Commit 38b407be authored May 08, 2020 by Michael Ellerman

powerpc/spufs: Rework fcheck() usage



Currently the spu coredump code triggers an RCU warning:

  =============================
  WARNING: suspicious RCU usage
  5.7.0-rc3-01755-g7cd49f0b7ec7 #1 Not tainted
  -----------------------------
  include/linux/fdtable.h:95 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 2, debug_locks = 1
  1 lock held by spu-coredump/1343:
   #0: c0000007fa22f430 (sb_writers#2){.+.+}-{0:0}, at: .do_coredump+0x1010/0x13c8

  stack backtrace:
  CPU: 0 PID: 1343 Comm: spu-coredump Not tainted 5.7.0-rc3-01755-g7cd49f0b7ec7 #1
  Call Trace:
    .dump_stack+0xec/0x15c (unreliable)
    .lockdep_rcu_suspicious+0x120/0x144
    .coredump_next_context+0x148/0x158
    .spufs_coredump_extra_notes_size+0x54/0x190
    .elf_coredump_extra_notes_size+0x34/0x50
    .elf_core_dump+0xe48/0x19d0
    .do_coredump+0xe50/0x13c8
    .get_signal+0x864/0xd88
    .do_notify_resume+0x158/0x3c8
    .interrupt_exit_user_prepare+0x19c/0x208
    interrupt_return+0x14/0x1c0

This comes from fcheck_files() via fcheck().

It's pretty clearly documented that fcheck() must be wrapped with
rcu_read_lock(), adding that fixes the RCU warning.

hch points out that once we've released the RCU read lock the file may
be closed and freed, which would leave us with a pointer to a freed
spu_context.

To avoid that, take a reference to the spu_context while we hold the
RCU read lock, and drop that reference later once we're done with the
context.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200508130633.2532759-1-mpe@ellerman.id.au

parent 9d1ebe9a

arch/powerpc/platforms/cell/spufs/coredump.c

+16 −3

Original line number	Diff line number	Diff line
		@@ -66,13 +66,20 @@ static int match_context(const void v, struct file file, unsigned fd)
		*/
		static struct spu_context coredump_next_context(int fd)
		{
		struct spu_context *ctx;
		struct file *file;
		int n = iterate_fd(current->files, *fd, match_context, NULL);
		if (!n)
		return NULL;
		*fd = n - 1;

		rcu_read_lock();
		file = fcheck(*fd);
		return SPUFS_I(file_inode(file))->i_ctx;
		ctx = SPUFS_I(file_inode(file))->i_ctx;
		get_spu_context(ctx);
		rcu_read_unlock();

		return ctx;
		}

		int spufs_coredump_extra_notes_size(void)
		@@ -83,17 +90,23 @@ int spufs_coredump_extra_notes_size(void)
		fd = 0;
		while ((ctx = coredump_next_context(&fd)) != NULL) {
		rc = spu_acquire_saved(ctx);
		if (rc)
		if (rc) {
		put_spu_context(ctx);
		break;
		}

		rc = spufs_ctx_note_size(ctx, fd);
		spu_release_saved(ctx);
		if (rc < 0)
		if (rc < 0) {
		put_spu_context(ctx);
		break;
		}

		size += rc;

		/* start searching the next fd next time */
		fd++;
		put_spu_context(ctx);
		}

		return size;