[PATCH 1/7] bpf, x86: Store caller's ip in trampoline stack

Jiri Olsa
2021-07-07T19:46:33Z

Storing caller's ip in trampoline's stack. Trampoline programs
can reach the IP in (ctx - 8) address, so there's no change in
program's arguments interface.

The IP address is takes from [fp + 8], which is return address
from the initial 'call fentry' call to trampoline.

This IP address will be returned via bpf_get_func_ip helper
helper, which is added in following patches.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
arch/x86/net/bpf_jit_comp.c | 19 +++++++++++++++++++
include/linux/bpf.h | 5 +++++
2 files changed, 24 insertions(+)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index e835164189f1..c320b3ce7b58 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1951,6 +1951,9 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
if (flags & BPF_TRAMP_F_CALL_ORIG)
stack_size += 8; /* room for return value of orig_call */

+ if (flags & BPF_TRAMP_F_IP_ARG)
+ stack_size += 8; /* room for IP address argument */
+
if (flags & BPF_TRAMP_F_SKIP_FRAME)
/* skip patched call instruction and point orig_call to actual
* body of the kernel function.
@@ -1964,6 +1967,22 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
EMIT1(0x53); /* push rbx */

+ if (flags & BPF_TRAMP_F_IP_ARG) {
+ /* Store IP address of the traced function:
+ * mov rax, QWORD PTR [rbp + 8]
+ * sub rax, X86_PATCH_SIZE
+ * mov QWORD PTR [rbp - stack_size], rax
+ */
+ emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
+ EMIT4(0x48, 0x83, 0xe8, X86_PATCH_SIZE);
+ emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -stack_size);
+
+ /* Continue with stack_size for regs storage, stack will
+ * be correctly restored with 'leave' instruction.
+ */
+ stack_size -= 8;
+ }
+
save_regs(m, &prog, nr_args, stack_size);

if (flags & BPF_TRAMP_F_CALL_ORIG) {
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f309fc1509f2..6b3da9bc3d16 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)

+/* Store IP address of the caller on the trampoline stack,
+ * so it's available for trampoline's programs.
+ */
+#define BPF_TRAMP_F_IP_ARG 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