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

[Andrew Cooper <andrew dot cooper3 at citrix dot com>]

On 12/02/18 00:26, H.J. Lu wrote:
> 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.
>

Oops sorry yes - I should have given you the error as well, but it looks
like you're on top of the problem.  If there is anything else I can do
to help, please ask.

~Andrew