fbc:    4 0x7f4cc6073fbc:    4
 129: 0x7f4cc606df7c:    4 0x7f4cc606ff7c:    4 0x7f4cc6071f7c:    4 0x7f4cc6073f7c:    4
 257: 0x7f4cc606defc:    4 0x7f4cc606fefc:    4 0x7f4cc6071efc:    4 0x7f4cc6073efc:    4

And here, there's a problem: a block of 33 bytes could store a pointer, a double and an odd string, and *requires* an 8 bytes alignment which is not given by ElectricFence. 

The alignment problem is explained in efence(3) man page in third paragraph of WORD-ALIGNMENT AND OVERRUN DETECTION section : http://linux.die.net/man/3/efence

According to this man page "Unfortunately, malloc() is required to return word-aligned allocations," which is not exactly true regarding ABI and Open Group specification.

ElectricFence must aligned memory by default on the ABI requirements (16 bytes) in order to behave as expected by the applications.