[PATCH 09/19] bpf, x64: Allow to use caller address from stack

Jiri Olsa
2021-06-05T11:11:14Z

Currently we call the original function by using the absolute address
given at the JIT generation. That's not usable when having trampoline
attached to multiple functions. In this case we need to take the
return address from the stack.

Adding support to retrieve the original function address from the stack
by adding new BPF_TRAMP_F_ORIG_STACK flag for arch_prepare_bpf_trampoline
function.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
arch/x86/net/bpf_jit_comp.c | 13 +++++++++----
include/linux/bpf.h | 5 +++++
2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 2a2e290fa5d8..b77e6bd78354 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2013,10 +2013,15 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
if (flags & BPF_TRAMP_F_CALL_ORIG) {
restore_regs(m, &prog, nr_args, stack_size);

- /* call original function */
- if (emit_call(&prog, orig_call, prog)) {
- ret = -EINVAL;
- goto cleanup;
+ if (flags & BPF_TRAMP_F_ORIG_STACK) {
+ emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
+ EMIT2(0xff, 0xd0); /* call *rax */
+ } else {
+ /* call original function */
+ if (emit_call(&prog, orig_call, prog)) {
+ ret = -EINVAL;
+ goto cleanup;
+ }
}
/* remember return value in a stack for bpf prog to access */
emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 86dec5001ae2..16fc600503fb 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -554,6 +554,11 @@ struct btf_func_model {
*/
#define BPF_TRAMP_F_SKIP_FRAME BIT(2)

+/* Get original function from stack instead of from provided direct address.
+ * Makes sense for fexit programs only.
+ */
+#define BPF_TRAMP_F_ORIG_STACK BIT(3)
+
/* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
* bytes on x86. Pick a number to fit into BPF_IMAGE_SIZE / 2
*/
--
2.31.1