add framework to support archs without a native wait4 syscall

this commit should make no codegen change for existing archs, but is a
prerequisite for new archs including riscv32. the wait4 emulation
backend provides both cancellable and non-cancellable variants because
waitpid is required to be a cancellation point, but all of our other
uses are not, and most of them cannot be.

based on patch by Stefan O'Rear.

2 files changed, 67 insertions, 0 deletions

diff --git a/src/internal/emulate_wait4.c b/src/internal/emulate_wait4.c
new file mode 100644
index 00000000..f6303412
--- /dev/null
+++ b/src/internal/emulate_wait4.c
@@ -0,0 +1,55 @@
+#include <sys/wait.h>
+#include "syscall.h"
+
+#ifndef SYS_wait4
+hidden long __emulate_wait4(int pid, int *status, int options, void *kru, int cp)
+{
+	idtype_t t;
+	int r;
+	siginfo_t info;
+
+	info.si_pid = 0;
+	if (pid < -1) {
+		t = P_PGID;
+		pid = -pid;
+	} else if (pid == -1) {
+		t = P_ALL;
+	} else if (pid == 0) {
+		t = P_PGID;
+	} else {
+		t = P_PID;
+	}
+
+	if (cp) r = __syscall_cp(SYS_waitid, t, pid, &info, options|WEXITED, kru);
+	else r = __syscall(SYS_waitid, t, pid, &info, options|WEXITED, kru);
+
+	if (r<0) return r;
+
+	if (info.si_pid && status) {
+		int sw=0;
+		switch (info.si_code) {
+		case CLD_CONTINUED:
+			sw = 0xffff;
+			break;
+		case CLD_DUMPED:
+			sw = info.si_status&0x7f | 0x80;
+			break;
+		case CLD_EXITED:
+			sw = (info.si_status&0xff) << 8;
+			break;
+		case CLD_KILLED:
+			sw = info.si_status&0x7f;
+			break;
+		case CLD_STOPPED:
+		case CLD_TRAPPED:
+			/* see ptrace(2); the high bits of si_status can contain */
+			/* PTRACE_EVENT_ values which must be preserved */
+			sw = (info.si_status << 8) + 0x7f;
+			break;
+		}
+		*status = sw;
+	}
+
+	return info.si_pid;
+}
+#endif
diff --git a/src/internal/syscall.h b/src/internal/syscall.h
index 4a446157..33d981f9 100644
--- a/src/internal/syscall.h
+++ b/src/internal/syscall.h
@@ -391,6 +391,18 @@ static inline long __alt_socketcall(int sys, int sock, int cp, syscall_arg_t a,
 #define __sys_open_cp(...) __SYSCALL_DISP(__sys_open_cp,,__VA_ARGS__)
 #define sys_open_cp(...) __syscall_ret(__sys_open_cp(__VA_ARGS__))
 
+#ifdef SYS_wait4
+#define __sys_wait4(a,b,c,d) __syscall(SYS_wait4,a,b,c,d)
+#define __sys_wait4_cp(a,b,c,d) __syscall_cp(SYS_wait4,a,b,c,d)
+#else
+hidden long __emulate_wait4(int, int *, int, void *, int);
+#define __sys_wait4(a,b,c,d) __emulate_wait4(a,b,c,d,0)
+#define __sys_wait4_cp(a,b,c,d) __emulate_wait4(a,b,c,d,1)
+#endif
+
+#define sys_wait4(a,b,c,d) __syscall_ret(__sys_wait4(a,b,c,d))
+#define sys_wait4_cp(a,b,c,d) __syscall_ret(__sys_wait4_cp(a,b,c,d))
+
 hidden void __procfdname(char __buf[static 15+3*sizeof(int)], unsigned);
 
 hidden void *__vdsosym(const char *, const char *);