Messages in this thread | | | Date | Tue, 21 Mar 2017 16:14:46 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/5] x86: Implement __WARN using UD0 |
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On Tue, Mar 21, 2017 at 09:03:40AM -0500, Josh Poimboeuf wrote: > On Fri, Mar 17, 2017 at 10:19:19PM +0100, Peter Zijlstra wrote:
> > +/* > > + * Since some emulators terminate on UD2, we cannot use it for WARN. > > + * Since various instruction decoders disagree on the length of UD1, > > + * we cannot use it either. So use UD0 for WARN. > > + * > > + * (binutils knows about "ud1" but {en,de}codes it as 2 bytes, whereas > > + * our kernel decoder thinks it takes a ModRM byte, which seems consistent > > + * with various things like the Intel SDM instruction encoding rules) > > + */ > > + > > +#define ASM_UD0 ".byte 0x0f, 0xff" > > +#define ASM_UD1 ".byte 0x0f, 0xb9" /* + ModRM */ > > +#define ASM_UD2 ".byte 0x0f, 0x0b" > > Thas ASM_UD1 macro isn't used anywhere.
I have it there for completeness sake, and documentation purposes.
> > --- a/arch/x86/kernel/traps.c > > +++ b/arch/x86/kernel/traps.c > > @@ -169,6 +169,41 @@ void ist_end_non_atomic(void) > > preempt_disable(); > > } > > > > +int is_valid_bugaddr(unsigned long addr) > > +{ > > + unsigned short ud; > > + > > +#ifdef CONFIG_X86_32 > > + if (addr < PAGE_OFFSET) > > + return 0; > > +#else > > + if ((long)addr > 0) > > + return 0; > > +#endif > > I think comparing with TASK_SIZE would be more correct and it wouldn't > need an ifdef.
Dunno about more correct (the TIF_ADDR32 case is irrelevant here), but yes, that would do away with the #ifdef.
> > + if (probe_kernel_address((unsigned short *)addr, ud)) > > + return 0; > > + > > + return ud == 0x0b0f || ud == 0xff0f; > > +} > > This code would be easier to grok if these were defines IMO.
Would be nice if I could use the same defines; but I can't see how I can make that happen :/ But sure; I suppose I can add more defines.
> Also, now that some of the BUG-specific functions are now also related
They already were, its just that x86 only now will use the WARN part of it. I don't feel that should be part of this patch.
> to WARN, they should probably be renamed to describe their new purpose, > like: > > "report_bug" -> "report_bug_or_warning" > "fixup_bug" -> "fixup_bug_or_warning" > > On a related note, if warn and bug are going to continue to use two > separate ud instructions for the foreseeable future, report_bug() could > be cleaned up a bit: e.g., for a ud0 instruction, it doesn't make sense > to call find_bug().
I'm sure you'll break $random arch if you go futz with that. Also, I think you mean UD2, since that's BUG. We actually need the bug_entry for WARNs (aka UD0).
Also, you're now optimizing the BUG() code; I don't think anybody cares about saving a few cycles there. It shouldn't happen in the first place.
> > +static int fixup_bug(struct pt_regs *regs, int trapnr) > > +{ > > + if (trapnr != X86_TRAP_UD) > > + return 0; > > + > > + switch (report_bug(regs->ip, regs)) { > > + case BUG_TRAP_TYPE_NONE: > > + case BUG_TRAP_TYPE_BUG: > > + break; > > + > > + case BUG_TRAP_TYPE_WARN: > > + regs->ip += 2; > > + return 1; > > For self-documentation purposes, maybe use a define for the length of > the ud0 instruction?
Well, UD0 and UD2 really. LENGTH_UD0_OR_UD2 is a bit of a fail, name wise.
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