mips: define syscall_get_error()

syscall_get_error() is required to be implemented on all architectures
in addition to already implemented syscall_get_nr(),
syscall_get_arguments(), syscall_get_return_value(), and
syscall_get_arch() functions in order to extend the generic ptrace API
with PTRACE_GET_SYSCALL_INFO request.

Link: http://lkml.kernel.org/r/20190510152803.GC28558@altlinux.org
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Acked-by: Paul Burton <paul.burton@mips.com>
Cc: Elvira Khabirova <lineprinter@altlinux.org>
Cc: Eugene Syromyatnikov <esyr@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Greentime Hu <greentime@andestech.com>
Cc: Helge Deller <deller@gmx.de>	[parisc]
Cc: James E.J. Bottomley <jejb@parisc-linux.org>
Cc: kbuild test robot <lkp@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vincent Chen <deanbo422@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/mips/include/asm/syscall.h

@@ -89,6 +89,12 @@ static inline unsigned long mips_get_syscall_arg(unsigned long *arg,
 	unreachable();
 }
 
+static inline long syscall_get_error(struct task_struct *task,
+				     struct pt_regs *regs)
+{
+	return regs->regs[7] ? -regs->regs[2] : 0;
+}
+
 static inline long syscall_get_return_value(struct task_struct *task,
 					    struct pt_regs *regs)
 {