Re: [PATCH 7/3] signal: Deliver all of the perf_data in si_perf

[Marco Elver]

On Mon, 3 May 2021 at 21:38, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>
> > On Sun, May 02, 2021 at 01:39:16PM -0500, Eric W. Biederman wrote:
> >
> >> The one thing that this doesn't do is give you a 64bit field
> >> on 32bit architectures.
> >>
> >> On 32bit builds the layout is:
> >>
> >>      int si_signo;
> >>      int si_errno;
> >>      int si_code;
> >>      void __user *_addr;
> >>
> >> So I believe if the first 3 fields were moved into the _sifields union
> >> si_perf could define a 64bit field as it's first member and it would not
> >> break anything else.
> >>
> >> Given that the data field is 64bit that seems desirable.
> >
> > The data field is fundamentally an address, it is internally a u64
> > because the perf ring buffer has u64 alignment and it saves on compat
> > crap etc.
> >
> > So for the 32bit/compat case the high bits will always be 0 and
> > truncating into an unsigned long is fine.
>
> I see why it is fine to truncate the data field into an unsigned long.
>
> Other than technical difficulties in extending siginfo_t is there any
> reason not to define data as a __u64?

No -- like I pointed at earlier, si_perf used to be __u64, but we
can't because of the siginfo_t limitation. What we have now is fine,
and not worth dwelling over given siginfo limitations.

Thanks,
-- Marco