Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass

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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.

-- 
Catalin



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