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

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Date:   Sat, 9 Jun 2018 02:13:12 +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 Fri, Jun 08, 2018 at 03:33:36PM -0700, Palmer Dabbelt wrote:
> On Thu, 07 Jun 2018 09:51:33 PDT (-0700), luc.vanoostenryck@...il.com wrote:
> > On Thu, Jun 07, 2018 at 09:30:19AM -0700, Palmer Dabbelt wrote:
> > > diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
> > > index 58fb2877c865..bd51e47ebd44 100644
> > > --- a/arch/riscv/lib/uaccess.S
> > > +++ b/arch/riscv/lib/uaccess.S
> > > @@ -13,7 +13,15 @@ _epc:
> > > 	.previous
> > > 	.endm
> > > 
> > > -ENTRY(__copy_user)
> > > +/* __asm_copy_to_user and __asm_copy_from_user are actually the same function,
> > > + * they're just provided as two different symbols to C code so sparse doesn't
> > > + * yell about casting between two different address spaces. */
> > > +.global __asm_copy_to_user
> > > +.set __asm_copy_to_user,__asm_copy_tofrom_user
> > > +.global __asm_copy_from_user
> > > +.set __asm_copy_from_user,__asm_copy_tofrom_user
> > > +
> > > +ENTRY(__asm_copy_tofrom_user)
> > 
> > I don't think that the size (as reported by objdump, for example) will
> > be correct or even present for __asm_copy_to_user & __asm_copy_to_user.
> > 
> > What can be done is:
> > 	ENTRY(__asm_copy_to_user)
> > 	ENTRY(__asm_copy_from_user)
> > 
> > 	<function definition>
> > 
> > 	ENDPROC(__asm_copy_to_user)
> > 	ENDPROC(__asm_copy_from_user)
> > 
> Thanks.  Do you mind checking to make sure this works and submitting a patch?

Not at all.
I should have done it already when I sent the previous email.

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.

Regards,
-- Luc