Re: [PATCH] MIPS: boot: Fix unaligned access with CONFIG_MIPS_RAW_APPENDED_DTB

[Paul Cercueil]

Hi Nick,

Le mer. 16 déc. 2020 à 18:08, Nick Desaulniers 
<ndesaulniers@xxxxxxxxxx> a écrit :
> On Wed, Dec 16, 2020 at 3:40 PM Paul Cercueil <paul@xxxxxxxxxxxxxxx> 
> 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()?

Indeed.

> >  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@xxxxxxxxxxxxxxx> # v4.7
> >  Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
>
> 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);

Actually that would not work (I did try that), since the 4-byte size 
appended to the compressed payload is inside the *(.image) section. The 
code that appends it (in scripts/Makefile.lib, I think) doesn't seem to 
take care about aligning it to a 4-byte offset. I have no idea why it 
does with GCC and doesn't with Clang, and I have no idea why the 
compressed payload's size isn't aligned either.

> 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@xxxxxxxxxx>

Thanks.

Cheers,
-Paul

> >  ---
> >   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