lib: always clear size in crypt_safe_free
Writing into allocated memory right before calling free can be optimized away by smart compilers. To prevent this, a volatile access must be performed. This happens already in crypt_safe_memzero.
It was difficult to provoke GCC to remove the assignment, but I was able to find a way to prove the theory:
- Build cryptsetup with: CFLAGS="-flto -O3 -g" ./configure --enable-static
- Create main.c:
#include <libcryptsetup.h>
int
main(void) {
char *x = crypt_safe_alloc(64);
crypt_safe_free(x);
return 0;
}
- Build the program with: gcc -O3 -flto -static -o main main.c -lcryptsetup
- Disassemble: objdump -d main
My output on an amd64 system is:
0000000000401670 <main>:
401670: 41 54 push %r12
401672: bf f0 03 00 00 mov $0x3f0,%edi
401677: 55 push %rbp
401678: 48 83 ec 08 sub $0x8,%rsp
40167c: e8 ff 4d 01 00 callq 416480 <__libc_malloc>
401681: 48 85 c0 test %rax,%rax
401684: 74 2f je 4016b5 <main+0x45>
401686: 48 c7 00 e8 03 00 00 movq $0x3e8,(%rax)
40168d: 4c 8d 60 08 lea 0x8(%rax),%r12
401691: 48 89 c5 mov %rax,%rbp
401694: be e8 03 00 00 mov $0x3e8,%esi
401699: 4c 89 e7 mov %r12,%rdi
40169c: e8 4f 76 01 00 callq 418cf0 <explicit_bzero>
4016a1: 48 8b 75 00 mov 0x0(%rbp),%rsi
4016a5: 4c 89 e7 mov %r12,%rdi
4016a8: e8 43 76 01 00 callq 418cf0 <explicit_bzero>
4016ad: 48 89 ef mov %rbp,%rdi
4016b0: e8 3b 54 01 00 callq 416af0 <__free>
4016b5: 48 83 c4 08 add $0x8,%rsp
4016b9: 31 c0 xor %eax,%eax
4016bb: 5d pop %rbp
4016bc: 41 5c pop %r12
4016be: c3 retq
4016bf: 90 nop
You can see that the memory allocation and explicit_bzero calls were not optimized away. But the size assignment disappeared.
Compiling without -O3 or without -flto does not inline the calls and keeps the assignment. Also the shared library shipped with my distribution has the assignment.
Edited by Samanta Navarro