linux-kernel - Re: mmotm 2020-05-13-20-30 uploaded (objtool warnings)

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Date:   Fri, 29 May 2020 15:57:51 +0200
From:   Christoph Hellwig <hch@....de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, broonie@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-next@...r.kernel.org, mhocko@...e.cz,
        mm-commits@...r.kernel.org, sfr@...b.auug.org.au,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        viro@...IV.linux.org.uk, hch@....de, x86@...nel.org
Subject: Re: mmotm 2020-05-13-20-30 uploaded (objtool warnings)

On Thu, May 28, 2020 at 07:20:05PM +0200, Peter Zijlstra wrote:
> > on x86_64:
> > 
> > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_from_user()+0x2a4: call to memset() with UACCESS enabled
> > arch/x86/lib/csum-wrappers_64.o: warning: objtool: csum_and_copy_to_user()+0x243: return with UACCESS enabled
> 
> Urgh, that's horrible code. That's got plain stac()/clac() calls on
> instead of the regular uaccess APIs.

Does it?  If this is from the code in linux-next, then the code does a
user_access_begin/end in csum_and_copy_{from,to}_user, then uses
unsafe_{get,put}_user inside those function itself.  But then they call
csum_partial_copy_generic with the __user casted away, but without any
comment on why this is safe.