On Tue, 2019-02-26 at 18:13 +0100, Richard Biener wrote: > > > 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. > > I wonder how to distinguish the latter two - does one really need > to test the size of ElfW(Nhdr).n_namesz for example?
I think the second one is only used on HPUX. Everything else uses the 4 byte words variant. I have only encountered the traditional note types and the new GNU Properties notes (on Fedora, I don't believe any other distro has, yet?, adopted them). > Why was the GNU Property one chosen this way?! All I can do is point you at the "consensus" document: https://sourceware.org/ml/binutils/2018-09/msg00282.html and the replies to that. > Is the first case (traditional > GNU note) with p_align == 8 invalid? Yes, I believe so. > That is, is testing p_align > really the correct way to determine how the individual parts are > aligned? I guess not. I do think that is the only way. If the PT_NOTE segment or SHT_NOTE segment has an alignment of 8 then it is a GNU Properties note with the new layout (at least on GNU systems). Cary did propose some additional constraints which might be helpful: https://sourceware.org/ml/binutils/2018-09/msg00359.html > So - how do I identify a GNU Property note vs. a traditional > note vs. a gabi one? > > Why was the third one added?! (I guess I asked that already...) Yeah... See above. Cheers, Mark
