Re: [PATCH -next] powerpc/mm: check base flags in ioremap_prot

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Nanyong Sun <sunnanyong@huawei.com>,
	mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org,
	akpm@linux-foundation.org, npiggin@gmail.com,
	christophe.leroy@c-s.fr
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] powerpc/mm: check base flags in ioremap_prot
Date: Fri, 3 Sep 2021 11:16:56 +0200	[thread overview]
Message-ID: <90aa2b67-24c8-4a5f-d91a-b562054d5c5d@csgroup.eu> (raw)
In-Reply-To: <20210903090339.3671524-1-sunnanyong@huawei.com>

Le 03/09/2021 à 11:03, Nanyong Sun a écrit :
> Some drivers who call ioremap_prot without setting base flags like
> ioremap_prot(addr, len, 0) may work well before
> commit 56f3c1413f5c ("powerpc/mm: properly set PAGE_KERNEL flags in
> ioremap()"), but now they will get a virtual address "successfully"
> from ioremap_prot and badly fault on memory access later because that
> patch also dropped the hack adding of base flags for ioremap_prot.
> 
> So return NULL and throw a warning if the caller of ioremap_prot did
> not set base flags properly. Why not just hack adding PAGE_KERNEL flags
> in the ioremap_prot, because most scenarios can be covered by high level
> functions like ioremap(), ioremap_coherent(), ioremap_cache()...
> so it is better to keep max flexibility for this low level api.

As far as I can see, there is no user of this fonction that sets flags to 0 in the kernel tree.

Did you find any ? If you did, I think it is better to fix the caller.

Christophe

> 
> Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
> ---
>   arch/powerpc/mm/ioremap.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/mm/ioremap.c b/arch/powerpc/mm/ioremap.c
> index 57342154d2b0..b7eda0f0d04d 100644
> --- a/arch/powerpc/mm/ioremap.c
> +++ b/arch/powerpc/mm/ioremap.c
> @@ -46,6 +46,10 @@ void __iomem *ioremap_prot(phys_addr_t addr, unsigned long size, unsigned long f
>   	pte_t pte = __pte(flags);
>   	void *caller = __builtin_return_address(0);
>   
> +	/* The caller should set base page flags properly */
> +	if (WARN_ON((flags & _PAGE_PRESENT) == 0))

This probably doesn't work for some plateforms like book3s/64. You should use helpers like 
pte_present().

See the comment at 
https://elixir.bootlin.com/linux/v5.14/source/arch/powerpc/include/asm/book3s/64/pgtable.h#L591

> +		return NULL;
> +
>   	/* writeable implies dirty for kernel addresses */
>   	if (pte_write(pte))
>   		pte = pte_mkdirty(pte);
> 