[bpf-next,v4,15/27] bpf, x64: Allow to use caller address from stack

Commit Message

Jiri Olsa Aug. 26, 2021, 7:39 p.m. UTC

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.

Basically we take the return address of the 'fentry' call:

   function + 0: call fentry    # stores 'function + 5' address on stack
   function + 5: ...

The 'function + 5' address will be used as the address for the
original function to call.

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

Patch

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 0fe6aacef3db..9f31197780ae 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2024,10 +2024,15 @@  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 f0f548f8f391..a5c3307d49c6 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -584,6 +584,11 @@  struct btf_func_model {
  */
 #define BPF_TRAMP_F_IP_ARG		BIT(3)
 
+/* Get original function from stack instead of from provided direct address.
+ * Makes sense for fexit programs only.
+ */
+#define BPF_TRAMP_F_ORIG_STACK		BIT(4)
+
 /* 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
  */