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authorKP Singh <kpsingh@google.com>2020-03-04 20:18:49 +0100
committerAlexei Starovoitov <ast@kernel.org>2020-03-04 13:41:05 -0800
commitae24082331d9bbaae283aafbe930a8f0eb85605a (patch)
tree606f33bff29d3661e0a72921237c74162daabeb8 /kernel/bpf/btf.c
parent7e639208e88d60abf83d48dfda4c0ad325a77b58 (diff)
downloadlinux-ae24082331d9bbaae283aafbe930a8f0eb85605a.tar.bz2
bpf: Introduce BPF_MODIFY_RETURN
When multiple programs are attached, each program receives the return value from the previous program on the stack and the last program provides the return value to the attached function. The fmod_ret bpf programs are run after the fentry programs and before the fexit programs. The original function is only called if all the fmod_ret programs return 0 to avoid any unintended side-effects. The success value, i.e. 0 is not currently configurable but can be made so where user-space can specify it at load time. For example: int func_to_be_attached(int a, int b) { <--- do_fentry do_fmod_ret: <update ret by calling fmod_ret> if (ret != 0) goto do_fexit; original_function: <side_effects_happen_here> } <--- do_fexit The fmod_ret program attached to this function can be defined as: SEC("fmod_ret/func_to_be_attached") int BPF_PROG(func_name, int a, int b, int ret) { // This will skip the original function logic. return 1; } The first fmod_ret program is passed 0 in its return argument. Signed-off-by: KP Singh <kpsingh@google.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Andrii Nakryiko <andriin@fb.com> Acked-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20200304191853.1529-4-kpsingh@chromium.org
Diffstat (limited to 'kernel/bpf/btf.c')
-rw-r--r--kernel/bpf/btf.c3
1 files changed, 2 insertions, 1 deletions
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 787140095e58..30841fb8b3c0 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3710,7 +3710,8 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
nr_args--;
}
- if (prog->expected_attach_type == BPF_TRACE_FEXIT &&
+ if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
+ prog->expected_attach_type == BPF_MODIFY_RETURN) &&
arg == nr_args) {
if (!t)
/* Default prog with 5 args. 6th arg is retval. */