On Tue, 2024-04-02 at 09:43 +0800, maobibo wrote: > > Sorry for the late reply, but I think it may be a bit non-constructive > > to repeatedly submit the same code without due explanation in our > > previous review threads. Let me try to recollect some of the details > > though... > Because your review comments about hypercall method is wrong, I need not > adopt it. Again it's unfair to say so considering the lack of LVZ documentation. /* snip */ > > 1. T0-T7 are scratch registers during SYSCALL ABI, this is what you > suggest, does there exist information leaking to user space from T0-T7 > registers? It's not a problem. When syscall returns RESTORE_ALL_AND_RET is invoked despite T0-T7 are not saved. So a "junk" value will be read from the leading PT_SIZE bytes of the kernel stack for this thread. The leading PT_SIZE bytes of the kernel stack is dedicated for storing the struct pt_regs representing the reg file of the thread in the userspace. Thus we may only read out the userspace T0-T7 value stored when the same thread was interrupted or trapped last time, or 0 (if the thread was never interrupted or trapped before). And it's impossible to read some data used by the kernel internally, or some data of another thread. But indeed there is some improvement here. Zeroing these registers seems cleaner than reading out the junk values, and also faster (move $t0, $r0 is faster than ld.d $t0, $sp, PT_R12). Not sure if it's worthy to violate Huacai's "keep things simple" aspiration though. -- Xi Ruoyao <xry111@xxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
