samples/seccomp: fix endianness bug in LO_ARG define

The LO_ARG define needs to consider endianness also for 32 bit builds.

The "bpf_fancy" test case didn't work on s390 in 32 bit and compat mode
because the LO_ARG define resulted in a BPF program which read the upper
halve of the 64 bit system call arguments instead of the lower halves.

Signed-off-by: Heiko Carstens <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

samples/seccomp/bpf-helper.h

···
 #define FIND_LABEL(labels, label) seccomp_bpf_label((labels), #label)
 
 #define EXPAND(...) __VA_ARGS__
+
+/* Ensure that we load the logically correct offset. */
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
+#elif __BYTE_ORDER == __BIG_ENDIAN
+#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
+#else
+#error "Unknown endianness"
+#endif
+
 /* Map all width-sensitive operations */
 #if __BITS_PER_LONG == 32
 
···
 #define JLE(x, jt) JLE32(x, EXPAND(jt))
 #define JA(x, jt) JA32(x, EXPAND(jt))
 #define ARG(i) ARG_32(i)
-#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
 
 #elif __BITS_PER_LONG == 64
 
 /* Ensure that we load the logically correct offset. */
 #if __BYTE_ORDER == __LITTLE_ENDIAN
 #define ENDIAN(_lo, _hi) _lo, _hi
-#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
 #define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
 #elif __BYTE_ORDER == __BIG_ENDIAN
 #define ENDIAN(_lo, _hi) _hi, _lo
-#define LO_ARG(idx) offsetof(struct seccomp_data, args[(idx)]) + sizeof(__u32)
 #define HI_ARG(idx) offsetof(struct seccomp_data, args[(idx)])
-#else
-#error "Unknown endianness"
 #endif
 
 union arg64 {