linux-hardening - Re: replacing memcpy() calls with direct assignment

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Date:   Tue, 21 Jun 2022 21:05:36 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Coccinelle <cocci@...teme.lip6.fr>,
        linux-hardening@...r.kernel.org,
        Julia Lawall <Julia.Lawall@...ia.fr>
Subject: Re: replacing memcpy() calls with direct assignment

On Tue, Jun 21, 2022 at 11:37:10AM -0700, Kees Cook wrote:
> Hello Coccinelle gurus! :)
> 
> I recently spent way too long looking at a weird bug in Clang that I
> eventually worked around by just replacing a memcpy() with a direct
> assignment. It really was very mechanical, and seems like it might be a
> common code pattern in the kernel. Swapping these would make the code
> much more readable, I think. Here's the example:
> 
> 
> https://lore.kernel.org/linux-hardening/20220616052312.292861-1-keescook@chromium.org/
> 
> -		memcpy(&host_image->image_section_info[i],
> -		       &fw_image->fw_section_info[i],
> -		       sizeof(struct fw_section_info_st));
> +		host_image->image_section_info[i] = fw_image->fw_section_info[i];

Ick, that hids the fact that you are doing a potentially huge memory
copy here.

And would it also prevent the compiler from using our optimized memcpy()
function and replacing it with whatever it wanted to use instead?

What clang bug does this fix such that it warrants us hiding this
information away from the developers?

thanks,

greg k-h