linux-kernel - Re: [PATCH 07/11] signal/arm64: Document conflicts with SI

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Message-ID: <20180117171729.GJ22781@e103592.cambridge.arm.com>
Date:   Wed, 17 Jan 2018 17:17:29 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Nicolas Pitre <nico@...aro.org>,
        Tony Lindgren <tony@...mide.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Tyler Baicar <tbaicar@...eaurora.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        James Morse <james.morse@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Olof Johansson <olof@...om.net>,
        Santosh Shilimkar <santosh.shilimkar@...com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 07/11] signal/arm64: Document conflicts with SI_USER and
 SIGFPE, SIGTRAP, SIGBUS

On Mon, Jan 15, 2018 at 11:23:03AM -0600, Eric W. Biederman wrote:
> Dave Martin <Dave.Martin@....com> writes:
> 
> > On Thu, Jan 11, 2018 at 06:59:36PM -0600, Eric W. Biederman wrote:

[...]

> >> Possible ABI fixes include:
> >> - Send the signal without siginfo
> >> - Don't generate a signal

[...]

> >> - Possibly assign and use an appropriate si_code
> >> - Don't handle cases which can't happen
> >
> > I think a mixture of these two is the best approach.
> >
> > In any case, si_code == 0 here doesn't seem to have any explicit meaning.
> > I think we can translate all of the arm64 faults to proper si_codes --
> > see my sketch below.  Probably means a bit more thought though.

[...]

> >> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c

[...]

> >> @@ -607,70 +607,70 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> >>  }
> >>  
> >>  static const struct fault_info fault_info[] = {
> >> -	{ do_bad,		SIGBUS,  0,		"ttbr address size fault"	},
> >> -	{ do_bad,		SIGBUS,  0,		"level 1 address size fault"	},
> >> -	{ do_bad,		SIGBUS,  0,		"level 2 address size fault"	},
> >> -	{ do_bad,		SIGBUS,  0,		"level 3 address size fault"	},

If I convert this kind of thing to SIGKILL there really is nothing
sensible to put in si_code, except possibly SI_KERNEL (indicating that
the kill did not come from userspace).  Even so, it hardly seems worth
filling in fields like si_pid and si_uid just to make this "correct".

In any case, if siginfo is never seen by userspace for SIGKILL this is
moot.

Obviously, siginfo is never copied to the user stack in that case, but
is it also guaranteed not to be visible to userspace by other means?
For ptrace I'm hoping not, since SIGKILL should nuke the tracee
immediately instead of being reported to the tracer as a
signal-delivery-stop -- so the tracer should get WIFSIGNALED() &&
WTERMSIG() == SIGKILL.  A subsequent PTRACE_GETSIGINFO would fail with
ESRCH.

Does that match your understanding?

If so, there is some merit in not pretending to pass a reall value
for si_code.

Should si_code simply be ignored for the SIGKILL case?

Cheers
---Dave