um: add mmap/mremap OS calls

For the upcoming shared-memory time-travel external
optimisations, we need to be able to mmap/mremap.
Add the necessary OS calls.

Link: https://patch.msgid.link/20240702192118.ca4472963638.Ic2da1d3a983fe57340c1b693badfa9c5bd2d8c61@changeid
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/include/shared/os.h

···
 extern int os_sendmsg_fds(int fd, const void *buf, unsigned int len,
 			  const int *fds, unsigned int fds_num);
 int os_poll(unsigned int n, const int *fds);
+void *os_mmap_rw_shared(int fd, size_t size);
+void *os_mremap_rw_shared(void *old_addr, size_t old_size, size_t new_size);
 
 /* start_up.c */
 extern void os_early_checks(void);

arch/um/os-Linux/file.c

···
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
 #include <sys/un.h>
+#include <sys/mman.h>
 #include <sys/types.h>
 #include <sys/eventfd.h>
 #include <poll.h>
···
 	}
 
 	return -EIO;
+}
+
+void *os_mmap_rw_shared(int fd, size_t size)
+{
+	void *res = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
+	if (res == MAP_FAILED)
+		return NULL;
+
+	return res;
+}
+
+void *os_mremap_rw_shared(void *old_addr, size_t old_size, size_t new_size)
+{
+	void *res;
+
+	res = mremap(old_addr, old_size, new_size, MREMAP_MAYMOVE, NULL);
+
+	if (res == MAP_FAILED)
+		return NULL;
+
+	return res;
 }