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

Song Liu
2023-08-24T21:57:47Z

On Thu, Aug 24, 2023 at 6:31 AM Puranjay Mohan <puranjay12@gmail.com> 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@gmail.com>
> Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
> ---
> 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
>