linux-kernel - Re: [PATCH] MIPS: boot: Fix unaligned access with CONFIG_MIPS_RAW_APPENDED

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Message-ID: <CAKwvOdnmt7v=+QdZbVYw9fDTeAhhHn0X++aLBa3uQVp7Gp=New@mail.gmail.com>
Date:   Wed, 16 Dec 2020 18:08:42 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nathan Chancellor <natechancellor@...il.com>, od@...c.me,
        linux-mips@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH] MIPS: boot: Fix unaligned access with CONFIG_MIPS_RAW_APPENDED_DTB

On Wed, Dec 16, 2020 at 3:40 PM Paul Cercueil <paul@...pouillou.net> wrote:
>
> The compressed payload is not necesarily 4-byte aligned, at least when
> compiling with Clang. In that case, the 4-byte value appended to the
> compressed payload that corresponds to the uncompressed kernel image
> size must be read using get_unaligned_le().

Should it be get_unaligned_le32()?

>
> This fixes Clang-built kernels not booting on MIPS (tested on a Ingenic
> JZ4770 board).
>
> Fixes: b8f54f2cde78 ("MIPS: ZBOOT: copy appended dtb to the end of the kernel")
> Cc: <stable@...r.kernel.org> # v4.7
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>

Hi Paul, thanks for the patch (and for testing with Clang)!
Alternatively, we could re-align __image_end to the next 4B multiple
via:

diff --git a/arch/mips/boot/compressed/ld.script
b/arch/mips/boot/compressed/ld.script
index 0ebb667274d6..349919eff5fb 100644
--- a/arch/mips/boot/compressed/ld.script
+++ b/arch/mips/boot/compressed/ld.script
@@ -27,6 +27,7 @@ SECTIONS
                /* Put the compressed image here */
                __image_begin = .;
                *(.image)
+               . = ALIGN(4);
                __image_end = .;
                CONSTRUCTORS
                . = ALIGN(16);

The tradeoff being up to 3 wasted bytes of padding in the compressed
image, vs fetching one value slower (assuming unaligned loads are
slower than aligned loads MIPS, IDK).  I doubt decompress_kernel is
called repeatedly, so let's take the byte saving approach of yours by
using unaligned loads!

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/mips/boot/compressed/decompress.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/mips/boot/compressed/decompress.c b/arch/mips/boot/compressed/decompress.c
> index c61c641674e6..47c07990432b 100644
> --- a/arch/mips/boot/compressed/decompress.c
> +++ b/arch/mips/boot/compressed/decompress.c
> @@ -117,7 +117,7 @@ void decompress_kernel(unsigned long boot_heap_start)
>                 dtb_size = fdt_totalsize((void *)&__appended_dtb);
>
>                 /* last four bytes is always image size in little endian */
> -               image_size = le32_to_cpup((void *)&__image_end - 4);
> +               image_size = get_unaligned_le32((void *)&__image_end - 4);
>
>                 /* copy dtb to where the booted kernel will expect it */
>                 memcpy((void *)VMLINUX_LOAD_ADDRESS_ULL + image_size,
> --
> 2.29.2
>


-- 
Thanks,
~Nick Desaulniers