Re: [PATCH v2 2/2] xen: Fix latent check-endbr.sh bug with 32bit build environments

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.07.2022 15:26, Andrew Cooper wrote:
> While Xen's current VMA means it works, the mawk fix (i.e. using $((0xN)) in
> the shell) isn't portable in 32bit shells.  See the code comment for the fix.
> 
> The fix found a second latent bug.  Recombining $vma_hi/lo should have used
> printf "%s%08x" and only worked previously because $vma_lo had bits set in
> it's top nibble.  Combining with the main fix, %08x becomes %07x.
> 
> Fixes: $XXX patch 1
> Reported-by: Jan Beulich <JBeulich@xxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with, I guess, ...

> --- a/xen/tools/check-endbr.sh
> +++ b/xen/tools/check-endbr.sh
> @@ -61,19 +61,36 @@ ${OBJDUMP} -j .text $1 -d -w | grep '     endbr64 *$' | 
> cut -f 1 -d ':' > $VALID &
>  #    the lower bits, rounding integers to the nearest 4k.
>  #
>  #    Instead, use the fact that Xen's .text is within a 1G aligned region, 
> and
> -#    split the VMA in half so AWK's numeric addition is only working on 32 
> bit
> -#    numbers, which don't lose precision.
> +#    split the VMA so AWK's numeric addition is only working on <32 bit
> +#    numbers, which don't lose precision.  (See point 5)
>  #
>  # 4) MAWK doesn't support plain hex constants (an optional part of the POSIX
>  #    spec), and GAWK and MAWK can't agree on how to work with hex constants 
> in
>  #    a string.  Use the shell to convert $vma_lo to decimal before passing to
>  #    AWK.
>  #
> +# 5) Point 4 isn't fully portable.  POSIX only requires that $((0xN)) be
> +#    evaluated as long, which in 32bit shells turns negative if bit 31 of the
> +#    VMA is set.  AWK then interprets this negative number as a double before
> +#    adding the offsets from the binary grep.
> +#
> +#    Instead of doing an 8/8 split with vma_hi/lo, do a 9/7 split.
> +#
> +#    The consequence of this is that for all offsets, $vma_lo + offset needs
> +#    to be less that 256M (i.e. 7 nibbles) so as to be successfully 
> recombined
> +#    with the 9 nibbles of $vma_hi.  This is fine; .text is at the start of a
> +#    1G aligned region, and Xen is far far smaller than 256M, but leave 
> safety
> +#    check nevertheless.
> +#
>  eval $(${OBJDUMP} -j .text $1 -h |
> -    $AWK '$2 == ".text" {printf "vma_hi=%s\nvma_lo=%s\n", substr($4, 1, 8), 
> substr($4, 9, 16)}')
> +    $AWK '$2 == ".text" {printf "vma_hi=%s\nvma_lo=%s\n", substr($4, 1, 9), 
> substr($4, 10, 16)}')
>  
>  ${OBJCOPY} -j .text $1 -O binary $TEXT_BIN
>  
> +bin_sz=$(stat -c '%s' $TEXT_BIN)
> +[ "$bin_sz" -ge $(((1 << 28) - $vma_lo)) ] &&
> +    { echo "$MSG_PFX Error: .text offsets can exceed 256M" >&2; exit 1; }

... s/can/cannot/ ?

Jan