Re: [PATCH/RESEND] recordmcount: arm: Implement make_nop

[Ard Biesheuvel]

On 15 November 2016 at 23:53, Stephen Boyd <sboyd@xxxxxxxxxxxxxx> wrote:
> On 11/15, Ard Biesheuvel wrote:
>> On 15 November 2016 at 19:18, Stephen Boyd <sboyd@xxxxxxxxxxxxxx> wrote:
>> > On 11/15, Ard Biesheuvel wrote:
>> >> On 19 October 2016 at 00:42, Stephen Boyd <sboyd@xxxxxxxxxxxxxx> wrote:
>> >> >
>> >> > +static unsigned char ideal_nop4_arm_le[4] = { 0x00, 0x00, 0xa0, 0xe1 }; /* mov r0, r0 */
>> >> > +static unsigned char ideal_nop4_arm_be[4] = { 0xe1, 0xa0, 0x00, 0x00 }; /* mov r0, r0 */
>> >>
>> >> Shouldn't you be taking the difference between BE8 and BE32 into
>> >> account here? IIRC, BE8 uses little endian encoding for instructions.
>> >
>> > I admit I haven't tested on a pre-armv6 CPU so I haven't come
>> > across the case of a BE32 CPU. But from what I can tell that
>> > doesn't matter.
>> >
>> > According to scripts/Makefile.build, cmd_record_mcount only runs
>> > the recordmcount program if CONFIG_FTRACE_MCOUNT_RECORD=y. That
>> > config is defined as:
>> >
>> >         config FTRACE_MCOUNT_RECORD
>> >                 def_bool y
>> >                 depends on DYNAMIC_FTRACE
>> >                 depends on HAVE_FTRACE_MCOUNT_RECORD
>> >
>> >
>> > And in arch/arm/Kconfig we see that DYNAMIC_FTRACE is selected:
>> >
>> >         select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
>> >
>> > which means that FTRACE_MCOUNT_RECORD can't be set when
>> > CPU_ENDIAN_BE32 is set.
>> >
>> > Do you agree that BE32 is not a concern here?
>> >
>>
>> Yes. But that implies then that you should not be using big-endian
>> instruction encodings at all, and simply use the _le variants for both
>> LE and BE8
>
> Ok. I understand what you're getting at now.
>
> I believe the linker is the one that does the instruction endian
> swap to little endian. So everything is built as big-endian data
> and instructions in the assembler phase and then when the linker
> runs to generate the final vmlinux elf file it does the swaps to
> make instructions little endian. recordmcount runs on the object
> files and not the vmlinux file.
>

Very interesting, I did not know that.

> For example, the do_undefinstr() function in
> arch/arm/kernel/traps.c is one place we nop out. On an le host
> and an le build without this patch I see:
>
> (This is all ARM, not thumb)
>
> 00000000 <do_undefinstr>:
>    0:   e1a0c00d        mov     ip, sp
>    4:   e92dd9f0        push    {r4, r5, r6, r7, r8, fp, ip, lr, pc}
>    8:   e24cb004        sub     fp, ip, #4
>    c:   e24dd08c        sub     sp, sp, #140    ; 0x8c
>   10:   e52de004        push    {lr}            ; (str lr, [sp, #-4]!)
>   14:   ebfffffe        bl      0 <__gnu_mcount_nc>
>
> After this patch on an le host and le build I see:
>
> 00000000 <do_undefinstr>:
>    0:   e1a0c00d        mov     ip, sp
>    4:   e92dd9f0        push    {r4, r5, r6, r7, r8, fp, ip, lr, pc}
>    8:   e24cb004        sub     fp, ip, #4
>    c:   e24dd08c        sub     sp, sp, #140    ; 0x8c
>   10:   e1a00000        nop                     ; (mov r0, r0)
>   14:   e1a00000        nop                     ; (mov r0, r0)
>
> So far so good. Similarly, with this patch and an le host and be
> build I see:
>
> 00000000 <do_undefinstr>:
>    0:   e1a0c00d        mov     ip, sp
>    4:   e92dd9f0        push    {r4, r5, r6, r7, r8, fp, ip, lr, pc}
>    8:   e24cb004        sub     fp, ip, #4
>    c:   e24dd08c        sub     sp, sp, #140    ; 0x8c
>   10:   e1a00000        nop                     ; (mov r0, r0)
>   14:   e1a00000        nop                     ; (mov r0, r0)
>
> but with *_le instead of *_be used a be build I see:
>
> 00000000 <do_undefinstr>:
>    0:   e1a0c00d        mov     ip, sp
>    4:   e92dd9f0        push    {r4, r5, r6, r7, r8, fp, ip, lr, pc}
>    8:   e24cb004        sub     fp, ip, #4
>    c:   e24dd08c        sub     sp, sp, #140    ; 0x8c
>   10:   0000a0e1        andeq   sl, r0, r1, ror #1
>   14:   0000a0e1        andeq   sl, r0, r1, ror #1
>
> I confirmed this by looking at the hexdump of the .exception.text
> section for the traps.o object file and the .text section of the
> vmlinux file. Basically objcopy the .exception.text of traps.o to
> get the first few instructions of the do_undefinstr() function:
>
> $ hexdump -C traps.o
> 00000000  e1 a0 c0 0d e9 2d d9 f0  e2 4c b0 04 e2 4d d0 8c
>
> And then objcopy the .text section in vmlinux and seek to the
> same function offset (there are a bunch of zeroes in front of it
> for padding):
>
> $ hexdump -C vmlinux
> ...
> 00001000  0d c0 a0 e1 f0 d9 2d e9  04 b0 4c e2 8c d0 4d e2
>
> As can be seen everything is swapped from the original object
> file in big-endian to be in little endian.
>
> Does that allay your concerns?
>

Yes, it does. Thanks