On Fri, Mar 29, 2013 at 02:31:57PM +0200, Mikolaj Golub wrote: > On Fri, Mar 29, 2013 at 11:22:45AM +0200, Konstantin Belousov wrote: > > On Thu, Mar 28, 2013 at 11:18:21PM +0200, Mikolaj Golub wrote: > > > On Thu, Mar 28, 2013 at 12:51:34PM +0200, Konstantin Belousov wrote: > > > > > > > In the generic Elf 64bit draft specification I have, the notes sections > > > > are specified to consists of entries, each of which is an array of > > > > 8-byte > > > > words. I think we are right using the 8-byte alignment. > > > > > > I have impression many implementations use 4-byte alignment. E.g. in > > > NetBSD: > > > > > > sys/kern/core_elf32.c: > > > > > > #define ELFROUNDSIZE 4 /* XXX Should it be sizeof(Elf_Word)? */ > > > #define elfround(x) roundup((x), ELFROUNDSIZE) > > Note that this is core_elf32. I am concerned with the 64-bit cores. > > core_elf64.c: > > #define ELFSIZE 64 > > #include "core_elf32.c" Also, the 4-bytes alignment is described in the comments in the glibc csu/abi-note.S.
>
> > >
> > > Also, we have inconsistency with imgactl_elf.c/parse_notes(), which
> > > uses 4-byte alignment:
> > >
> > > note = (const Elf_Note *)((const char *)(note + 1) +
> > > roundup2(note->n_namesz, sizeof(Elf32_Addr)) +
> > > roundup2(note->n_descsz, sizeof(Elf32_Addr)));
> > >
> > > I suppose there were no issues before, because accidentally the sizes
> > > of all notes we had were 8 bytes aligned.
> > Indeed, both ABI and NOINIT notes have size which is multiple of 8.
> >
> > >
> > > Now, when I add new notes it will break things. I don't have strong
> > > opinion, it will be ok for me to leave 8-byte alignment and fix
> > > issues, just want to have strong support here :-)
> > Well, while the issue is discussed and decided, you could just make
> > your new notes size be multiple of 8 too.
>
> I thought about this too. Then I need to be more caerful when
> extracting stats from notes, because the length returned by
> procstat_core_get() can be more than a real payload.
>
> Ok, I will try this way.
>
> I could add length to the note header, which is currently contains
> only structsize, so it would became something like:
>
> struct {
> int structsize;
> int lenght;
> }
>
> But not sure it is worth doing, especially if the forced 8-bit
> alignment is a temporary mesure.
No, it is definitely not worth it.
I inspected imgact_elf.c:parse_note(), imgact_elf.c:putnote() and
rtld.c:digest_notes(). Only putnote() uses 8-byte alignment.
Every other OS and our !coredump code assumes 4-byte alignment.
Does changing the putnote() to align on the 4-byte boundary cause
real change in the core file notes layout ?
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