samples/bpf: add a test for bpf_override_return

This adds a basic test for bpf_override_return to verify it works.  We
override the main function for mounting a btrfs fs so it'll return
-ENOMEM and then make sure that trying to mount a btrfs fs will fail.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 8 insertions, 2 deletions

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 4c223ab30293..cf446c25c0ec 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -677,6 +677,10 @@ union bpf_attr {
  *     @buf: buf to fill
  *     @buf_size: size of the buf
  *     Return : 0 on success or negative error code
+ *
+ * int bpf_override_return(pt_regs, rc)
+ *      @pt_regs: pointer to struct pt_regs
+ *      @rc: the return value to set
  */
 #define __BPF_FUNC_MAPPER(FN)           \
         FN(unspec),                     \
@@ -736,7 +740,8 @@ union bpf_attr {
         FN(xdp_adjust_meta),            \
         FN(perf_event_read_value),      \
         FN(perf_prog_read_value),       \
-        FN(getsockopt),
+        FN(getsockopt),                 \
+        FN(override_return),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h
index fd9a17fa8a8b..33cb00e46c49 100644
--- a/tools/testing/selftests/bpf/bpf_helpers.h
+++ b/tools/testing/selftests/bpf/bpf_helpers.h
@@ -82,7 +82,8 @@ static int (*bpf_perf_event_read_value)(void *map, unsigned long long flags,
 static int (*bpf_perf_prog_read_value)(void *ctx, void *buf,
                                        unsigned int buf_size) =
         (void *) BPF_FUNC_perf_prog_read_value;
-
+static int (*bpf_override_return)(void *ctx, unsigned long rc) =
+        (void *) BPF_FUNC_override_return;
 
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions