On 18/07/2022 10:11, Jan Beulich wrote:
> 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 <[email protected]>
>> Signed-off-by: Andrew Cooper <[email protected]>
> Reviewed-by: Jan Beulich <[email protected]>
Thanks, but...
> 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/ ?
Why? "Can" is correct here. If the offsets can't exceed 256M, then
everything is good.
~Andrew
