linux-kernel - Re: [RFC PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE

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Date:   Mon, 25 Apr 2022 03:47:38 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE_SIZE

On domenica 24 aprile 2022 12:48:06 CEST Fabio M. De Francesco wrote:
> Call VM_BUG_ON() if offset + len > PAGE_SIZE.
> 
> This is an RFC patch because I'm not sure if we actually need to 
BUG_ON(),

Actually I considered VM_BUG_ON() for consistency with the other functions 
in highmem.h

What about VM_WARN_ON_ONCE() here and also in memset_page?

Thanks,

Fabio

> so I'd like to know what Maintainers and other developers think about 
this
> proposal.
> 
> Please note that memset_page() uses VM_BUG_ON() to prevent the same 
issue.
> 
> Furthermore, I also cannot understand why memset_page() does not call
> flush_dcache_page() after memset() but I suppose that the different
> implementations of these two functions should be addressed in another 
RFC.
> 
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
>  include/linux/highmem.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index c3d562b5f0c1..bdb16d5f8148 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -349,6 +349,8 @@ static inline void memcpy_to_page(struct page *page, 
size_t offset,
>  static inline void memzero_page(struct page *page, size_t offset, size_t 
len)
>  {
>  	char *addr = kmap_local_page(page);
> +
> +	VM_BUG_ON(offset + len > PAGE_SIZE);
>  	memset(addr + offset, 0, len);
>  	flush_dcache_page(page);
>  	kunmap_local(addr);
> -- 
> 2.34.1
> 
>