The old assembly generated for the syscall intrinsic did not specify
clobber constraints. This adds RCX and R11 (that are clobbered by
the instruction itself), and memory (that is clobbered by some
system calls) to the assembly constraints.
Note: This is still incorrect on FreeBSD, which clobbers more registers
and uses the carry flag instead of -errno in rax to indicate an error.
This call is intended to provide the ability to securely scrub memory
without compiler interference, in a similar manner to explicit_bzero,
memset_s, SecureZeroMemory.
The approach taken is a volatile memset followed by a seqentially
consistent memory fence, to prevent the call from being optimized away
by DSE, and from being reordered. An identical approach is currently
being used by the zeroize Rust crate, and is effective in practice.
LLVM IR output:
```
; Function Attrs: nounwind
define internal i8* @mem.zero_explicit(i8* %0, i64 %1) #0 {
decls:
call void @llvm.memset.p0i8.i64(i8* %0, i8 0, i64 %1, i1 true)
fence seq_cst
ret i8* %0
}
```
Yawning Angel