[PATCH 3.12 075/123] m68k: Define asmlinkage_protect

From: Jiri Slaby
Date: Wed Oct 28 2015 - 10:12:35 EST

From: Andreas Schwab <schwab@xxxxxxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


commit 8474ba74193d302e8340dddd1e16c85cc4b98caf upstream.

Make sure the compiler does not modify arguments of syscall functions.
This can happen if the compiler generates a tailcall to another
function. For example, without asmlinkage_protect sys_openat is compiled
into this function:

clr.l %d0
move.w 18(%sp),%d0
move.l %d0,16(%sp)
jbra do_sys_open

Note how the fourth argument is modified in place, modifying the register
%d4 that gets restored from this stack slot when the function returns to
user-space. The caller may expect the register to be unmodified across
system calls.

Signed-off-by: Andreas Schwab <schwab@xxxxxxxxxxxxxx>
Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
arch/m68k/include/asm/linkage.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/arch/m68k/include/asm/linkage.h b/arch/m68k/include/asm/linkage.h
index 5a822bb790f7..066e74f666ae 100644
--- a/arch/m68k/include/asm/linkage.h
+++ b/arch/m68k/include/asm/linkage.h
@@ -4,4 +4,34 @@
#define __ALIGN .align 4
#define __ALIGN_STR ".align 4"

+ * Make sure the compiler doesn't do anything stupid with the
+ * arguments on the stack - they are owned by the *caller*, not
+ * the callee. This just fools gcc into not spilling into them,
+ * and keeps it from doing tailcall recursion and/or using the
+ * stack slots for temporaries, since they are live and "used"
+ * all the way to the end of the function.
+ */
+#define asmlinkage_protect(n, ret, args...) \
+ __asmlinkage_protect##n(ret, ##args)
+#define __asmlinkage_protect_n(ret, args...) \
+ __asm__ __volatile__ ("" : "=r" (ret) : "0" (ret), ##args)
+#define __asmlinkage_protect0(ret) \
+ __asmlinkage_protect_n(ret)
+#define __asmlinkage_protect1(ret, arg1) \
+ __asmlinkage_protect_n(ret, "m" (arg1))
+#define __asmlinkage_protect2(ret, arg1, arg2) \
+ __asmlinkage_protect_n(ret, "m" (arg1), "m" (arg2))
+#define __asmlinkage_protect3(ret, arg1, arg2, arg3) \
+ __asmlinkage_protect_n(ret, "m" (arg1), "m" (arg2), "m" (arg3))
+#define __asmlinkage_protect4(ret, arg1, arg2, arg3, arg4) \
+ __asmlinkage_protect_n(ret, "m" (arg1), "m" (arg2), "m" (arg3), \
+ "m" (arg4))
+#define __asmlinkage_protect5(ret, arg1, arg2, arg3, arg4, arg5) \
+ __asmlinkage_protect_n(ret, "m" (arg1), "m" (arg2), "m" (arg3), \
+ "m" (arg4), "m" (arg5))
+#define __asmlinkage_protect6(ret, arg1, arg2, arg3, arg4, arg5, arg6) \
+ __asmlinkage_protect_n(ret, "m" (arg1), "m" (arg2), "m" (arg3), \
+ "m" (arg4), "m" (arg5), "m" (arg6))

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