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New .nops directive, to aid Linux alternatives patching?

H.J. Lu hjl.tools@gmail.com
Sun Feb 11 23:05:00 GMT 2018 
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


-- 
H.J.
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