[PATCH] MIPS: Don't declare __current_thread_info globally

[Paul Burton <paulburton@kernel.org>]

Declaring __current_thread_info as a global register variable has the
effect of preventing GCC from saving & restoring its value in cases
where the ABI would typically do so.

To quote GCC documentation:

> If the register is a call-saved register, call ABI is affected: the
> register will not be restored in function epilogue sequences after the
> variable has been assigned. Therefore, functions cannot safely return
> to callers that assume standard ABI.

When our position independent VDSO is built for the n32 or n64 ABIs all
functions it exposes should be preserving the value of $gp/$28 for their
caller, but in the presence of the __current_thread_info global register
variable GCC stops doing so & simply clobbers $gp/$28 when calculating
the address of the GOT.

In cases where the VDSO returns success this problem will typically be
masked by the caller in libc returning & restoring $gp/$28 itself, but
that is by no means guaranteed. In cases where the VDSO returns an error
libc will typically contain a fallback path which will now fail
(typically with a bad memory access) if it attempts anything which
relies upon the value of $gp/$28 - eg. accessing anything via the GOT.

Fix this by moving the declaration of __current_thread_info inside the
current_thread_info() function, demoting it from global register
variable to local register variable & avoiding inadvertently creating a
non-standard calling ABI for the VDSO.

Signed-off-by: Paul Burton <paulburton@kernel.org>
Reported-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Fixes: ebb5e78cc634 ("MIPS: Initial implementation of a VDSO")
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Brauner <christian.brauner@canonical.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org> # v4.4+
---
 arch/mips/include/asm/thread_info.h | 4 ++--
 arch/mips/kernel/relocate.c         | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
index 4993db40482c..aceefc3f9a1a 100644
--- a/arch/mips/include/asm/thread_info.h
+++ b/arch/mips/include/asm/thread_info.h
@@ -50,10 +50,10 @@ struct thread_info {
 }
 
 /* How to get the thread information struct from C.  */
-register struct thread_info *__current_thread_info __asm__("$28");
-
 static inline struct thread_info *current_thread_info(void)
 {
+	register struct thread_info *__current_thread_info __asm__("$28");
+
 	return __current_thread_info;
 }
 
diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c
index 3d80a51256de..c9afdc39b003 100644
--- a/arch/mips/kernel/relocate.c
+++ b/arch/mips/kernel/relocate.c
@@ -296,6 +296,7 @@ static inline int __init relocation_addr_valid(void *loc_new)
 
 void *__init relocate_kernel(void)
 {
+	register struct thread_info *__current_thread_info __asm__("$28");
 	void *loc_new;
 	unsigned long kernel_length;
 	unsigned long bss_length;
-- 
2.24.0