On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote: > The copy_mount_options() function takes a user pointer argument but not > a size. It tries to read up to a PAGE_SIZE. However, copy_from_user() is > not guaranteed to return all the accessible bytes if, for example, the > access crosses a page boundary and gets a fault on the second page. To > work around this, the current copy_mount_options() implementations > performs to copy_from_user() passes, first to the end of the current > page and the second to what's left in the subsequent page. > > Some architectures like arm64 can guarantee an exact copy_from_user() > depending on the size (since the arch function performs some alignment > on the source register). Introduce an arch_has_exact_copy_from_user() > function and allow copy_mount_options() to perform the user access in a > single pass. > > While this function is not on a critical path, the single-pass behaviour > is required for arm64 MTE (memory tagging) support where a uaccess can > trigger intra-page faults (tag not matching). With the current > implementation, if this happens during the first page, the function will > return -EFAULT. Do you know how much extra overhead we'd incur if we read at must one tag granule at a time, instead of PAGE_SIZE? I'm guessing that in practice strcpy_from_user() type operations copy much less than a page most of the time, so what we lose in uaccess overheads we _might_ regain in less redundant copying. Would need behchmarking though. [...] Cheers ---Dave
