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Re: [RFC] Change PCH "checksum"


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?

Richard.

> Cheers,
> 
> Mark
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)


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