linux-kernel - Re: [PATCH 3/3] riscv: fix __user annotation for __copy

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Message-ID: <mhng-b01e0f6b-07fd-4271-a256-c09024675db8@palmer-si-x1c4>
Date:   Mon, 11 Jun 2018 12:01:37 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     luc.vanoostenryck@...il.com
CC:     atish.patra@....com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, albert@...ive.com
Subject:     Re: [PATCH 3/3] riscv: fix __user annotation for __copy_user()

On Sat, 09 Jun 2018 14:42:12 PDT (-0700), luc.vanoostenryck@...il.com wrote:
> On Sat, Jun 09, 2018 at 01:00:08PM -0700, Palmer Dabbelt wrote:
>> On Fri, 08 Jun 2018 17:13:12 PDT (-0700), luc.vanoostenryck@...il.com wrote:
>> > I tried it and ... the preprocessed asm is as expected:
>> > 	.globl __asm_copy_to_user ; .balign 4 ; __asm_copy_to_user:
>> > 	.globl __asm_copy_from_user ; .balign 4 ; __asm_copy_from_user:
>> >
>> >
>> > 	 li t6, 0x00040000
>> > 	 csrs sstatus, t6
>> > 	 ...
>> >
>> > But the nm -S returns different sizes for them:
>> > 	0000000000000004 000000000000006c T __asm_copy_from_user
>> > 	0000000000000002 000000000000006e T __asm_copy_to_user
>> >
>> > and the object code is:
>> > 	0000000000000000 <__asm_copy_to_user-0x2>:
>> > 	   0:   0001                    nop
>> >
>> > 	0000000000000002 <__asm_copy_to_user>:
>> > 	   2:   0001                    nop
>> >
>> > 	0000000000000004 <__asm_copy_from_user>:
>> > 	   4:   00040fb7                lui     t6,0x40
>> > 	   8:   100fa073                csrs    sstatus,t6
>> > 	   ...
>> >
>> > Why these unnneded nops?
>> > Is this a known problem of my toolchain (I use a plain gcc 7.3 +
>> > binutils 2.29, both configured as riscv64-none-elf)?
>> >
>> > If I remove the two ENTRY() and use instead:
>> > 	.globl __asm_copy_to_user ; __asm_copy_to_user:
>> > 	.globl __asm_copy_from_user ; __asm_copy_from_user:
>> > (IOW, I drop the .balign) then I get the expected result.
>> > But well, this seems unrelated to the double ENTRY.
>> >
>> > I can't test it more for now because I've some link errors (which,
>> > I understand are probably solved in the riscv tree of binutils).
>> >
>> > I'll send you the patch anyway since, as far as I understand the changes
>> > specific to this copy_to/from_user is OK.
>>
>> I think it's probably a bug in binutils-2.29 that should be fixed by
>> 2.30 -- IIRC we had some bugs that looked like this and they got
>> fixed, though it might be just in master (so 2.31).
>
> I've tried binutils-2.30 and riscv-binutils-gdb, both still have
> the problem and master binutils-gdb doesn't compile for me.
> OTOH, everything is fine if I disabled CONFIG_RISCV_ISA_C.

OK, I'll try and figure out what's going on.  We've had a handful of headaches 
trying to get things like '.align 2; .align 2' to actually produce no NOPs for 
the second alignment directive, which is surprisingly complicated due to the 
aggressive linker relaxation we do.

>> Either way it looks innocuous WRT the patch.
>
> Indeed.
> With this, the RISC-V arch should be sparse clean.
> I'll recheck after -rc1.

This will be part of the PR that I tag today, so I anticipate it'll be in.

Thanks!