[PATCH 34/64] fortify: Detect struct member overflows in memcpy() at compile-time

[Rasmus Villemoes]

On 27/07/2021 22.58, Kees Cook wrote:

> At its core, FORTIFY_SOURCE uses the compiler's __builtin_object_size()
> internal[0] to determine the available size at a target address based on
> the compile-time known structure layout details. It operates in two
> modes: outer bounds (0) and inner bounds (1). In mode 0, the size of the
> enclosing structure is used. In mode 1, the size of the specific field
> is used. For example:
> 
> 	struct object {
> 		u16 scalar1;	/* 2 bytes */
> 		char array[6];	/* 6 bytes */
> 		u64 scalar2;	/* 8 bytes */
> 		u32 scalar3;	/* 4 bytes */
> 	} instance;
> 
>
> __builtin_object_size(instance.array, 0) == 18, since the remaining size
> of the enclosing structure starting from "array" is 18 bytes (6 + 8 + 4).

I think the compiler would usually end up making that struct size 24,
with 4 bytes of trailing padding (at least when alignof(u64) is 8). In
that case, does __builtin_object_size(instance.array, 0) actually
evaluate to 18, or to 22? A quick test on x86-64 suggests the latter, so
the memcpy(, , 20) would not be a violation.

Perhaps it's better to base the example on something which doesn't have
potential trailing padding - so either add another 4 byte member, or
also make scalar2 u32.

Rasmus