prefer new socket syscalls, fallback to SYS_socketcall only if needed

a number of users performing seccomp filtering have requested use of
the new individual syscall numbers for socket syscalls, rather than
the legacy multiplexed socketcall, since the latter has the arguments
all in memory where they can't participate in filter decisions.

previously, some archs used the multiplexed socketcall if it was
historically all that was available, while other archs used the
separate syscalls. the intent was that the latter set only include
archs that have "always" had separate socket syscalls, at least going
back to linux 2.6.0. however, at least powerpc, powerpc64, and sh were
wrongly included in this set, and thus socket operations completely
failed on old kernels for these archs.

with the changes made here, the separate syscalls are always
preferred, but fallback code is compiled for archs that also define
SYS_socketcall. two such archs, mips (plain o32) and microblaze,
define SYS_socketcall despite never having needed it, so it's now
undefined by their versions of syscall_arch.h to prevent inclusion of
useless fallback code.

some archs, where the separate syscalls were only added after the
addition of SYS_accept4, lack SYS_accept. because socket calls are
always made with zeros in the unused argument positions, it suffices
to just use SYS_accept4 to provide a definition of SYS_accept, and
this is done to make happy the macro machinery that concatenates the
socket call name onto __SC_ and SYS_.

arch/i386/syscall_arch.h

@@ -87,5 +87,3 @@ static inline long __syscall6(long n, long a1, long a2, long a3, long a4, long a
 #define VDSO_CGT32_VER "LINUX_2.6"
 #define VDSO_CGT_SYM "__vdso_clock_gettime64"
 #define VDSO_CGT_VER "LINUX_2.6"
-
-#define SYSCALL_USE_SOCKETCALL

arch/m68k/syscall_arch.h

@@ -87,5 +87,4 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 	return d0;
 }
 
-#define SYSCALL_USE_SOCKETCALL
 #define SYSCALL_IPC_BROKEN_MODE

arch/microblaze/syscall_arch.h

@@ -95,3 +95,5 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 }
 
 #define SYSCALL_IPC_BROKEN_MODE
+
+#undef SYS_socketcall

arch/mips/syscall_arch.h

@@ -149,3 +149,5 @@ static inline long __syscall7(long n, long a, long b, long c, long d, long e, lo
 
 #define SO_SNDTIMEO_OLD 0x1005
 #define SO_RCVTIMEO_OLD 0x1006
+
+#undef SYS_socketcall

arch/s390x/syscall_arch.h

@@ -72,5 +72,3 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 	register long r7 __asm__("r7") = f;
 	__asm_syscall("+r"(r2), "r"(r1), "r"(r3), "r"(r4), "r"(r5), "r"(r6), "r"(r7));
 }
-
-#define SYSCALL_USE_SOCKETCALL

src/internal/syscall.h

@@ -2,6 +2,7 @@
 #define _INTERNAL_SYSCALL_H
 
 #include <features.h>
+#include <errno.h>
 #include <sys/syscall.h>
 #include "syscall_arch.h"
 
@@ -57,15 +58,22 @@ hidden long __syscall_ret(unsigned long),
 #define __syscall_cp(...) __SYSCALL_DISP(__syscall_cp,__VA_ARGS__)
 #define syscall_cp(...) __syscall_ret(__syscall_cp(__VA_ARGS__))
 
-#ifndef SYSCALL_USE_SOCKETCALL
-#define __socketcall(nm,a,b,c,d,e,f) __syscall(SYS_##nm, a, b, c, d, e, f)
-#define __socketcall_cp(nm,a,b,c,d,e,f) __syscall_cp(SYS_##nm, a, b, c, d, e, f)
-#else
-#define __socketcall(nm,a,b,c,d,e,f) __syscall(SYS_socketcall, __SC_##nm, \
-    ((long [6]){ (long)a, (long)b, (long)c, (long)d, (long)e, (long)f }))
-#define __socketcall_cp(nm,a,b,c,d,e,f) __syscall_cp(SYS_socketcall, __SC_##nm, \
-    ((long [6]){ (long)a, (long)b, (long)c, (long)d, (long)e, (long)f }))
-#endif
+static inline long __alt_socketcall(int sys, int sock, int cp, long a, long b, long c, long d, long e, long f)
+{
+	long r;
+	if (cp) r = __syscall_cp(sys, a, b, c, d, e, f);
+	else r = __syscall(sys, a, b, c, d, e, f);
+	if (r != -ENOSYS) return r;
+#ifdef SYS_socketcall
+	if (cp) r = __syscall_cp(SYS_socketcall, sock, ((long[6]){a, b, c, d, e, f}));
+	else r = __syscall(SYS_socketcall, sock, ((long[6]){a, b, c, d, e, f}));
+#endif
+	return r;
+}
+#define __socketcall(nm, a, b, c, d, e, f) __alt_socketcall(SYS_##nm, __SC_##nm, 0, \
+	(long)(a), (long)(b), (long)(c), (long)(d), (long)(e), (long)(f))
+#define __socketcall_cp(nm, a, b, c, d, e, f) __alt_socketcall(SYS_##nm, __SC_##nm, 1, \
+	(long)(a), (long)(b), (long)(c), (long)(d), (long)(e), (long)(f))
 
 /* fixup legacy 16-bit junk */
 
@@ -338,6 +346,12 @@ hidden long __syscall_ret(unsigned long),
 #define __SC_recvmmsg    19
 #define __SC_sendmmsg    20
 
+/* This is valid only because all socket syscalls are made via
+ * socketcall, which always fills unused argument slots with zeros. */
+#ifndef SYS_accept
+#define SYS_accept SYS_accept4
+#endif
+
 #ifndef SO_RCVTIMEO_OLD
 #define SO_RCVTIMEO_OLD  20
 #endif