On Wed, Feb 16, 2022 at 5:19 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > From: Arnd Bergmann <arnd@xxxxxxxx> > > There are many different ways that access_ok() is defined across > architectures, but in the end, they all just compare against the > user_addr_max() value or they accept anything. > > Provide one definition that works for most architectures, checking > against TASK_SIZE_MAX for user processes or skipping the check inside > of uaccess_kernel() sections. > > For architectures without CONFIG_SET_FS(), this should be the fastest > check, as it comes down to a single comparison of a pointer against a > compile-time constant, while the architecture specific versions tend to > do something more complex for historic reasons or get something wrong. This isn't actually optimal. On x86, TASK_SIZE_MAX is a bizarre constant that has a very specific value to work around a bug^Wdesign error^Wfeature of Intel CPUs. TASK_SIZE_MAX is the maximum address at which userspace is permitted to allocate memory, but there is a huge gap between user and kernel addresses, and any value in the gap would be adequate for the comparison. If we wanted to optimize this, simply checking the high bit (which x86 can do without any immediate constants at all) would be sufficient and, for an access known to fit in 32 bits, one could get even fancier and completely ignore the size of the access. (For accesses not known to fit in 32 bits, I suspect some creativity could still come up with a construction that's substantially faster than the one in your patch.) So there's plenty of room for optimization here. (This is not in any respect a NAK -- it's just an observation that this could be even better.)
