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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.07.2022 11:31, Andrew Cooper wrote:
> On 18/07/2022 10:11, Jan Beulich wrote:
>> On 15.07.2022 15:26, Andrew Cooper wrote:
>>> --- 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.

Hmm, the wording then indeed is ambiguous. I read "can" as "are allowed
to", when we mean "aren't allowed to". Maybe ".text is 256M or more in
size"? If you mention "offsets", then I think the check should be based
on actually observing an offset which is too large (which .text size
alone doesn't guarantee will happen).

Jan