On Mon, Nov 19, 2018 at 05:17:26AM +0000, Jethro Beekman wrote: > On 2018-11-18 18:32, Jarkko Sakkinen wrote: > > On Sun, Nov 18, 2018 at 09:15:48AM +0200, Jarkko Sakkinen wrote: > > > On Thu, Nov 01, 2018 at 10:53:40AM -0700, Andy Lutomirski wrote: > > > > Hi all- > > > > > > > > The people working on SGX enablement are grappling with a somewhat > > > > annoying issue: the x86 EENTER instruction is used from user code and > > > > can, as part of its normal-ish operation, raise an exception. It is > > > > also highly likely to be used from a library, and signal handling in > > > > libraries is unpleasant at best. > > > > > > > > There's been some discussion of adding a vDSO entry point to wrap > > > > EENTER and do something sensible with the exceptions, but I'm > > > > wondering if a more general mechanism would be helpful. > > > > > > I haven't really followed all of this discussion because I've been busy > > > working on the patch set but for me all of these approaches look awfully > > > complicated. > > > > > > I'll throw my own suggestion and apologize if this has been already > > > suggested and discarded: return-to-AEP. > > > > > > My idea is to do just a small extension to SGX AEX handling. At the > > > moment hardware will RAX, RBX and RCX with ERESUME parameters. We can > > > fill extend this by filling other three spare registers with exception > > > information. > > > > > > AEP handler can then do whatever it wants to do with this information > > > or just do ERESUME. > > > > A correction here. In practice this will add a requirement to have a bit > > more complicated AEP code (check the regs for exceptions) than before > > and not just bytes for ENCLU. > > > > e.g. AEP handler should be along the lines > > > > 1. #PF (or #UD or) happens. Kernel fills the registers when it cannot > > handle the exception and returns back to user space i.e. to the > > AEP handler. > > 2. Check the registers containing exception information. If they have > > been filled, take whatever actions user space wants to take. > > 3. Otherwise, just ERESUME. > > > > From my point of view this is making the AEP parameter useful. Its > > standard use is just weird (always point to a place just containing > > ENCLU bytes, why the heck it even exists). > > I like this solution. Keeps things simple. One question: when an exception > occurs, how does the kernel know whether to set special registers or send a > signal? Yes, and AFAIK people do in many cases people want to do something else than just direct ERESUME in AEP handler so would neither be a major bummer for user space. If I remember correctly you have such? You can check the cases that we have for SIGSEGV (namely EPCM conflict) from Sean's patch 08/23. I'm open for expanding the scope. It is the easy part after there is consensus for the handling mechanism :-) /Jarkko
