Re: [PATCH bpf-next v2 1/3] riscv: extend patch_text_nosync() for multiple pages

[Puranjay Mohan]

Hi Song,

On Thu, Aug 24, 2023 at 11:57 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> On Thu, Aug 24, 2023 at 6:31 AM Puranjay Mohan <puranjay12@xxxxxxxxx> wrote:
> >
> > The patch_insn_write() function currently doesn't work for multiple
> > pages of instructions, therefore patch_text_nosync() will fail with a
> > page fault if called with lengths spanning multiple pages.
> >
> > This commit extends the patch_insn_write() function to support multiple
> > pages by copying at max 2 pages at a time in a loop. This implementation
> > is similar to text_poke_copy() function of x86.
> >
> > Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> > Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> > ---
> >  arch/riscv/kernel/patch.c | 39 ++++++++++++++++++++++++++++++++++-----
> >  1 file changed, 34 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> > index 575e71d6c8ae..465b2eebbc37 100644
> > --- a/arch/riscv/kernel/patch.c
> > +++ b/arch/riscv/kernel/patch.c
> > @@ -53,12 +53,18 @@ static void patch_unmap(int fixmap)
> >  }
> >  NOKPROBE_SYMBOL(patch_unmap);
> >
> > -static int patch_insn_write(void *addr, const void *insn, size_t len)
> > +static int __patch_insn_write(void *addr, const void *insn, size_t len)
> >  {
> >         void *waddr = addr;
> >         bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
> >         int ret;
> >
> > +       /*
> > +        * Only two pages can be mapped at a time for writing.
> > +        */
> > +       if (len > 2 * PAGE_SIZE)
> > +               return -EINVAL;
>
> This check cannot guarantee __patch_insn_write touch at most two pages.

Yes, I just realised this can span 3 pages if len = 2 * PAGE_SIZE and
offset_in_page(addr) > 0.

> Maybe use
>
>     if (len + offset_in_page(addr) > 2 * PAGE_SIZE)
>         return -EINVAL;
> ?

Will fix it in the next version.

>
> Thanks,
> Song
>
> >         /*
> >          * Before reaching here, it was expected to lock the text_mutex
> >          * already, so we don't need to give another lock here and could
> > @@ -74,7 +80,7 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
> >                 lockdep_assert_held(&text_mutex);
> >
> >         if (across_pages)
> > -               patch_map(addr + len, FIX_TEXT_POKE1);
> > +               patch_map(addr + PAGE_SIZE, FIX_TEXT_POKE1);
> >
> >         waddr = patch_map(addr, FIX_TEXT_POKE0);
> >
> > @@ -87,15 +93,38 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
> >
> >         return ret;
> >  }
> > -NOKPROBE_SYMBOL(patch_insn_write);
> > +NOKPROBE_SYMBOL(__patch_insn_write);
> >  #else
> > -static int patch_insn_write(void *addr, const void *insn, size_t len)
> > +static int __patch_insn_write(void *addr, const void *insn, size_t len)
> >  {
> >         return copy_to_kernel_nofault(addr, insn, len);
> >  }
> > -NOKPROBE_SYMBOL(patch_insn_write);
> > +NOKPROBE_SYMBOL(__patch_insn_write);
> >  #endif /* CONFIG_MMU */
> >
> > +static int patch_insn_write(void *addr, const void *insn, size_t len)
> > +{
> > +       size_t patched = 0;
> > +       size_t size;
> > +       int ret = 0;
> > +
> > +       /*
> > +        * Copy the instructions to the destination address, two pages at a time
> > +        * because __patch_insn_write() can only handle len <= 2 * PAGE_SIZE.
> > +        */
> > +       while (patched < len && !ret) {
> > +               size = min_t(size_t,
> > +                            PAGE_SIZE * 2 - offset_in_page(addr + patched),
> > +                            len - patched);
> > +               ret = __patch_insn_write(addr + patched, insn + patched, size);
> > +
> > +               patched += size;
> > +       }
> > +
> > +       return ret;
> > +}
> > +NOKPROBE_SYMBOL(patch_insn_write);
> > +
> >  int patch_text_nosync(void *addr, const void *insns, size_t len)
> >  {
> >         u32 *tp = addr;
> > --
> > 2.39.2
> >

Thanks,
Puranjay