On Fri, Oct 01, 2021 at 01:27:06PM +0100, Mark Rutland wrote: > > So we may need to get rid of .fixup altogether. Especially for arches > > which support livepatch. > > > > We can replace some of the custom .fixup handlers with generic handlers > > like x86 does, which do the fixup work in exception context. This > > generally works better for more generic work like putting an error code > > in a certain register and resuming execution at the subsequent > > instruction. > > I reckon even ignoring the unwind problems this'd be a good thing since > it'd save on redundant copies of the fixup logic that happen to be > identical, and the common cases like uaccess all fall into this shape. > > As for how to do that, in the past Peter and I had come up with some > assembler trickery to get the name of the error code register encoded > into the extable info: > > https://lore.kernel.org/lkml/20170207111011.GB28790@leverpostej/ > https://lore.kernel.org/lkml/20170207160300.GB26173@leverpostej/ > https://lore.kernel.org/lkml/20170208091250.GT6515@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > ... but maybe that's already solved on x86 in a different way? That's really cool :-) But it might be overkill for x86's needs. For the exceptions which rely on handlers rather than anonymous .fixup code, the register assumptions are hard-coded in the assembler constraints. I think that works well enough. > > However a lot of the .fixup code is rather custom and doesn't > > necessarily work well with that model. > > Looking at arm64, even where we'd need custom handlers it does appear we > could mostly do that out-of-line in the exception handler. The more > exotic cases are largely in out-of-line asm functions, where we can move > the fixups within the function, after the usual return. > > I reckon we can handle the fixups for load_unaligned_zeropad() in the > exception handler. > > Is there anything specific that you think is painful in the exception > handler? Actually, after looking at all the x86 .fixup usage, I think we can make this two-pronged approach work. Either move the .fixup code to an exception handler (with a hard-coded assembler constraint register) or put it in the function (out-of-line where possible). I'll try to work up some patches (x86 only of course). -- Josh
