On 2020-04-02 14:05, Ard Biesheuvel wrote: > On Thu, 2 Apr 2020 at 13:50, Peter Smith <Peter.Smith@xxxxxxx> wrote: >> >> > I take it this implies that the LLVM linker does not support the >> > R_ARM_ALU_PC_Gn relocations? Since otherwise, adrl could simply be >> > expanded to a pair of adds with the appropriate relocations, letting >> > the linker fix up the immediates (and the ADD vs SUB bits) >> >> Not at the moment. I have a patch in review to add the G0 variants for these in Arm state at reviews.llvm.org/D75349 . As far as I know LLVM MC does not have support for generating the relocations either. This could be added though. I agree that using the G* relocations with a pair of add/sub instructions would be the ideal solution. The adrl psuedo is essentially that but implemented at assembly time. I think it would be possible to implement in LLVM but at the time (4+ years ago) I wasn't confident in finding someone that would think that adrl support was worth the disruption, for example the current Arm assembly backend can only produce 1 instruction as output and adrl requires two. >> >> I'd be happy to look at group relocation support in LLD, I haven't got a lot of spare time so progress is likely to be slow though. >> > > For Linux, I have proposed another approach in the past, which is to > define a (Linux-local) adr_l macro with unlimited range [0], which > basically comes down to place relative movw/movt pairs for v7+, and > something along the lines of > > ldr <reg>, 222f > 111: add <reg>, <reg>, pc > .subsection 1 > 222: .long <sym> - (111b + 8) > .previous Just to confirm: The instance at hand today seems to be working fine without adrl, so I guess we are fine here, do you agree? There are a couple more instances of adrl in arch/arm/crypto/, maybe that is where the adr_l macro could come in. -- Stefan > > for v6 and earlier. Could you comment on whether Clang's integrated > assembler could support anything like this? > > > Thanks, > Ard. > > > > [0] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?h=arm-kaslr-latest&id=fd440f1131553a5201ce3b94905419bd067b93b3
