On Thu, Jan 23, 2020 at 10:03 AM Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > We used to have a read/write argument to the old "verify_area()" and > "access_ok()" model, and it was a mistake. It was due to odd i386 user > access issues. We got rid of it. I'm not convinced this is any better > - it looks very similar and for odd ppc access issues. If the mode (read or write) were made visible to the trap handler, I'd find that useful for machine check recovery. If I'm in the middle of a copy_from_user() and I get a machine check reading poison from a user address ... then I could try to recover in the same way as for the user accessing the poison (offline the page, SIGBUS the task). But if the poison is in kernel memory and we are doing a copy_to_user(), then we are hosed (or would need some more complex recovery plan). [Note that we only get recoverable machine checks on loads... writes are posted, so if something goes wrong it isn't synchronous with the store instruction that initiated it] -Tony
