Re: [Xen-devel] [PATCH v2 3/8] xen/x86: manually build xen.mb.efi binary

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.07.18 at 16:02, <daniel.kiper@xxxxxxxxxx> wrote:
> On Thu, Jul 05, 2018 at 02:18:03AM -0600, Jan Beulich wrote:
>> >>> On 04.07.18 at 18:35, <daniel.kiper@xxxxxxxxxx> wrote:
>> > On Wed, Jul 04, 2018 at 09:27:43AM -0600, Jan Beulich wrote:
>> >> >>> On 04.07.18 at 16:01, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> > On Mon, Jun 25, 2018 at 09:36:07AM -0600, Jan Beulich wrote:
>> >> >> >>> On 19.06.18 at 16:35, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> >> > @@ -582,6 +587,12 @@ static void __init efi_arch_memory_setup(void)
>> >> >> >      if ( !efi_enabled(EFI_LOADER) )
>> >> >> >          return;
>> >> >> >
>> >> >> > +    if ( efi_enabled(EFI_MB_LOADER) )
>> >> >> > +        for ( pte = __page_tables_start; pte < __page_tables_end;
>> >> >> > +              pte += ( pte != (intpte_t *)l2_identmap ) ? 1 : 4 * 
>> >> >> > L2_PAGETABLE_ENTRIES )
>> >> >>
>> >> >> Please avoid explicit casts - &l2e_get_intpte(l2_identmap[0]) or
>> >> >> something along those lines ought to work here. Same for
>> >> >> 4 * L2_PAGETABLE_ENTRIES - you mean ARRAY_SIZE() there.
>> >> >
>> >> > OK.
>> >> >
>> >> >> Also this whole code block needs a comment, to explain what it
>> >> >> does and also why l2_identmap needs skipping.
>> >> >>
>> >> >> Furthermore - isn't this off by one, and you process l2_identmap[0]
>> >> >> this way, skipping the rest _plus_ the first following entry? I think
>> >> >
>> >> > The code mimics similar code in head.S.
>> >>
>> >> I can't see a similar off-by-1 in head.S.
>> >
>> >  662         /*
>> >  663          * Update frame addresses in page tables excluding l2_identmap
>> >  664          * without its first entry which points to l1_identmap.
>> >  665          */
>> >  666         mov     $((__page_tables_end-__page_tables_start)/8),%ecx
>> >  667         mov     $(((l2_identmap-__page_tables_start)/8)+1),%edx
>> >  668 1:      cmp     
>> > $((l2_identmap+l2_identmap_sizeof-__page_tables_start)/8),%ecx
>> >  669         cmove   %edx,%ecx
>> >  670         testl   $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
>> >  671         jz      2f
>> >  672         add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
>> >  673 2:      loop    1b
>>
>> Well - this is the code in question, but you fail to point out where
>> the off-by-1 is.
> 
> Line 667, 668 and 669.

I don't think so, no. Note the -8 in lines 670 and 672.

>> >> >> > @@ -271,6 +284,9 @@ SECTIONS
>> >> >> >         *(.data.rel)
>> >> >> >         *(.data.rel.*)
>> >> >> >         CONSTRUCTORS
>> >> >> > +       /* PE file must end at XEN_FILE_ALIGN boundary. */
>> >> >> > +       . = ALIGN(XEN_FILE_ALIGN);
>> >> >> > +       __pe_text_raw_end = .;
>> >> >>
>> >> >> Is this really a requirement on the file, or just on the label?
>> >> >
>> >> > File, so, probably it can be moved behind the label. Though it means
>> >> > that __pe_text_raw_end will not point to the real end of .text section.
>> >>
>> >> This is an answer contradicting itself: If the requirement indeed
>> >> is on the file, then things need to remain as is. I'm wondering
>> >> though what entity would enforce this requirement (if such
>> >> exists in the first place).
>> >
>> > I am not sure what kind of entity you think about.
>>
>> Taking your comment, there must be (a) something said in the spec
>> and (b) its "violation" leading to problems. I guess if I dug carefully
>> enough I might be able to find (a), so it is (b) that I'm asking about.
> 
> Microsoft Portable Executable and Common Object File Format
> Specification, Revision 11, says this:
> 
>   FileAlignment: The alignment factor (in bytes) that is used to align
>   the raw data of sections in the image file. The value should be a power
>   of 2 between 512 and 64 K, inclusive. The default is 512. If the
>   SectionAlignment is less than the architecture’s page size, then
>   FileAlignment must match SectionAlignment.

Well, as said before - there's a variety of Windows drivers with much
smaller alignment (often 32).

> And e.g. at least sbsign is very picky and complains if sections are
> not aligned correctly in the PE file.

That's possibly relevant, and would make up for a good reason here.
In which case, with the comment minimally extended, I'm fine with the
change as still visible above.

Jan


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