Re: New .nops directive, to aid Linux alternatives patching?

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sun, Feb 11, 2018 at 4:07 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sun, Feb 11, 2018 at 3:28 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Sun, Feb 11, 2018 at 3:05 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Sun, Feb 11, 2018 at 10:58 AM, Andrew Cooper
>>> <andrew.cooper3@citrix.com> wrote:
>>>> On 11/02/2018 17:19, H.J. Lu wrote:
>>>>> On Sun, Feb 11, 2018 at 8:45 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> On Sun, Feb 11, 2018 at 8:25 AM, Andrew Cooper
>>>>>> <andrew.cooper3@citrix.com> wrote:
>>>>>>> On 11/02/2018 00:59, H.J. Lu wrote:
>>>>>>>>>> Please try users/hjl/nop branch:
>>>>>>>>>>
>>>>>>>>>> https://github.com/hjl-tools/binutils-gdb/tree/users/hjl/nop
>>>>>>>>> Oh - thankyou!  I was about to ask if there were any pointers to get
>>>>>>>>> started hacking on binutils.
>>>>>>>>>
>>>>>>>>> As for the functionality, there are unfortunately some issues.  Given
>>>>>>>>> this source:
>>>>>>>>>
>>>>>>>>>         .text
>>>>>>>>> single:
>>>>>>>>>         nop
>>>>>>>>>
>>>>>>>>> pseudo_1:
>>>>>>>>>         .nop 1
>>>>>>>>>
>>>>>>>>> pseudo_8:
>>>>>>>>>         .nop 8
>>>>>>>>>
>>>>>>>>> pseudo_8_4:
>>>>>>>>>         .nop 8, 4
>>>>>>>>>
>>>>>>>>> pseudo_20:
>>>>>>>>>         .nop 20
>>>>>>>>>
>>>>>>>>> I get the following disassembly:
>>>>>>>>>
>>>>>>>>> 0000000000000000 <single>:
>>>>>>>>>    0:    90                       nop
>>>>>>>>>
>>>>>>>>> 0000000000000001 <pseudo_1>:
>>>>>>>>>    1:    66 90                    xchg   %ax,%ax
>>>>>>>>>
>>>>>>>>> 0000000000000003 <pseudo_8>:
>>>>>>>>>    3:    66 0f 1f 84 00 00 00     nopw   0x0(%rax,%rax,1)
>>>>>>>>>    a:    00 00
>>>>>>>>>
>>>>>>>>> 000000000000000c <pseudo_8_4>:
>>>>>>>>>    c:    90                       nop
>>>>>>>>>    d:    0f 1f 40 00              nopl   0x0(%rax)
>>>>>>>>>   11:    0f 1f 40 00              nopl   0x0(%rax)
>>>>>>>>>
>>>>>>>>> 0000000000000015 <pseudo_20>:
>>>>>>>>>   15:    90                       nop
>>>>>>>>>   16:    66 2e 0f 1f 84 00 00     nopw   %cs:0x0(%rax,%rax,1)
>>>>>>>>>   1d:    00 00 00
>>>>>>>>>   20:    66 2e 0f 1f 84 00 00     nopw   %cs:0x0(%rax,%rax,1)
>>>>>>>>>   27:    00 00 00
>>>>>>>>>
>>>>>>>>> The MAX_NOP part looks to be working as intended (including reducing
>>>>>>>>> below the default of 10), but there appears to be an off-by-one
>>>>>>>>> somewhere, as one too many nops are emitted in the block.
>>>>>>>>>
>>>>>>>>> Furthermore, attempting to use .nop 30 yields:
>>>>>>>>>
>>>>>>>>> /tmp/ccI2Eakp.s: Assembler messages:
>>>>>>>>> /tmp/ccI2Eakp.s: Fatal error: can't write 145268933551616 bytes to
>>>>>>>>> section .text of nops.o: 'Bad value'
>>>>>>>> Please try my branch again.  It should be fixed.
>>>>>>> Thanks.  All of that looks to be in order.
>>>>>>>
>>>>>>> However, when trying to build larger examples, I've started hitting:
>>>>>>>
>>>>>>> /tmp/ccvxOy2v.s: Assembler messages:
>>>>>>> /tmp/ccvxOy2v.s: Internal error in md_convert_frag at
>>>>>>> ../../gas/config/tc-i386.c:9510.
>>>>>>>
>>>>>>> Which is the gas_assert (fragP->fr_var != BFD_RELOC_X86_NOP); you've added.
>>>>>>>
>>>>>>> It occurs when the calculation of the number of nops to insert evaluates
>>>>>>> to 0, and a simple ".nop 0" managed to reproduce the issue.  The
>>>>>>> calculation evaluating to 0 is a side effect of the existing logic to
>>>>>>> evaluate how much, if an, padding is required, and follows this kind of
>>>>>>> pattern:
>>>>>>>
>>>>>> It should be fixed now.  I also added 11-byte nop for 64-bit:
>>>>>>
>>>>>> 67 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:0x0(%eax,%eax,1)
>>>>>>
>>>>> I implemented:
>>>>>
>>>>> .nop SIZE [, MAX_NOP]
>>>>>
>>>>> where the maximum size is 255 bytes.  Should we go with
>>>>>
>>>>> .nop MAX_SIZE, SIZE [, MAX_NOP]
>>>>>
>>>>> to support more than 255 bytes?
>>>>
>>>> If you were to do that, why not simply remove the 255 maximum limit,
>>>> rather than having a user pass two identical numbers?  That said, I
>>>> think the current implementation with 255 is probably fine; My example
>>>> of ~45 is pushing it, but I expect that any example trying to use 64 or
>>>> more almost certainly has a better way to do the same thing.
>>>>
>>>> As for your latest branch, I've found one very curious failure which I'm
>>>> at a loss to explain.  Its all building fine, except for one single
>>>> RSB-stuffing alternative in VT-x vmexit handler.  The alternative in
>>>> question should be 0 +21 nops padding, optionally replaced with 21 bytes
>>>> of actual RSB-stuffing, and several identical copies of this alternative
>>>> elsewhere appear to be working correctly.
>>>>
>>>> Using your latest branch, when building using .skip, everything works
>>>> correctly, but when building with .nop, the calculation believes that
>>>> there are only 3 bytes of padding necessary, and trip the assertion that
>>>> the replacement length is not longer than original length.
>>>>
>>>> At a guess, I'd say that something is suspect with the relocation
>>>> calculations, but I have no idea what.
>>>>
>>>> I haven't managed to miniaturise the repro any further than this:
>>>>
>>>> Grab
>>>> http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=shortlog;h=refs/heads/alternatives-v1
>>>> which is a branch cleaning up a load of our alternatives handling, and
>>>> has support for .nop, and use the following build rune from the root of
>>>> the tree:
>>>>
>>>> (cd xen/arch/x86/hvm/vmx;
>>>> PATH=/local/bin/gcc-ret/bin:/local/bin/nops-binutils/bin:$PATH gcc
>>>> -D__ASSEMBLY__ -m64 -DBUILD_ID -fno-strict-aliasing -Wall
>>>> -Wstrict-prototypes -Wdeclaration-after-statement
>>>> -Wno-unused-but-set-variable -Wno-unused-local-typedefs -O1
>>>> -fno-omit-frame-pointer -nostdinc -fno-builtin -fno-common -Werror
>>>> -Wredundant-decls -Wno-pointer-arith -pipe -g -D__XEN__ -include
>>>> ../../../../include/xen/config.h '-D__OBJECT_FILE__="entry.o"'
>>>> -Wa,--strip-local-absolute -MMD -MF ./.entry.o.d -I../../../../include
>>>> -I../../../../include/asm-x86/mach-generic
>>>> -I../../../../include/asm-x86/mach-default -DXEN_IMG_OFFSET=0x200000
>>>> '-D__OBJECT_LABEL__=arch$x86$hvm$vmx$entry.o' -msoft-float
>>>> -fno-stack-protector -fno-exceptions -Wnested-externs -DHAVE_GAS_VMX
>>>> -DHAVE_GAS_SSE4_2 -DHAVE_GAS_EPT -DHAVE_GAS_RDRAND -DHAVE_GAS_FSGSBASE
>>>> -DHAVE_GAS_RDSEED -DHAVE_GAS_LONG_NOPS -U__OBJECT_LABEL__
>>>> -DHAVE_GAS_QUOTED_SYM '-D__OBJECT_LABEL__=arch/x86/hvm/vmx/entry.o'
>>>> -mno-red-zone -fpic -fno-asynchronous-unwind-tables -mno-sse
>>>> -mskip-rax-setup -DGCC_HAS_VISIBILITY_ATTRIBUTE
>>>> -mindirect-branch=thunk-extern -mindirect-branch-register
>>>> -DCONFIG_INDIRECT_THUNK -Wa,-I../../../../include -c entry.S -o entry.o)
>>>>
>>>> vmx's entry.S is fairly small, and in this example, I happen to be using
>>>> one of your repoline branch versions of from "gcc (GCC) 7.2.1
>>>> 20171218".  Substitute the PATH as appropriate, and the interesting bits
>>>> of the ALTERNATIVE implementation are all in
>>>> xen/include/asm-x86/alternative-asm.h
>>>
>>> Is this the error message you saw:
>>>
>>> gcc -D__ASSEMBLY__ -m64 -DBUILD_ID -fno-strict-aliasing -Wall
>>> -Wstrict-prototypes -Wdeclaration-after-statement
>>> -Wno-unused-but-set-variable -Wno-unused-local-typedefs -O1
>>> -fno-omit-frame-pointer -nostdinc -fno-builtin -fno-common -Werror
>>> -Wredundant-decls -Wno-pointer-arith -pipe -g -D__XEN__ -include
>>> /export/ssd/git/kernel.org/xen/xen/include/xen/config.h
>>> '-D__OBJECT_FILE__="entry.o"' -Wa,--strip-local-absolute -MMD -MF
>>> ./.entry.o.d -I/export/ssd/git/kernel.org/xen/xen/include
>>> -I/export/ssd/git/kernel.org/xen/xen/include/asm-x86/mach-generic
>>> -I/export/ssd/git/kernel.org/xen/xen/include/asm-x86/mach-default
>>> -DXEN_IMG_OFFSET=0x200000
>>> '-D__OBJECT_LABEL__=arch$x86$hvm$vmx$entry.o' -msoft-float
>>> -fno-stack-protector -fno-exceptions -Wnested-externs -DHAVE_GAS_VMX
>>> -DHAVE_GAS_SSE4_2 -DHAVE_GAS_EPT -DHAVE_GAS_RDRAND -DHAVE_GAS_FSGSBASE
>>> -DHAVE_GAS_RDSEED -DHAVE_GAS_LONG_NOPS -U__OBJECT_LABEL__
>>> -DHAVE_GAS_QUOTED_SYM '-D__OBJECT_LABEL__=arch/x86/hvm/vmx/entry.o'
>>> -mno-red-zone -fpic -fno-asynchronous-unwind-tables -mno-sse
>>> -mskip-rax-setup -DGCC_HAS_VISIBILITY_ATTRIBUTE
>>> -mindirect-branch=thunk-extern -mindirect-branch-register
>>> -DCONFIG_INDIRECT_THUNK
>>> -Wa,-I/export/ssd/git/kernel.org/xen/xen/include -c entry.S
>>> entry.S: Assembler messages:
>>> entry.S:41: Error: value of 292 too large for field of 1 byte at 1
>>>
>>>
>>
>> I need a small testcase to work on assembler.  Please double
>> check to verify that your change is correct.
>>
>
> Does it look a testcase?
>
> .macro mknops nr_bytes
> #ifdef NOP
>     .nop \nr_bytes, 9
> #else
>     .skip \nr_bytes, 9
> #endif
> .endm
>
> .L_orig_s:
> .L_orig_e:
>      mknops (-(((.L_repl_e1 - .L_repl_s1) - (.L_orig_e - .L_orig_s)) >
> 0) * ((.L_repl_e1 - .L_repl_s1) - (.L_orig_e - .L_orig_s)))
> .L_orig_p:
>
>     .byte 0xff + (.L_repl_e1 - .L_repl_s1) - (.L_orig_p - .L_orig_s)
>     .section .altinstr_replacement, "ax", @progbits
> .L_repl_s1:
> .L_fill_rsb_loop:
>     jnz .L_fill_rsb_loop
>     mov %rax, %rsp
> .L_repl_e1:
>
> [hjl@gnu-bdx-1 vmx]$ gcc -c y.S -DNOP
> y.S: Assembler messages:
> y.S:14: Error: value of 257 too large for field of 1 byte at 3
> [hjl@gnu-bdx-1 vmx]$ gcc -c y.S
> [hjl@gnu-bdx-1 vmx]$
>
>

This is because I used a machine dependent relax state in
assembler to implement this so that I only need to change
the x86 specific part of assembler.  But it is also used to
implement branches.  They won't work together.

I need to add a new relax state.

-- 
H.J.