Re: [patch 0/3] add rt_tgsigqueueinfo syscall [RESEND]

[Roland McGrath <roland-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

I don't see any rationale for rt_tgsigqueueinfo and rt_sigqueueinfo to
differ in their treatment of si_pid/si_uid (whatever that is).  It just
seems like common sense that they would match.

Oleg and/or Sukadev have some patches floating around (maybe all in -mm?)
that relate to setting those.

I notice that POSIX says that si_pid and si_uid have reliable values
whenever si_code <= 0 (what we call SI_FROMUSER() in asm/siginfo.h).
(POSIX only has sigqueue() and kill() et al to send these, so a POSIX
application never explicitly supplies the values.  libc/libpthread do.
The {t,tg,}kill syscalls all set si_pid to tgid already.)
This means a POSIX-conformant application might check si_pid and si_uid and
rely on them not being forged by some other process/user.  Firstly this
means that si_pid must be the POSIX PID, i.e. tgid, not the Linux TID
(which is not a useful value in POSIX interfaces).  Secondly it means the
kernel should guarantee the correctness of these values (at least when
crossing processes, might as well do always).

I don't recall if the pending changes already use tgid, I think they do.

I think what both calls should do is set si_pid to tgid and si_uid to uid
whenever SI_FROMUSER().  This satisfies the POSIX and security trust
concern, and makes them uniform.  (In Sukadev's version, what si_pid value
they fill in here depends on pid_ns details of sender and recipient.)

Vis a vis Sukadev's changes, I also notice that si_uid ought to be
translated for the recipient user_ns.


Thanks,
Roland
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