Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs

On Mon, 2 Jul 2018, Mathieu Desnoyers wrote:

quoted

Platforms with 32 bit word size only guarantee atomicity of a 32 bit
write or RMV instruction.

Special instructions may exist on a platform to perform 64 bit atomic
updates. We use cmpxchg64 f.e. on Intel 32 bit platforms to guarantee
atomicity8.

So use the macros that we have to guarantee 64 bit ops and you should be
fine. See linux/arch/x86/include/asm/atomic64_32.h

We are talking about user-space here. What we need is a single instruction
atomic store, similar to what WRITE_ONCE() does in the kernel. The discussion
is about whether doing the user-space equivalent of a WRITE_ONCE() to a u64
on a 32-bit architecture should be considered to provide single-copy atomicity
on the low 32 bits.

Right. You would need to make this work for userspace. atomic64_32.h is a
good reference as to which instructions provide 64 bit atomicity on 32
bit platforms.