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Re: [RFC] Change PCH "checksum"


On Tue, 2019-02-26 at 15:36 +0100, Richard Biener wrote:
> On Tue, 26 Feb 2019, Mark Wielaard wrote:
> 
> > On Tue, 2019-02-26 at 09:33 +0100, Richard Biener wrote:
> > > On Mon, 25 Feb 2019, Mark Wielaard wrote:
> > > > Since the introduction of GNU Property notes this is (sadly) no
> > > > longer
> > > > the correct way to iterate through ELF notes. The padding of
> > > > names
> > > > and
> > > > desc  might now depend on the alignment of the PT_NOTE segment.
> > > > https://sourceware.org/ml/binutils/2018-09/msg00359.html
> > > 
> > > Ick, that's of course worse ;)  So it's not entirely clear what
> > > the correct thing to do is - from how I read the mail at the
> > > above
> > > link only iff sh_align of the note section is exactly 8 the above
> > > ALIGN would use 8 byte alignment and else 4 is correct
> > > (independent
> > > on sh_align).  Or can I assume sh_align of the note section is
> > > "correct" for all existing binaries?  Note also the eventual
> > > difference
> > > between note sections and note program headers which have
> > > another,
> > > possibly different(?) alignment?  It's of course "easy" to
> > > replace
> > > 4 above by info->dlpi_phdr[i].p_align (but the align field
> > > differs
> > > in width between elfclass 32 and 64 ... :/).
> > > 
> > > So - is merely changing the re-alignment from 4 to 
> > > info->dlpi_phdr[i].p_align "correct"?
> > 
> > Yes, you will have multiple note segments one that combines the 4
> > padded notes and one that combines the 8 padded notes.
> > Some tools put 0 or 1 in the align field, so you might want to use
> > (completely untested):
> > align = (p_align <= 4) ? 4 : 8;
> > offset += ALIGN ((ALIGN (sizeof (uint32_t) * 3 + namesz, align)
> >                   + descsz), align);
> 
> That would mean when p_align == 8 the note name isn't 8-aligned
> but just 4-aligned?  That is, sizeof (Elf*_Nhdr) == 12, and the
> name starts right after that instead of being aligned according
> to p_align?  That sounds odd...  So p_align only applies to
> the descriptor?

Yes, it is that odd. There are 3 kinds of ELF notes.

The traditional ones as used by GNU and Solaris, which use 4 byte words
for everything whether in ELFCLASS32 or ELFCLASS64 and which are 4 byte
aligned themselves.

The gabi ones, which are similar for ELFCLASS32 but for ELFCLASS64 all
words are 8 bytes and 8 bytes aligned themselves (as used by HPUX).

And the new style GNU Property notes, only used in ELFCLASS64, which
use 4 byte words for the first 3 fields, immediately followed by the
name bytes, padded so that desc is 8 bytes aligned and the note as a
whole is 8 byte aligned.

Cheers,

Mark


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