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

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Date:   Tue, 12 Jun 2018 05:00:08 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Palmer Dabbelt <palmer@...ive.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 Mon, Jun 11, 2018 at 12:01:37PM -0700, Palmer Dabbelt wrote:
> 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.

OK. I imagine indeed but note that no linker is involved here so,
if the problem is still present, it must already be in the assembler.
 
> > 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.

Cool! 

-- Luc