signal/powerpc: Document conflicts with SI_USER and SIGFPE and SIGTRAP

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 and TRAP_FIXME, siginfo_layout() will now return
SIL_FAULT and the appropriate fields will be reliably copied.

Possible ABI fixes includee:
- 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
Cc: Paul Mackerras <paulus@samba.org>
Cc: Kumar Gala <kumar.gala@freescale.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:  linuxppc-dev@lists.ozlabs.org
Ref: 9bad068c24d7 ("[PATCH] ppc32: support for e500 and 85xx")
Ref: 0ed70f6105ef ("PPC32: Provide proper siginfo information on various exceptions.")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

arch/powerpc/include/uapi/asm/siginfo.h

@@ -18,4 +18,19 @@
 #undef NSIGTRAP
 #define NSIGTRAP	4
 
+/*
+ * SIGFPE si_codes
+ */
+#ifdef __KERNEL__
+#define FPE_FIXME	0	/* Broken dup of SI_USER */
+#endif /* __KERNEL__ */
+
+/*
+ * SIGTRAP si_codes
+ */
+#ifdef __KERNEL__
+#define TRAP_FIXME	0	/* Broken dup of SI_USER */
+#endif /* __KERNEL__ */
+
+
 #endif	/* _ASM_POWERPC_SIGINFO_H */

arch/powerpc/kernel/traps.c

@@ -917,7 +917,7 @@ void unknown_exception(struct pt_regs *regs)
 	printk("Bad trap at PC: %lx, SR: %lx, vector=%lx\n",
 	       regs->nip, regs->msr, regs->trap);
 
-	_exception(SIGTRAP, regs, 0, 0);
+	_exception(SIGTRAP, regs, TRAP_FIXME, 0);
 
 	exception_exit(prev_state);
 }
@@ -939,7 +939,7 @@ bail:
 
 void RunModeException(struct pt_regs *regs)
 {
-	_exception(SIGTRAP, regs, 0, 0);
+	_exception(SIGTRAP, regs, TRAP_FIXME, 0);
 }
 
 void single_step_exception(struct pt_regs *regs)
@@ -978,7 +978,7 @@ static void emulate_single_step(struct pt_regs *regs)
 
 static inline int __parse_fpscr(unsigned long fpscr)
 {
-	int ret = 0;
+	int ret = FPE_FIXME;
 
 	/* Invalid operation */
 	if ((fpscr & FPSCR_VE) && (fpscr & FPSCR_VX))
@@ -1929,7 +1929,7 @@ void SPEFloatingPointException(struct pt_regs *regs)
 	extern int do_spe_mathemu(struct pt_regs *regs);
 	unsigned long spefscr;
 	int fpexc_mode;
-	int code = 0;
+	int code = FPE_FIXME;
 	int err;
 
 	flush_spe_to_thread(current);
@@ -1998,7 +1998,7 @@ void SPEFloatingPointRoundException(struct pt_regs *regs)
 		printk(KERN_ERR "unrecognized spe instruction "
 		       "in %s at %lx\n", current->comm, regs->nip);
 	} else {
-		_exception(SIGFPE, regs, 0, regs->nip);
+		_exception(SIGFPE, regs, FPE_FIXME, regs->nip);
 		return;
 	}
 }