[ARM PATCH] 1565/1: syscall macros clobbering returned error value

Patch from Nicolas Pitre

In both 2.5.70-rmk1 and 2.4.19-rmk7 the syscall macros are clobering
the returned error value when building library code.

Example code:

#include <linux/unistd.h>
#include <errno.h>
extern int fake_syscall(int x, int y, int z);
_syscall3(int, fake_syscall, int, x, int, y, int, z)

Current generated code:

fake_syscall:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        str     lr, [sp, #-4]!
        swi     __NR_fake_syscall
        cmn     r0, #126
        ldrls   pc, [sp], #4
        bl      __errno_location
        rsb     r3, r0, #0
        str     r3, [r0, #0]
        mvn     r0, #0
        ldr     pc, [sp], #4

In the code above, whenever the return value is an error code, it
is lost due to the call to __errno_location. And because of the
asm("r0") constraint on the variable __res the compiler continues
using r0 for it even if it's now a pointer value.  errno ends up with
a totally bogus value.

With the patch below the above code becomes:

fake_syscall:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        stmfd   sp!, {r4, lr}
        swi     __NR_fake_syscall
        cmn     r0, #126
        mov     r4, r0
        bls     .L3
        bl      __errno_location
        rsb     r3, r4, #0
        str     r3, [r0, #0]
        mvn     r4, #0
.L3:
        mov     r0, r4
        ldmfd   sp!, {r4, pc}

which is correct.

Oh and added a small estetic change for generated code too.

include/asm-arm/unistd.h

@@ -303,7 +303,7 @@
 	"swi	0\n\t"					\
 	"pop	{r7}"
 #else
-#define __syscall(name) "swi\t" __sys1(__NR_##name) "\n\t"
+#define __syscall(name) "swi\t" __sys1(__NR_##name) ""
 #endif
 #endif
 
@@ -318,24 +318,28 @@ do {									\
 
 #define _syscall0(type,name)						\
 type name(void) {							\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	:"=r" (__res)							\
+	: "=r" (__res_r0)						\
 	:								\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }
 
 #define _syscall1(type,name,type1,arg1) 				\
 type name(type1 arg1) { 						\
   register long __r0 __asm__("r0") = (long)arg1;			\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	: "=r" (__res)							\
+	: "=r" (__res_r0)						\
 	: "r" (__r0)							\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }
 
@@ -343,12 +347,14 @@ type name(type1 arg1) { 						\
 type name(type1 arg1,type2 arg2) {					\
   register long __r0 __asm__("r0") = (long)arg1;			\
   register long __r1 __asm__("r1") = (long)arg2;			\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	: "=r" (__res)							\
+	: "=r" (__res_r0)						\
 	: "r" (__r0),"r" (__r1) 					\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }
 
@@ -358,12 +364,14 @@ type name(type1 arg1,type2 arg2,type3 arg3) {				\
   register long __r0 __asm__("r0") = (long)arg1;			\
   register long __r1 __asm__("r1") = (long)arg2;			\
   register long __r2 __asm__("r2") = (long)arg3;			\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	: "=r" (__res)							\
+	: "=r" (__res_r0)						\
 	: "r" (__r0),"r" (__r1),"r" (__r2)				\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }
 
@@ -374,12 +382,14 @@ type name(type1 arg1, type2 arg2, type3 arg3, type4 arg4) {		\
   register long __r1 __asm__("r1") = (long)arg2;			\
   register long __r2 __asm__("r2") = (long)arg3;			\
   register long __r3 __asm__("r3") = (long)arg4;			\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	: "=r" (__res)							\
+	: "=r" (__res_r0)						\
 	: "r" (__r0),"r" (__r1),"r" (__r2),"r" (__r3)			\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }
   
@@ -391,12 +401,14 @@ type name(type1 arg1, type2 arg2, type3 arg3, type4 arg4, type5 arg5) {	\
   register long __r2 __asm__("r2") = (long)arg3;			\
   register long __r3 __asm__("r3") = (long)arg4;			\
   register long __r4 __asm__("r4") = (long)arg5;			\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	: "=r" (__res)							\
+	: "=r" (__res_r0)						\
 	: "r" (__r0),"r" (__r1),"r" (__r2),"r" (__r3),"r" (__r4)	\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }
 
@@ -408,12 +420,14 @@ type name(type1 arg1, type2 arg2, type3 arg3, type4 arg4, type5 arg5, type6 arg6
   register long __r3 __asm__("r3") = (long)arg4;			\
   register long __r4 __asm__("r4") = (long)arg5;			\
   register long __r5 __asm__("r5") = (long)arg6;			\
-  register long __res __asm__("r0");					\
+  register long __res_r0 __asm__("r0");					\
+  long __res;								\
   __asm__ __volatile__ (						\
   __syscall(name)							\
-	: "=r" (__res)							\
+	: "=r" (__res_r0)						\
 	: "r" (__r0),"r" (__r1),"r" (__r2),"r" (__r3), "r" (__r4),"r" (__r5)		\
 	: "lr");							\
+  __res = __res_r0;							\
   __syscall_return(type,__res);						\
 }