Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text_free()

[Ard Biesheuvel]

On Fri, 24 Jul 2020 at 12:27, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
>
> > Use text_alloc() and text_free() instead of module_alloc() and
> > module_memfree() when an arch provides them.
> >
> > Cc: linux-mm@xxxxxxxxx
> > Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> > ---
> >  kernel/kprobes.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index 4e46d96d4e16..611fcda9f6bf 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -40,6 +40,7 @@
> >  #include <asm/cacheflush.h>
> >  #include <asm/errno.h>
> >  #include <linux/uaccess.h>
> > +#include <linux/vmalloc.h>
> >
> >  #define KPROBE_HASH_BITS 6
> >  #define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
> > @@ -111,12 +112,20 @@ enum kprobe_slot_state {
> >
> >  void __weak *alloc_insn_page(void)
> >  {
> > +#ifdef CONFIG_ARCH_HAS_TEXT_ALLOC
> > +     return text_alloc(PAGE_SIZE);
> > +#else
> >       return module_alloc(PAGE_SIZE);
> > +#endif
> >  }
> >
> >  void __weak free_insn_page(void *page)
> >  {
> > +#ifdef CONFIG_ARCH_HAS_TEXT_ALLOC
> > +     text_free(page);
> > +#else
> >       module_memfree(page);
> > +#endif
> >  }
>
> I've read the observations in the other threads, but this #ifdef
> jungle is silly, it's a de-facto open coded text_alloc() with a
> module_alloc() fallback...
>

Also, as I attempted to explain before, there is no reason to allocate
kasan shadow for any of these use cases, so cloning module_alloc() to
implement text_alloc() is not the correct approach even on x86.

I suppose module_alloc() could be reimplemented in terms of
text_alloc() in this case, but simply relabelling it like this seems
inappropriate on all architectures.