On Thu, Jan 14, 2021 at 06:10:13PM +0000, Mark Rutland wrote: > On Wed, Jan 13, 2021 at 04:25:41PM -0600, Josh Poimboeuf wrote: > > On Wed, Jan 13, 2021 at 08:23:15PM +0000, Mark Brown wrote: > > > On Wed, Jan 13, 2021 at 01:33:13PM -0600, Josh Poimboeuf wrote: > > > > > > > I think it's worth mentioning a little more about objtool. There are a > > > > few passing mentions of objtool's generation of metadata (i.e. ORC), but > > > > objtool has another relevant purpose: stack validation. That's > > > > particularly important when it comes to frame pointers. > > > > > > > For some architectures like x86_64 and arm64 (but not powerpc/s390), > > > > it's far too easy for a human to write asm and/or inline asm which > > > > violates frame pointer protocol, silently causing the violater's callee > > > > to get skipped in the unwind. Such architectures need objtool > > > > implemented for CONFIG_STACK_VALIDATION. > > > > > > This basically boils down to just adding a statement saying "you may > > > need to depend on objtool" I think? > > > > Right, but maybe it would be a short paragraph or two. > > I reckon that's a top-level section between requirements and > consideration along the lines of: > > 3. Compile-time analysis > ======================== > > To ensure that kernel code can be correctly unwound in all cases, > architectures may need to verify that code has been compiled in a manner > expected by the unwinder. For example, an unwinder may expect that > functions manipulate the stack pointer in a limited way, or that all > functions use specific prologue and epilogue sequences. Architectures > with such requirements should verify the kernel compilation using > objtool. > > In some cases, an unwinder may require metadata to correctly unwind. > Where necessary, this metadata should be generated at build time using > objtool. Sounds good to me. -- Josh
