Fix crashes reading /proc/<pid> of a task that is exiting

fs/fd.c

@@ -143,7 +143,10 @@ static int fdtable_expand(struct fdtable *table, fd_t max) {
 }
 
 struct fd *fdtable_get(struct fdtable *table, fd_t f) {
-    if (f < 0 || (unsigned) f >= current->files->size)
+    // Bounds-check the table being indexed, not the calling task's. They are
+    // not always the same one: /proc/<pid>/fd reads another task's table, and
+    // fdtable_release walks a table the caller has already let go of.
+    if (f < 0 || (unsigned) f >= table->size)
         return NULL;
     return table->files[f];
 }

fs/proc/pid.c

@@ -269,7 +269,15 @@ static struct proc_dir_entry proc_pid_fd;
 static bool proc_pid_fd_readdir(struct proc_entry *entry, unsigned long *index, struct proc_entry *next_entry) {
     struct task *task = proc_get_task(entry);
     if (task == NULL)
-        return _ESRCH;
+        // This returns "is there another entry", not an error code. _ESRCH is
+        // nonzero, so it reads as true and proc_readdir goes on to use a
+        // next_entry that was never filled in.
+        return false;
+    if (task->files == NULL) {
+        // Exited, not yet reaped: do_exit dropped the fd table already.
+        proc_put_task(task);
+        return false;
+    }
     lock(&task->files->lock);
     while (*index < task->files->size && task->files->files[*index] == NULL)
         (*index)++;
@@ -289,9 +297,15 @@ static int proc_pid_fd_readlink(struct proc_entry *entry, char *buf) {
     struct task *task = proc_get_task(entry);
     if (task == NULL)
         return _ESRCH;
+    if (task->files == NULL) {
+        proc_put_task(task);
+        return _ESRCH;
+    }
     lock(&task->files->lock);
     struct fd *fd = fdtable_get(task->files, entry->fd);
-    int err = generic_getpath(fd, buf);
+    int err = _ENOENT;
+    if (fd != NULL)
+        err = generic_getpath(fd, buf);
     unlock(&task->files->lock);
     proc_put_task(task);
     return err;
@@ -302,7 +316,9 @@ static int proc_pid_exe_readlink(struct proc_entry *entry, char *buf) {
     if (task == NULL)
         return _ESRCH;
     lock(&task->general_lock);
-    int err = generic_getpath(task->mm->exefile, buf);
+    int err = _ESRCH;
+    if (task->mm != NULL)
+        err = generic_getpath(task->mm->exefile, buf);
     unlock(&task->general_lock);
     proc_put_task(task);
     return err;
@@ -329,6 +345,10 @@ static int proc_pid_cwd_readlink(struct proc_entry *entry, char *buf) {
     struct task *task = proc_get_task(entry);
     if (task == NULL)
         return _ESRCH;
+    if (task->fs == NULL) {
+        proc_put_task(task);
+        return _ESRCH;
+    }
     lock(&task->fs->lock);
     int err = generic_getpath(task->fs->pwd, buf);
     unlock(&task->fs->lock);

kernel/exit.c

@@ -50,12 +50,21 @@ noreturn void do_exit(int status) {
     }
 
     // release all our resources
-    mm_release(current->mm);
+    // These are read by other threads through /proc/<pid>, which holds
+    // pids_lock for the whole access, so detach them under pids_lock -- a
+    // reader then sees either a pointer that is still ours to hand out, or
+    // NULL. The actual releasing has to happen outside the lock.
+    lock(&pids_lock);
+    struct mm *mm = current->mm;
+    struct fdtable *files = current->files;
+    struct fs_info *fs = current->fs;
     current->mm = NULL;
-    fdtable_release(current->files);
     current->files = NULL;
-    fs_info_release(current->fs);
     current->fs = NULL;
+    unlock(&pids_lock);
+    mm_release(mm);
+    fdtable_release(files);
+    fs_info_release(fs);
     // sighand must be released below so it can be protected by pids_lock
     // since it can be accessed by other threads