Yes, the pointer provided by the caller, coming from the RPC buffer, may not
actually be safe to dereference. Try this with /run/shm as tmpfs with the crash server configured to dump cores:

#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

#define name "/run/shm/test.txt"
int main(void) {
	int fd = open(name, O_RDWR|O_CREAT, 0777);
	if (ftruncate(fd, 4096))
		perror("fruncate");
	char *c = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
	if (c == MAP_FAILED)
		perror("mmap");
	if (close(fd))
		perror("close");
	if (unlink(name))
		perror("unlink");
	memset(c, 0, 4096);
	if (munmap(c, 4096))
		perror("munmap");

	return 0;
}

It will make *ext2fs* crash, because

- removing a file from tmpfs make its memory object go away, thus making *c
  unwritable (it's not the bug at stake, the program here is meant to crash)
- the crash server uses vm_read to read the process memory to write the
  core. GNU Mach achieves it by playing with virtual memory.
- the crash server uses vm_write to write this to the FS. GNU Mach passes the
  RPC data out of line by playing with virtual memory.
- ext2fs eventually tries to copy from the RPC data, assumed to be safe, to the
  memory object, here backed by the pager. But the data is actually not safe.


That probably needs to be fixed at the mig layer, to make sure incoming
out-of-line data is accessible before handing it to the routine?

Index: hurd-debian/libpager/pager-memcpy.c
===================================================================
--- hurd-debian.orig/libpager/pager-memcpy.c
+++ hurd-debian/libpager/pager-memcpy.c
@@ -124,11 +124,14 @@ pager_memcpy (struct pager *pager, memor
 	      __sync_synchronize();
 
 	      if (prot == VM_PROT_READ)
-		memcpy (other, (const void *) window + pageoff, copy_count);
+		err = hurd_safe_copyout (other, (const void *) window + pageoff, copy_count);
 	      else
-		memcpy ((void *) window + pageoff, other, copy_count);
+		err = hurd_safe_copyin ((void *) window + pageoff, other, copy_count);
 	      vm_deallocate (mach_task_self (), window, window_size);
 
+	      if (err)
+		return err;
+	      
 	      offset += copy_count;
 	      other += copy_count;
 	      to_copy -= copy_count;