From: Oleg Nesterov > Sent: 16 December 2024 10:13 > > David, > > let me say first that my understanding of this magic is very limited, > please correct me. I only (half) understand what the 'magic' has to accomplish and some of the pitfalls. I've copied linux-mm - someone there might know more. > On 12/16, David Laight wrote: > > > > It all depends on how hard __replace_page() tries to be atomic. > > The page has to change from one backed by the executable to a private > > one backed by swap - otherwise you can't write to it. > > This is what uprobe_write_opcode() does, And will be enough for single byte changes - they'll be picked up at some point after the change. > > But the problems arise when the instruction prefetch unit has read > > part of the 5-byte instruction (it might even only read half a cache > > line at a time). > > I'm not sure how long the pipeline can sit in that state - but I > > can do a memory read of a PCIe address that takes ~3000 clocks. > > (And a misaligned AVX-512 read is probably eight 8-byte transfers.) > > > > So I think you need to force an interrupt while the PTE is invalid. > > And that need to be simultaneous on all cpu running that process. > > __replace_page() does ptep_get_and_clear(old_pte) + flush_tlb_page(). > > That's not enough? I doubt it. As I understand it. The hardware page tables will be shared by all the threads of a process. So unless you hard synchronise all the cpu (and flush the TLB) while the PTE is being changed there is always the possibility of a cpu picking up the new PTE before the IPI that (I presume) flush_tlb_page() generates is processed. If that happens when the instruction you are patching is part-read into the instruction decode buffer then you'll execute a mismatch of the two instructions. I can't remember the outcome of discussions about live-patching kernel code - and I'm sure that was aligned 32bit writes. > > > Stopping the process using ptrace would do it. > > Not an option :/ Thought you'd say that. David > > Oleg. - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
