bpf: Move getsockopt retval to struct bpf_cg_run_ctx

The retval value is moved to struct bpf_cg_run_ctx for ease of access
in different prog types with different context structs layouts. The
helper implementation (to be added in a later patch in the series) can
simply perform a container_of from current->bpf_ctx to retrieve
bpf_cg_run_ctx.

Unfortunately, there is no easy way to access the current task_struct
via the verifier BPF bytecode rewrite, aside from possibly calling a
helper, so a pointer to current task is added to struct bpf_sockopt_kern
so that the rewritten BPF bytecode can access struct bpf_cg_run_ctx with
an indirection.

For backward compatibility, if a getsockopt program rejects a syscall
by returning 0, an -EPERM will be generated, by having the
BPF_PROG_RUN_ARRAY_CG family macros automatically set the retval to
-EPERM. Unlike prior to this patch, this -EPERM will be visible to
ctx->retval for any other hooks down the line in the prog array.

Additionally, the restriction that getsockopt filters can only set
the retval to 0 is removed, considering that certain getsockopt
implementations may return optlen. Filters are now able to set the
value arbitrarily.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/73b0325f5c29912ccea7ea57ec1ed4d388fc1d37.1639619851.git.zhuyifei@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 71fa57b88bfc..d23e999dc032 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1356,7 +1356,10 @@ struct bpf_sockopt_kern {
 	s32		level;
 	s32		optname;
 	s32		optlen;
-	s32		retval;
+	/* for retval in struct bpf_cg_run_ctx */
+	struct task_struct *current_task;
+	/* Temporary "register" for indirect stores to ppos. */
+	u64		tmp_reg;
 };
 
 int copy_bpf_fprog_from_user(struct sock_fprog *dst, sockptr_t src, int len);