Re: [PATCH 10/12] signal: Redefine signinfo so 64bit fields are possible

[Peter Collingbourne <pcc@google.com>]

On Mon, May 3, 2021 at 8:42 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> Marco Elver <elver@google.com> writes:
>
> > On Mon, 3 May 2021 at 23:04, Eric W. Biederman <ebiederm@xmission.com> wrote:
> >> "Eric W. Beiderman" <ebiederm@xmission.com> writes:
> >> > From: "Eric W. Biederman" <ebiederm@xmission.com>
> >> >
> >> > The si_perf code really wants to add a u64 field.  This change enables
> >> > that by reorganizing the definition of siginfo_t, so that a 64bit
> >> > field can be added without increasing the alignment of other fields.
> >
> > If you can, it'd be good to have an explanation for this, because it's
> > not at all obvious -- some future archeologist will wonder how we ever
> > came up with this definition of siginfo...
> >
> > (I see the trick here is that before the union would have changed
> > alignment, introducing padding after the 3 ints -- but now because the
> > 3 ints are inside the union the union's padding no longer adds padding
> > for these ints.  Perhaps you can explain it better than I can. Also
> > see below.)
>
> Yes.  The big idea is adding a 64bit field into the second union
> in the _sigfault case will increase the alignment of that second
> union to 64bit.
>
> In the 64bit case the alignment is already 64bit so it is not an
> issue.
>
> In the 32bit case there are 3 ints followed by a pointer.  When the
> 64bit member is added the alignment of _segfault becomes 64bit.  That
> 64bit alignment after 3 ints changes the location of the 32bit pointer.
>
> By moving the 3 preceding ints into _segfault that does not happen.
>
>
>
> There remains one very subtle issue that I think isn't a problem
> but I would appreciate someone else double checking me.
>
>
> The old definition of siginfo_t on 32bit almost certainly had 32bit
> alignment.  With the addition of a 64bit member siginfo_t gains 64bit
> alignment.  This difference only matters if the 64bit field is accessed.
> Accessing a 64bit field with 32bit alignment will cause unaligned access
> exceptions on some (most?) architectures.
>
> For the 64bit field to be accessed the code needs to be recompiled with
> the new headers.  Which implies that when everything is recompiled
> siginfo_t will become 64bit aligned.
>
>
> So the change should be safe unless someone is casting something with
> 32bit alignment into siginfo_t.

How about if someone has a field of type siginfo_t as an element of a
struct? For example:

struct foo {
  int x;
  siginfo_t y;
};

With this change wouldn't the y field move from offset 4 to offset 8?

Peter