Hi Janis, On Wed, Oct 12, 2022 at 10:56:01PM +0200, Janis Schoetterl-Glausch wrote: > Add cmpxchg functionality similar to that in cmpxchg.h except that the > target is a user space address and that the address' storage key is > matched with the access_key argument in order to honor key-controlled > protection. > The access is performed by changing to the secondary-spaces mode and > setting the PSW key for the duration of the compare and swap. > > Signed-off-by: Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> > --- > > > Possible variations: > * check the assumptions made in cmpxchg_user_key_size and error out > * call functions called by copy_to_user > * access_ok? is a nop > * should_fail_usercopy? > * instrument_copy_to_user? doesn't make sense IMO > * don't be overly strict in cmpxchg_user_key > > > arch/s390/include/asm/uaccess.h | 189 ++++++++++++++++++++++++++++++++ > 1 file changed, 189 insertions(+) So finally I send the uaccess/cmpxchg patches in reply to this mail. Sorry for the long delay! The first three patches are not required for the functionality you need, but given that I always stress that the code should be consistent I include them anyway. The changes are probably quite obvious: - Keep uaccess cmpxchg code more or less identical to regular cmpxchg code. I wasn't able to come up with a readable code base which could be used for both variants. - Users may only use the cmpxchg_user_key() macro - _not_ the inline function, which is an internal API. This will require that you need to add a switch statement and couple of casts within the KVM code, but shouldn't have much of an impact on the generated code. - Cause link error for non-integral sizes, similar to other uaccess functions. - cmpxchg_user_key() has now a simple return value: 0 or -EFAULT, and writes the old value to a location provided by a pointer. This is quite similar to the futex code. Users must compare the old and expected value to figure out if something was exchanged. Note that this is in most cases more efficient than extracting the condition code from the PSW with ipm, since nowadays we have instructions like compare and branch relative on condition, etc. - Couple of other minor changes which I forgot. Code is untested (of course :) ). Please give it a try and let me know if this is good enough for your purposes. I also did not limit the number of retries for the one and two byte scenarion. Before doing that we need to have proof that there really is a problem. Maybe Nico or you will give this a try. Thanks, Heiko
