New .nops directive, to aid Linux alternatives patching?

H.J. Lu hjl.tools@gmail.com
Mon Feb 12 13:44:00 GMT 2018


On Mon, Feb 12, 2018 at 3:26 AM, Andrew Cooper
<andrew.cooper3@citrix.com> wrote:
> 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.
>

It is fixed now.

BTW, linker doesn't support "-r -s" on object generated by "gcc -g3".
-g2 works.  I am working on a fix.


-- 
H.J.



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