Re: [PATCH 5/7] x86/ioremap: prevent additions against the NULL pointer

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Mar 13, 2025 at 05:21:13PM +0000, Andrew Cooper wrote:
> On 13/03/2025 3:30 pm, Roger Pau Monne wrote:
> > This was reported by clang UBSAN as:
> >
> > UBSAN: Undefined behaviour in arch/x86/mm.c:6297:40
> > applying zero offset to null pointer
> > [...]
> > Xen call trace:
> >     [<ffff82d040303662>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
> >     [<ffff82d040304aa3>] F __ubsan_handle_pointer_overflow+0xcb/0x100
> >     [<ffff82d0406ebbc0>] F ioremap_wc+0xc8/0xe0
> >     [<ffff82d0406c3728>] F video_init+0xd0/0x180
> >     [<ffff82d0406ab6f5>] F console_init_preirq+0x3d/0x220
> >     [<ffff82d0406f1876>] F __start_xen+0x68e/0x5530
> >     [<ffff82d04020482e>] F __high_start+0x8e/0x90
> >
> > Fix bt_ioremap() and ioremap{,_wc}() to not add the offset if the returned
> > pointer from __vmap() is NULL.
> >
> > Fixes: d0d4635d034f ('implement vmap()')
> > Fixes: f390941a92f1 ('x86/DMI: fix table mapping when one lives above 1Mb')
> > Fixes: 81d195c6c0e2 ('x86: introduce ioremap_wc()')
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>, with one style fix.
> 
> It's unfortunate, because C23 makes this one case (add 0 to NULL
> pointer) explicitly well defined to avoid corner cases like this.  Oh well.

Interesting, so they added a new type (nullptr_t) that has a single
possible value (nullptr), and hence arithmetic operations against it
always result in nullptr.  That's helpful to prevent this kind of
bugs.

> > diff --git a/xen/arch/x86/dmi_scan.c b/xen/arch/x86/dmi_scan.c
> > index 2fcc485295eb..a05492037519 100644
> > --- a/xen/arch/x86/dmi_scan.c
> > +++ b/xen/arch/x86/dmi_scan.c
> > @@ -119,8 +120,10 @@ static const void *__init bt_ioremap(paddr_t addr, unsigned int len)
> >      if ( system_state < SYS_STATE_boot )
> >          return __acpi_map_table(addr, len);
> >  
> > -    return __vmap(&mfn, PFN_UP(offs + len), 1, 1, PAGE_HYPERVISOR_RO,
> > -                  VMAP_DEFAULT) + offs;
> > +    va = __vmap(&mfn, PFN_UP(offs + len), 1, 1, PAGE_HYPERVISOR_RO,
> > +	        VMAP_DEFAULT);
> 
> You've got mixed tabs/spaces here.

Thanks, vim autodetection is a bit confused with this file because it
uses both hard and soft tabs, fixed now.

Roger.