libsel4muslcsys: Implement sys_write wrapper

This wraps sys_writev the same way that sys_read is implemented.

libsel4muslcsys/src/sys_io.c

diff --git a/libsel4muslcsys/src/sys_io.c b/libsel4muslcsys/src/sys_io.c
index bd94f4e..4544371 100644
--- a/libsel4muslcsys/src/sys_io.c
+++ b/libsel4muslcsys/src/sys_io.c
@@ -326,6 +326,17 @@
     return ret;
 }
 
+long sys_write(va_list ap)
+{
+
+    int fd = va_arg(ap, int);
+    void *buf = va_arg(ap, void *);
+    size_t count = va_arg(ap, size_t);
+    /* construct an iovec and call writev */
+    struct iovec iov = {.iov_base = buf, .iov_len = count };
+    return writev(fd, &iov, 1);
+}
+
 long sys_readv(va_list ap)
 {
     int fd = va_arg(ap, int);

libsel4muslcsys/src/syscalls.h

diff --git a/libsel4muslcsys/src/syscalls.h b/libsel4muslcsys/src/syscalls.h
index 1761c56..9101f8d 100644
--- a/libsel4muslcsys/src/syscalls.h
+++ b/libsel4muslcsys/src/syscalls.h
@@ -39,6 +39,7 @@
 long sys_mmap2(va_list ap);
 long sys_mmap(va_list ap);
 long sys_mremap(va_list ap);
+long sys_write(va_list ap);
 long sys_writev(va_list ap);
 long sys_madvise(va_list ap);
 

libsel4muslcsys/src/vsyscall.c

diff --git a/libsel4muslcsys/src/vsyscall.c b/libsel4muslcsys/src/vsyscall.c
index 666e5ad..021948b 100644
--- a/libsel4muslcsys/src/vsyscall.c
+++ b/libsel4muslcsys/src/vsyscall.c
@@ -120,6 +120,9 @@
 #endif
     [__NR_set_tid_address] = boot_set_tid_address,
     [__NR_writev] = boot_sys_writev,
+    /* We don't need a boot_sys_write variant as this implementation wraps
+     * whatever __NR_writev is set to. */
+    [__NR_write] = sys_write,
     [__NR_sched_yield] = sys_sched_yield,
     [__NR_exit] = sys_exit,
     [__NR_rt_sigprocmask] = sys_rt_sigprocmask,