MIPS: Fix MAX_REG_OFFSET

[ Upstream commit c44572e ]

Fix MAX_REG_OFFSET to point to the last register in 'pt_regs' and not to
the marker itself, which could allow regs_get_register() to return an
invalid offset.

Fixes: 40e084a ("MIPS: Add uprobes support.")
Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit eaa30e1d0ed229311c939e9f471f8e52632ae7b9)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>

Author: toblux

arch/mips/include/asm/ptrace.h

@@ -65,7 +65,8 @@ static inline void instruction_pointer_set(struct pt_regs *regs,
 
 /* Query offset/name of register from its name/offset */
 extern int regs_query_register_offset(const char *name);
-#define MAX_REG_OFFSET (offsetof(struct pt_regs, __last))
+#define MAX_REG_OFFSET \
+	(offsetof(struct pt_regs, __last) - sizeof(unsigned long))
 
 /**
  * regs_get_register() - get register value from its offset