Re: [PATCH 02/12] kasan, mm: optimize kmalloc poisoning

[Andrey Konovalov]

On Tue, Feb 2, 2021 at 5:25 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> > +#ifdef CONFIG_KASAN_GENERIC
> > +
> > +/**
> > + * kasan_poison_last_granule - mark the last granule of the memory range as
> > + * unaccessible
> > + * @addr - range start address, must be aligned to KASAN_GRANULE_SIZE
> > + * @size - range size
> > + *
> > + * This function is only available for the generic mode, as it's the only mode
> > + * that has partially poisoned memory granules.
> > + */
> > +void kasan_poison_last_granule(const void *address, size_t size);
> > +
> > +#else /* CONFIG_KASAN_GENERIC */
> > +
> > +static inline void kasan_poison_last_granule(const void *address, size_t size) { }

^

> > +
> > +#endif /* CONFIG_KASAN_GENERIC */
> > +
> >  /*
> >   * Exported functions for interfaces called from assembly or from generated
> >   * code. Declarations here to avoid warning about missing declarations.

> > @@ -96,6 +92,16 @@ void kasan_poison(const void *address, size_t size, u8 value)
> >  }
> >  EXPORT_SYMBOL(kasan_poison);
> >
> > +#ifdef CONFIG_KASAN_GENERIC
> > +void kasan_poison_last_granule(const void *address, size_t size)
> > +{
> > +     if (size & KASAN_GRANULE_MASK) {
> > +             u8 *shadow = (u8 *)kasan_mem_to_shadow(address + size);
> > +             *shadow = size & KASAN_GRANULE_MASK;
> > +     }
> > +}
> > +#endif
>
> The function declaration still needs to exist in the dead branch if
> !IS_ENABLED(CONFIG_KASAN_GENERIC). It appears in that case it's declared
> (in kasan.h), but not defined.  We shouldn't get linker errors because
> the optimizer should remove the dead branch. Nevertheless, is this code
> generally acceptable?

The function is defined as empty when !CONFIG_KASAN_GENERIC, see above.