Messages in this thread | | | Subject | Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user | From | Andy Lutomirski <> | Date | Sat, 16 Feb 2019 18:36:52 -0800 |
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> On Feb 16, 2019, at 3:47 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > >> On Sat, Feb 16, 2019 at 02:50:15PM -0800, Andy Lutomirski wrote: >> >> What is the actual problem? We’re not actually demand-faulting this data, are we? Are we just overrunning the buffer because the from_user helpers are too clever? Can we fix it for real by having the fancy helpers do *aligned* loads so that they don’t overrun the buffer? Heck, this might be faster, too. > > Unaligned _stores_ are not any cheaper, and you'd get one hell of > extra arithmetics from trying to avoid both. Check something > like e.g. memcpy() on alpha, where you really have to keep all > accesses aligned, both on load and on store side.
I think we should avoid unaligned loads and do unaligned stores instead.
I would general expect that unaligned stores are a bit cheaper since they don’t need to complete for the comparisons to happen.
But I maintain my claim that this code should not be overrunning its input buffer into the next page, since it could have observable side effects.
> > Can't we just pad the buffers a bit? Making sure that name_buf > and symlink_buf are _not_ followed by unmapped pages shouldn't > be hard. Both are allocated by kmalloc(), so... > > What am I missing here?
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