Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

[Josh Poimboeuf]

On Tue, Oct 17, 2017 at 09:58:59AM -0400, Boris Ostrovsky wrote:
> On 10/17/2017 01:24 AM, Josh Poimboeuf wrote:
> > On Mon, Oct 16, 2017 at 02:18:48PM -0400, Boris Ostrovsky wrote:
> >> On 10/12/2017 03:53 PM, Boris Ostrovsky wrote:
> >>> On 10/12/2017 03:27 PM, Andrew Cooper wrote:
> >>>> On 12/10/17 20:11, Boris Ostrovsky wrote:
> >>>>> There is also another problem:
> >>>>>
> >>>>> [    1.312425] general protection fault: 0000 [#1] SMP
> >>>>> [    1.312901] Modules linked in:
> >>>>> [    1.313389] CPU: 0 PID: 1 Comm: init Not tainted 4.14.0-rc4+ #6
> >>>>> [    1.313878] task: ffff88003e2c0000 task.stack: ffffc9000038c000
> >>>>> [    1.314360] RIP: 10000e030:entry_SYSCALL_64_fastpath+0x1/0xa5
> >>>>> [    1.314854] RSP: e02b:ffffc9000038ff50 EFLAGS: 00010046
> >>>>> [    1.315336] RAX: 000000000000000c RBX: 000055f550168040 RCX:
> >>>>> 00007fcfc959f59a
> >>>>> [    1.315827] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
> >>>>> 0000000000000000
> >>>>> [    1.316315] RBP: 000000000000000a R08: 000000000000037f R09:
> >>>>> 0000000000000064
> >>>>> [    1.316805] R10: 000000001f89cbf5 R11: ffff88003e2c0000 R12:
> >>>>> 00007fcfc958ad60
> >>>>> [    1.317300] R13: 0000000000000000 R14: 000055f550185954 R15:
> >>>>> 0000000000001000
> >>>>> [    1.317801] FS:  0000000000000000(0000) GS:ffff88003f800000(0000)
> >>>>> knlGS:0000000000000000
> >>>>> [    1.318267] CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
> >>>>> [    1.318750] CR2: 00007fcfc97ab218 CR3: 000000003c88e000 CR4:
> >>>>> 0000000000042660
> >>>>> [    1.319235] Call Trace:
> >>>>> [    1.319700] Code: 51 50 57 56 52 51 6a da 41 50 41 51 41 52 41 53 48
> >>>>> 83 ec 30 65 4c 8b 1c 25 c0 d2 00 00 41 f7 03 df 39 08 90 0f 85 a5 00 00
> >>>>> 00 50 <ff> 15 9c 95 d0 ff 58 48 3d 4c 01 00 00 77 0f 4c 89 d1 ff 14 c5
> >>>>> [    1.321161] RIP: entry_SYSCALL_64_fastpath+0x1/0xa5 RSP: ffffc9000038ff50
> >>>>> [    1.344255] ---[ end trace d7cb8cd6cd7c294c ]---
> >>>>> [    1.345009] Kernel panic - not syncing: Attempted to kill init!
> >>>>> exitcode=0x0000000b
> >>>>>
> >>>>>
> >>>>> All code
> >>>>> ========
> >>>>>    0:    51                       push   %rcx
> >>>>>    1:    50                       push   %rax
> >>>>>    2:    57                       push   %rdi
> >>>>>    3:    56                       push   %rsi
> >>>>>    4:    52                       push   %rdx
> >>>>>    5:    51                       push   %rcx
> >>>>>    6:    6a da                    pushq  $0xffffffffffffffda
> >>>>>    8:    41 50                    push   %r8
> >>>>>    a:    41 51                    push   %r9
> >>>>>    c:    41 52                    push   %r10
> >>>>>    e:    41 53                    push   %r11
> >>>>>   10:    48 83 ec 30              sub    $0x30,%rsp
> >>>>>   14:    65 4c 8b 1c 25 c0 d2     mov    %gs:0xd2c0,%r11
> >>>>>   1b:    00 00
> >>>>>   1d:    41 f7 03 df 39 08 90     testl  $0x900839df,(%r11)
> >>>>>   24:    0f 85 a5 00 00 00        jne    0xcf
> >>>>>   2a:    50                       push   %rax
> >>>>>   2b:*    ff 15 9c 95 d0 ff        callq  *-0x2f6a64(%rip)        #
> >>>>> 0xffffffffffd095cd        <-- trapping instruction
> >>>>>   31:    58                       pop    %rax
> >>>>>   32:    48 3d 4c 01 00 00        cmp    $0x14c,%rax
> >>>>>   38:    77 0f                    ja     0x49
> >>>>>   3a:    4c 89 d1                 mov    %r10,%rcx
> >>>>>   3d:    ff                       .byte 0xff
> >>>>>   3e:    14 c5                    adc    $0xc5,%al
> >>>>>
> >>>>>
> >>>>> so the original 'cli' was replaced with the pv call but to me the offset
> >>>>> looks a bit off, no? Shouldn't it always be positive?
> >>>> callq takes a 32bit signed displacement, so jumping back by up to 2G is
> >>>> perfectly legitimate.
> >>> Yes, but
> >>>
> >>> ostr@workbase> nm vmlinux | grep entry_SYSCALL_64_fastpath
> >>> ffffffff817365dd t entry_SYSCALL_64_fastpath
> >>> ostr@workbase> nm vmlinux | grep " pv_irq_ops"
> >>> ffffffff81c2dbc0 D pv_irq_ops
> >>> ostr@workbase>
> >>>
> >>> so pv_irq_ops.irq_disable is about 5MB ahead of where we are now. (I
> >>> didn't mean that x86 instruction set doesn't allow negative
> >>> displacement, I was trying to say that pv_irq_ops always live further down)
> >> I believe the problem is this:
> >>
> >> #define PV_INDIRECT(addr)       *addr(%rip)
> >>
> >> The displacement that the linker computes will be relative to the where
> >> this instruction is placed at the time of linking, which is in
> >> .pv_altinstructions (and not .text). So when we copy it into .text the
> >> displacement becomes bogus.
> > apply_alternatives() is supposed to adjust that displacement based on
> > the new IP, though it could be messing that up somehow.  (See patch
> > 10/13.)
> >
> 
> That patch doesn't take into account the fact that replacement
> instructions may have to save/restore registers. So, for example,
> 
> 
> -        if (a->replacementlen && is_jmp(replacement[0]))
> +        } else if (a->replacementlen == 6 && *insnbuf == 0xff &&
> +               *(insnbuf+1) == 0x15) {
> +            /* indirect call */
> +            *(s32 *)(insnbuf + 2) += replacement - instr;
> +            DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
> +                *(s32 *)(insnbuf + 2),
> +                (unsigned long)instr + *(s32 *)(insnbuf + 2) + 6);
> +
> 
> doesn't do the adjustment of
> 
>   2a:    50                       push   %rax
>   2b:*    ff 15 9c 95 d0 ff        callq  *-0x2f6a64(%rip)
>   31:    58                       pop    %rax
> 
> because instbuf points to 'push' and not to 'call'.

Ah.  I forgot that asm paravirt patches put the saves/restores _in_ the
replacement, whereas in C code they're _outside_ the replacement.

Changing PV_INDIRECT to use absolute addressing would be a decent fix,
but I think that would break the PIE support Thomas Garnier has been
working on.

Maybe we can add a new field to the alternatives entry struct which
specifies the offset to the CALL instruction, so apply_alternatives()
can find it.

-- 
Josh