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
