2007/4/9, Ross Ridge <[EMAIL PROTECTED]> wrote:
Florian Weimer writes:
>Yeah, but that division is fairly expensive if it can't be performed
>at compile time. OTOH, if __compute_size is inlined in all places,
>code size does increase somewhat.
Well, I believe the assumption was that __compute_size would be inlined.
If you want to minimize code size and avoid the division then a library
function something like following might work:
void *__allocate_array(size_t num, size_t size, size_t max_num) {
if (num > max_num)
size = ~size_t(0);
else
size *= num;
return operator new[](size);
}
GCC would caclulate the constant "~size_t(0) / size" and pass it as the
third argument. You'ld be trading a multiply for a couple of constant
outgoing arguments, so the code growth should be small. Unfortunately,
you'd be trading what in most cases is a fast shift and maybe add or
two for slower multiply.
So long as whatever switch is used to enable this check isn't on by
default and its effect on code size and speed is documented, I don't
think it matters that much what those effects are. Anything that works
should make the people concerned about security happy. People more
concerned with size or speed aren't going to enable this feature.
Ross Ridge
Hi Ross Ridge,
I tuned it a little bit.
-----------------------------------------------------------------------------
#include <stddef.h>
void *__allocate_array_of_RossRidge(size_t num, size_t size, size_t max_num) {
if (num > max_num)
size = ~size_t(0);
else
size *= num;
return operator new[](size);
}
void *__allocate_array_of_JCPizarro(size_t num, size_t size, size_t max_num) {
if (num > max_num) return operator new[](~size_t(0));
return operator new[](size*num);
}
-----------------------------------------------------------------------------
_Z29__allocate_array_of_RossRidgejjj:
[ gcc v3.4.6 : 9 instructions ]
movl 4(%esp), %edx
cmpl 12(%esp), %edx
movl 8(%esp), %eax
orl $-1, %eax
imull %edx, %eax
pushl %eax
call _Znaj
popl %edx
ret
_Z29__allocate_array_of_RossRidgejjj:
[ gcc 4.1.3 20070326 (prerelease) : 8 instructions ]
movl 4(%esp), %eax
orl $-1, %ecx
cmpl 12(%esp), %eax
movl 8(%esp), %edx
movl %edx, %ecx
imull %eax, %ecx
movl %ecx, 4(%esp)
jmp _Znaj
_Z29__allocate_array_of_JCPizarrojjj:
[ gcc 4.1.3 20070326 (prerelease) and gcc 3.4.6 : 7 instructions ]
movl 4(%esp), %edx
cmpl 12(%esp), %edx
movl 8(%esp), %eax
movl $-1, 4(%esp)
imull %edx, %eax
movl %eax, 4(%esp)
jmp _Znaj
-----------------------------------------------------------------------------
J.C. Pizarro
