On Tue, Apr 28, 2020 at 03:06:27PM +0100, Catalin Marinas wrote: > On Mon, Apr 27, 2020 at 05:56:42PM +0100, Dave P Martin wrote: > > 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? > > Our copy routines already read 16 bytes at a time, so that's the tag > granule. With current copy_mount_options() we have the issue that it > assumes a fault in the first page is fatal. > > Even if we change it to a loop of smaller uaccess, we still have the > issue of unaligned accesses which can fail without reading all that's > possible (i.e. the access goes across a tag granule boundary). > > The previous copy_mount_options() implementation (from couple of months > ago I think) had a fallback to byte-by-byte, didn't have this issue. > > > 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. > > strncpy_from_user() has a fallback to byte by byte, so we don't have an > issue here. > > The above is only for synchronous accesses. For async, in v3 I disabled > such checks for the uaccess routines. Fair enough, I hadn't fully got my head around what's going on here. (But see my other reply.) I was suspicious about the WARN_ON(), but I see people are on top of that. Cheers ---Dave
