Re: [lkp-robot] [x86/kconfig] 81d3871900: BUG:unable_to_handle_kernel

[Joonsoo Kim]

On Tue, Oct 17, 2017 at 09:50:04AM +0200, Thomas Gleixner wrote:
> On Tue, 17 Oct 2017, Joonsoo Kim wrote:
> > On Wed, Oct 11, 2017 at 12:01:20PM -0500, Josh Poimboeuf wrote:
> > > > Looking at the panic, the code in slob_free() was:
> > > > 
> > > >    0:	e8 8d f7 ff ff       	callq  0xfffffffffffff792
> > > >    5:	48 ff 05 c9 8c 91 02 	incq   0x2918cc9(%rip)        # 0x2918cd5
> > > >    c:	85 c0                	test   %eax,%eax
> > > >    e:	75 51                	jne    0x61
> > > >   10:	49 0f bf c5          	movswq %r13w,%rax
> > > >   14:	48 ff 05 c2 8c 91 02 	incq   0x2918cc2(%rip)        # 0x2918cdd
> > > >   1b:	48 8d 3c 43          	lea    (%rbx,%rax,2),%rdi
> > > >   1f:	48 39 ef             	cmp    %rbp,%rdi
> > > >   22:	75 3d                	jne    0x61
> > > >   24:	48 ff 05 ba 8c 91 02 	incq   0x2918cba(%rip)        # 0x2918ce5
> > > >   2b:*	8b 6d 00             	mov    0x0(%rbp),%ebp		<-- trapping instruction
> > > >   2e:	66 85 ed             	test   %bp,%bp
> > > >   31:	7e 09                	jle    0x3c
> > > >   33:	48 ff 05 b3 8c 91 02 	incq   0x2918cb3(%rip)        # 0x2918ced
> > > >   3a:	eb 05                	jmp    0x41
> > > >   3c:	bd                   	.byte 0xbd
> > > >   3d:	01 00                	add    %eax,(%rax)
> > > > 
> > > > The slob_free() code tried to read four bytes at ffff88001c4afffe, and
> > > > ended up reading past the page into a bad area.  I think the bad address
> > > > (ffff88001c4afffe) was returned from slob_next() and it panicked trying
> > > > to read s->units in slob_units().
> > 
> > Hello,
> > 
> > It looks like a compiler bug. The code of slob_units() try to read two
> > bytes at ffff88001c4afffe. It's valid. But the compiler generates
> > wrong code that try to read four bytes.
> > 
> > static slobidx_t slob_units(slob_t *s) 
> > {
> >   if (s->units > 0)
> >     return s->units;
> >   return 1;
> > }
> > 
> > s->units is defined as two bytes in this setup.
> > 
> > Wrongly generated code for this part.
> > 
> > 'mov 0x0(%rbp), %ebp'
> > 
> > %ebp is four bytes.
> > 
> > I guess that this wrong four bytes read cross over the valid memory
> > boundary and this issue happend.
> > 
> > Proper code (two bytes read) is generated if different version of gcc
> > is used.
> 
> Which version fails to generate proper code and which versions work?
> 

gcc 4.8 and 4.9 fails to generate proper code. gcc 5.1 and
the latest version works fine.

I guess that this problem is related to the corner case of some
optimization feature since minor code change makes the result
different. And, with -O2, proper code is generated even if gcc 4.8 is
used.

Thanks.