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

[Song Liu]

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.
Maybe use

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

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
>