On Wed, Mar 06, 2019 at 02:39:33PM +0100, Dmitry Vyukov wrote: > On Wed, Mar 6, 2019 at 2:13 PM Peter Zijlstra <pet...@infradead.org> wrote:
> > annotated: > > > > 0000 0000000000000150 <__asan_load1>: > > 0000 150: 48 b8 ff ff ff ff ff movabs $0xffff7fffffffffff,%rax > > 0007 157: 7f ff ff > > 000a 15a: 53 push %rbx > > /\/\/\/\/\/\ > > This push is unpleasant on hot fast path. I think we need to move > whole report cold path into a separate noinline function as it is now, > and that function will do the magic with smap. Then this won't prevent > tail calling and won't affect fast-path codegen. It's a bit daft of GCC to do that anyway; since it only uses that rbx thing in the cold path at __asan_load1+0x30. But yes, that wants fixing or something. Then again; a kernel with KASAN on is unbearable slow anyway. > > 000b 15b: 48 8b 4c 24 08 mov 0x8(%rsp),%rcx > > 0010 160: 48 39 c7 cmp %rax,%rdi > > 0013 163: 76 24 jbe 189 <__asan_load1+0x39> > > 0015 165: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax > > 001c 16c: fc ff df > > 001f 16f: 48 89 fa mov %rdi,%rdx > > 0022 172: 48 c1 ea 03 shr $0x3,%rdx > > 0026 176: 0f b6 04 02 movzbl (%rdx,%rax,1),%eax > > 002a 17a: 84 c0 test %al,%al > > 002c 17c: 75 02 jne 180 <__asan_load1+0x30> > > 002e 17e: 5b pop %rbx > > 002f 17f: c3 retq ^^^ hot path, vvv cold path > > 0030 180: 89 fa mov %edi,%edx > > 0032 182: 83 e2 07 and $0x7,%edx > > 0035 185: 38 d0 cmp %dl,%al > > 0037 187: 7f f5 jg 17e <__asan_load1+0x2e> > > 0039 189: 9c pushfq > > 003a 18a: 5b pop %rbx > > 003b 18b: 90 nop > > 003c 18c: 90 nop > > 003d 18d: 90 nop > > 003e 18e: 31 d2 xor %edx,%edx > > 0040 190: be 01 00 00 00 mov $0x1,%esi > > 0045 195: e8 00 00 00 00 callq 19a <__asan_load1+0x4a> > > 0046 196: R_X86_64_PLT32 __kasan_report-0x4 > > 004a 19a: 53 push %rbx > > 004b 19b: 9d popfq > > 004c 19c: 5b pop %rbx > > 004d 19d: c3 retq > > +static __always_inline > > +void kasan_report(unsigned long addr, size_t size, bool is_write, unsigned > > long ip) > > +{ > > + unsigned long flags; > > + > > + flags = smap_save(); > > Previously you said that messing with smap here causes boot errors. > Shouldn't we do smap_save iff kasan_report_enabled? Otherwise we just > bail out, so no need to enable/disable smap. > > > + __kasan_report(addr, size, is_write, ip); > > + smap_restore(flags); > > + > > +} Ah, you think I booted this :-) Still, this is only PUSHF;CLAC, which I think should actually work really early. It was that #UD thing that didn't work early, simply because we'd not set up the exception vector yet when first this happens. > > --- a/mm/kasan/generic_report.c > > +++ b/mm/kasan/generic_report.c > > @@ -118,14 +118,14 @@ const char *get_bug_type(struct kasan_ac > > #define DEFINE_ASAN_REPORT_LOAD(size) \ > > void __asan_report_load##size##_noabort(unsigned long addr) \ > > { \ > > - kasan_report(addr, size, false, _RET_IP_); \ > > + __kasan_report(addr, size, false, _RET_IP_); \ > > Unless I am missing something, this seems to make this patch no-op. We > fixed kasan_report for smap, but here we now use __kasan_report which > is not fixed. So this won't work with smap again?.. I've not found callers of __asan_report_load* with AC=1 in the kernel yet. Under what condtions does GCC emit calls to these functions?