Stop Memsetting Structures

Post by mehrdadn » Sat, Apr 27, 2019, 7:44 PM UTC

Someone wrote: post #12

anyfoo wrote: post #2

Does that also zero the padding that may exist between elements of the structure (or at its end)? Because if not, not memset'ing will open you up to all kinds of info leaks, especially if you plan to send that struct over the network (or to another process, or from kernel to userspace...). EDIT: Actually I have to apologize, because I’m not certain what the C standard says about whether the padding bytes can change w…

Sending structures that include padding or that you don’t know the exact layout of over the network is another thing you shouldn’t do, period. Code doing that may break with a compiler change or a compilation flag change on one end of the connection, even if you can guarantee both ends use the same CPU.

Post by ThrowawayR2 » Sat, Apr 27, 2019, 7:44 PM UTC

cperciva wrote: post #7

Another reason to not memset structures: In the code struct foo { void * bar; } baz; ... memset(&baz, 0, sizeof(struct foo)); assert(baz.bar == NULL); it's possible for the assertion to fail, since NULL it not guaranteed to be represented in memory by a zero bytes.

Post by anyfoo » Sat, Apr 27, 2019, 7:45 PM UTC

Someone wrote: post #12

anyfoo wrote: post #2

Does that also zero the padding that may exist between elements of the structure (or at its end)? Because if not, not memset'ing will open you up to all kinds of info leaks, especially if you plan to send that struct over the network (or to another process, or from kernel to userspace...). EDIT: Actually I have to apologize, because I’m not certain what the C standard says about whether the padding bytes can change w…

Sending structures that include padding or that you don’t know the exact layout of over the network is another thing you shouldn’t do, period. Code doing that may break with a compiler change or a compilation flag change on one end of the connection, even if you can guarantee both ends use the same CPU.

Post by snops » Sat, Apr 27, 2019, 7:49 PM UTC

  enum color {RED, GREEN, BLUE};
  char *color_name[] = {
    [RED] = "Red",
    [GREEN] = "Green",
    [BLUE] = "Blue"
  };

  printf("%s",color_name[RED]);

Post by Someone » Sat, Apr 27, 2019, 7:54 PM UTC

dataflow wrote: post #14

anyfoo wrote: post #2

Does that also zero the padding that may exist between elements of the structure (or at its end)? Because if not, not memset'ing will open you up to all kinds of info leaks, especially if you plan to send that struct over the network (or to another process, or from kernel to userspace...). EDIT: Actually I have to apologize, because I’m not certain what the C standard says about whether the padding bytes can change w…

Nope it doesn't: //GCC: -O0 //MSVC: /Od #include #include struct S { unsigned char x; int y; }; int f(int f) { S a; memset(&a, f, sizeof(a)); unsigned char y[2]; memcpy(&y, &a, sizeof(y)); return y[1]; } int g() { S a = { 1, 2 }; unsigned char y[2]; memcpy(&y, &a, sizeof(y)); return y[1]; } int main(int argc, char *argv[]) { f(0xDD); printf("%#x\n", g()); f(0xFF); printf("%#x\n", g()); } It definitely seems like the…