[PATCH 10/19] bpf: Allow to store caller's ip as argument

Jiri Olsa
2021-06-05T11:11:21Z

When we will have multiple functions attached to trampoline
we need to propagate the function's address to the bpf program.

Adding new BPF_TRAMP_F_IP_ARG flag to arch_prepare_bpf_trampoline
function that will store origin caller's address before function's
arguments.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
arch/x86/net/bpf_jit_comp.c | 18 ++++++++++++++----
include/linux/bpf.h | 5 +++++
2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index b77e6bd78354..d2425c18272a 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1951,7 +1951,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
void *orig_call)
{
int ret, i, cnt = 0, nr_args = m->nr_args;
- int stack_size = nr_args * 8;
+ int stack_size = nr_args * 8, ip_arg = 0;
struct bpf_tramp_progs *fentry = &tprogs[BPF_TRAMP_FENTRY];
struct bpf_tramp_progs *fexit = &tprogs[BPF_TRAMP_FEXIT];
struct bpf_tramp_progs *fmod_ret = &tprogs[BPF_TRAMP_MODIFY_RETURN];
@@ -1975,6 +1975,9 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
*/
orig_call += X86_PATCH_SIZE;

+ if (flags & BPF_TRAMP_F_IP_ARG)
+ stack_size += 8;
+
prog = image;

EMIT1(0x55); /* push rbp */
@@ -1982,7 +1985,14 @@ 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 */

- save_regs(m, &prog, nr_args, stack_size);
+ if (flags & BPF_TRAMP_F_IP_ARG) {
+ emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
+ EMIT4(0x48, 0x83, 0xe8, X86_PATCH_SIZE); /* sub $X86_PATCH_SIZE,%rax*/
+ emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -stack_size);
+ ip_arg = 8;
+ }
+
+ save_regs(m, &prog, nr_args, stack_size - ip_arg);

if (flags & BPF_TRAMP_F_CALL_ORIG) {
/* arg1: mov rdi, im */
@@ -2011,7 +2021,7 @@ 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);
+ restore_regs(m, &prog, nr_args, stack_size - ip_arg);

if (flags & BPF_TRAMP_F_ORIG_STACK) {
emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
@@ -2052,7 +2062,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
}

if (flags & BPF_TRAMP_F_RESTORE_REGS)
- restore_regs(m, &prog, nr_args, stack_size);
+ restore_regs(m, &prog, nr_args, stack_size - ip_arg);

/* This needs to be done regardless. If there were fmod_ret programs,
* the return value is only updated on the stack and still needs to be
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 16fc600503fb..6cbf3c81c650 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -559,6 +559,11 @@ struct btf_func_model {
*/
#define BPF_TRAMP_F_ORIG_STACK BIT(3)

+/* First argument is IP address of the caller. Makes sense for fentry/fexit
+ * programs only.
+ */
+#define BPF_TRAMP_F_IP_ARG 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
*/
--
2.31.1