linux-kernel - Re: [PATCH] mm/mremap: fix BUILD_BUG_ON() error in get

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Date:   Wed, 3 Feb 2021 12:03:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Arnd Bergmann <arnd@...db.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Brian Geffon <bgeffon@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] mm/mremap: fix BUILD_BUG_ON() error in get_extent

On Wed, Feb 03, 2021 at 11:48:40AM -0700, Nathan Chancellor wrote:
> On Tue, Jan 12, 2021 at 12:16:34PM -0700, Nathan Chancellor wrote:
> > On Wed, Dec 30, 2020 at 04:40:40PM +0100, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@...db.de>
> > > 
> > > clang cannt evaluate this function argument at compile time
> > > when the function is not inlined, which leads to a link
> > > time failure:
> > > 
> > > ld.lld: error: undefined symbol: __compiletime_assert_414
> > > >>> referenced by mremap.c
> > > >>>               mremap.o:(get_extent) in archive mm/built-in.a
> > > 
> > > Mark the function as __always_inline to avoid it.
> > > 
> > > Fixes: 9ad9718bfa41 ("mm/mremap: calculate extent in one place")
> > > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > 
> > I would like to see some movement on getting this fixed in 5.11. As it
> > stands, this is one of three __compiletime_assert references with
> > CONFIG_UBSAN_UNSIGNED_OVERFLOW. If we want to keep the BUILD_BUG()
> > around, I think this is fine. Alternatively, turning it into a runtime
> > check would be fine too.
> > 
> > Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> 
> Ping? It is pretty late into the 5.11 cycle and this is still broken.

I think we should just do the __always_inline. Who can take this?

-Kees

> 
> Cheers,
> Nathan
> 
> > > ---
> > >  mm/mremap.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/mm/mremap.c b/mm/mremap.c
> > > index c5590afe7165..1cb464a07184 100644
> > > --- a/mm/mremap.c
> > > +++ b/mm/mremap.c
> > > @@ -336,8 +336,9 @@ enum pgt_entry {
> > >   * valid. Else returns a smaller extent bounded by the end of the source and
> > >   * destination pgt_entry.
> > >   */
> > > -static unsigned long get_extent(enum pgt_entry entry, unsigned long old_addr,
> > > -			unsigned long old_end, unsigned long new_addr)
> > > +static __always_inline unsigned long get_extent(enum pgt_entry entry,
> > > +			unsigned long old_addr, unsigned long old_end,
> > > +			unsigned long new_addr)
> > >  {
> > >  	unsigned long next, extent, mask, size;
> > >  
> > > -- 
> > > 2.29.2
> >  

-- 
Kees Cook