vfs: Avoid null dereference

[des]

Summary

In VOP_READDIR(), ap->a_eofflag may be null. Check that it isn't
before dereferencing it.

[markj]

Why not fix up the one VOP_READDIR caller which doesn't pass an eof pointer?

[des]

In D52386#1196251, @markj wrote:

Why not fix up the one VOP_READDIR caller which doesn't pass an eof pointer?

Because eofflag is documented to be optional and nullable, there may be third-party code that expects that to be the case, and removing the existing null checks would be more work than adding the handful that were missing.

[kib]

In D52386#1196301, @des wrote:

In D52386#1196251, @markj wrote:

Why not fix up the one VOP_READDIR caller which doesn't pass an eof pointer?

Because eofflag is documented to be optional and nullable, there may be third-party code that expects that to be the case, and removing the existing null checks would be more work than adding the handful that were missing.

Where is it documented, in VOP_READDIR.9? Then change the documentation as well.

We do not maintain neither KBI nor KPI compatibility for VFS (or VM), so this change should be even mergeable to stable branches.

[markj]

In D52386#1196305, @kib wrote:

In D52386#1196301, @des wrote:

In D52386#1196251, @markj wrote:

Why not fix up the one VOP_READDIR caller which doesn't pass an eof pointer?

Because eofflag is documented to be optional and nullable, there may be third-party code that expects that to be the case, and removing the existing null checks would be more work than adding the handful that were missing.

Where is it documented, in VOP_READDIR.9? Then change the documentation as well.

We do not maintain neither KBI nor KPI compatibility for VFS (or VM), so this change should be even mergeable to stable branches.

I agree, it is better to make the interface simpler even if it's a bit more work. Third-party code can easily adapt to the new semantics.