Date: Fri, 12 Jan 2018 23:29:44 +0100
From: Helge Deller <deller@gmx.de>
To: "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-parisc@vger.kernel.org,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        John David Anglin <dave.anglin@bell.net>
Cc: linux-kernel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
        Oleg Nesterov <oleg@redhat.com>, linux-arch@vger.kernel.org
Subject: Re: [PATCH 04/11] signal/parisc: Document a conflict with SI_USER
 with SIGFPE
Message-ID: <20180112222944.GA22642@ls3530.fritz.box>
References: <87373b6ghs.fsf@xmission.com>
 <20180112005940.23279-4-ebiederm@xmission.com>
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* Eric W. Biederman <ebiederm@xmission.com>:
> Setting si_code to 0 results in a userspace seeing an si_code of 0.
> This is the same si_code as SI_USER.  Posix and common sense requires
> that SI_USER not be a signal specific si_code.  As such this use of 0
> for the si_code is a pretty horribly broken ABI.
> 
> Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
> value of __SI_KILL and now sees a value of SIL_KILL with the result
> that uid and pid fields are copied and which might copying the si_addr
> field by accident but certainly not by design.  Making this a very
> flakey implementation.
> 
> Utilizing FPE_FIXME siginfo_layout will now return SIL_FAULT and the
> appropriate fields will reliably be copied.
> 
> This bug is 13 years old and parsic machines are no longer being built
> so I don't know if it possible or worth fixing it.  But it is at least
> worth documenting this so other architectures don't make the same
> mistake.


I think we should fix it, even if we now break the ABI.

It's about a "conditional trap" which needs to be handled by userspace.
I doubt there is any Linux code out which is utilizing this
parisc-specific trap.

I'd suggest to add a new FPE trap si_code (e.g. FPE_CONDTRAP).
While at it, maybe we should include the already existing FPE_MDAOVF
from the frv architecture, so that arch/frv/include/uapi/asm/siginfo.h
can go completely.

Suggested patch is below.

I'm willing to test the patch below on the parisc architecture for a few
weeks. And it will break arch/x86/kernel/signal_compat.c which needs
looking at then too.

Thoughts?

Helge


[PATCH] parisc: Add FPE_CONDTRAP for conditional trap handling

Posix and common sense requires that SI_USER not be a signal specific
si_code.  Thus add a new FPE_CONDTRAP si_code for conditional traps.

Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/arch/parisc/kernel/traps.c b/arch/parisc/kernel/traps.c
index 8453724b8009..13702f0f5ba1 100644
--- a/arch/parisc/kernel/traps.c
+++ b/arch/parisc/kernel/traps.c
@@ -627,9 +627,9 @@ void notrace handle_interruption(int code, struct pt_regs *regs)
 		   on condition  */
 		if(user_mode(regs)){
 			si.si_signo = SIGFPE;
-			/* Set to zero, and let the userspace app figure it out from
-			   the insn pointed to by si_addr */
-			si.si_code = 0;
+			/* Let userspace app figure out from the insn pointed
+			 * to by si_addr */
+			si.si_code = FPE_CONDTRAP;
 			si.si_addr = (void __user *) regs->iaoq[0];
 			force_sig_info(SIGFPE, &si, current);
 			return;
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index e447283b8f52..2b759fe42142 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -193,7 +193,9 @@ typedef struct siginfo {
 #define FPE_FLTRES	6	/* floating point inexact result */
 #define FPE_FLTINV	7	/* floating point invalid operation */
 #define FPE_FLTSUB	8	/* subscript out of range */
-#define NSIGFPE		8
+#define FPE_MDAOVF	9       /* media overflow */
+#define FPE_CONDTRAP	10      /* trap on condition */
+#define NSIGFPE		10
 
 /*
  * SIGSEGV si_codes