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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v3] powerpc: implement CONFIG_DEBUG_VIRTUAL
In-Reply-To: <32b00148-214c-44db-e9cf-fa4caa2547e8@c-s.fr>
References: <a42260e77d609a10667872c2e7bf14ceabcce050.1544562518.git.christophe.leroy@c-s.fr> <87va3zwq1t.fsf@concordia.ellerman.id.au> <32b00148-214c-44db-e9cf-fa4caa2547e8@c-s.fr>
Date:   Wed, 12 Dec 2018 21:35:59 +1100
Message-ID: <87o99rvxps.fsf@concordia.ellerman.id.au>
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Sender: linux-kernel-owner@vger.kernel.org
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Christophe Leroy <christophe.leroy@c-s.fr> writes:
> Le 12/12/2018 à 01:23, Michael Ellerman a écrit :
>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>> 
>>> This patch implements CONFIG_DEBUG_VIRTUAL to warn about
>>> incorrect use of virt_to_phys() and page_to_phys()
>>>
>>> Below is the result of test_debug_virtual:
>>>
>>> [    1.438746] WARNING: CPU: 0 PID: 1 at ./arch/powerpc/include/asm/io.h:808 test_debug_virtual_init+0x3c/0xd4
>>> [    1.448156] CPU: 0 PID: 1 Comm: swapper Not tainted 4.20.0-rc5-00560-g6bfb52e23a00-dirty #532
>>> [    1.457259] NIP:  c066c550 LR: c0650ccc CTR: c066c514
>>> [    1.462257] REGS: c900bdb0 TRAP: 0700   Not tainted  (4.20.0-rc5-00560-g6bfb52e23a00-dirty)
>>> [    1.471184] MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 48000422  XER: 20000000
>>> [    1.477811]
>>> [    1.477811] GPR00: c0650ccc c900be60 c60d0000 00000000 006000c0 c9000000 00009032 c7fa0020
>>> [    1.477811] GPR08: 00002400 00000001 09000000 00000000 c07b5d04 00000000 c00037d8 00000000
>>> [    1.477811] GPR16: 00000000 00000000 00000000 00000000 c0760000 c0740000 00000092 c0685bb0
>>> [    1.477811] GPR24: c065042c c068a734 c0685b8c 00000006 00000000 c0760000 c075c3c0 ffffffff
>>> [    1.512711] NIP [c066c550] test_debug_virtual_init+0x3c/0xd4
>>> [    1.518315] LR [c0650ccc] do_one_initcall+0x8c/0x1cc
>>> [    1.523163] Call Trace:
>>> [    1.525595] [c900be60] [c0567340] 0xc0567340 (unreliable)
>>> [    1.530954] [c900be90] [c0650ccc] do_one_initcall+0x8c/0x1cc
>>> [    1.536551] [c900bef0] [c0651000] kernel_init_freeable+0x1f4/0x2cc
>>> [    1.542658] [c900bf30] [c00037ec] kernel_init+0x14/0x110
>>> [    1.547913] [c900bf40] [c000e1d0] ret_from_kernel_thread+0x14/0x1c
>>> [    1.553971] Instruction dump:
>>> [    1.556909] 3ca50100 bfa10024 54a5000e 3fa0c076 7c0802a6 3d454000 813dc204 554893be
>>> [    1.564566] 7d294010 7d294910 90010034 39290001 <0f090000> 7c3e0b78 955e0008 3fe0c062
>>> [    1.572425] ---[ end trace 6f6984225b280ad6 ]---
>>> [    1.577467] PA: 0x09000000 for VA: 0xc9000000
>>> [    1.581799] PA: 0x061e8f50 for VA: 0xc61e8f50
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>> ---
>>>   v3: Added missing linux/mm.h
>>>       I realised that a driver may use DMA on stack after checking with virt_addr_valid(), so the new
>>>       verification might induce false positives. I remove it for now, will add it again later in a more
>>>       controled way.
>> 
>> What is this comment referring to?
>> 
>> I can't see any difference to v2 except the linux/mm.h include.
>
> v2 was:
>
>
> @@ -804,6 +806,11 @@ extern void __iounmap_at(void *ea, unsigned long size);
>    */
>   static inline unsigned long virt_to_phys(volatile void * address)
>   {
> +	if (IS_ENABLED(CONFIG_DEBUG_VIRTUAL) &&
> +	    !WARN_ON(IS_ENABLED(CONFIG_HAVE_ARCH_VMAP_STACK) && current->pid &&
> +		     object_is_on_stack((const void*)address)))
> +			WARN_ON(!virt_addr_valid(address));
> +
>   	return __pa((unsigned long)address);
>   }
>
>
> v3 is: (same as v1)
>
>
> @@ -804,6 +806,8 @@ extern void __iounmap_at(void *ea, unsigned long size);
>    */
>   static inline unsigned long virt_to_phys(volatile void * address)
>   {
> +	WARN_ON(IS_ENABLED(CONFIG_DEBUG_VIRTUAL) && !virt_addr_valid(address));
> +
>   	return __pa((unsigned long)address);
>   }

Right, sorry I must have been looking at v1 (which was already applied
in my tree).

> The idea in v2 was to detect objects on stack used for DMA before 
> activating CONFIG_VMAP_STACK, but if the driver uses virt_addr_valid() 
> to decide if it can DMA map it, then we'll get false positives.
> So I think this should be added with a dedicated DEBUG CONFIG option, 
> not implicitely.

Sounds good. I'll take v3.

cheers