Commit 7e6f3cd8 authored by Jiri Olsa's avatar Jiri Olsa Committed by Alexei Starovoitov
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bpf, x86: Store caller's ip in trampoline stack 



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: default avatarJiri Olsa <jolsa@kernel.org>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210714094400.396467-2-jolsa@kernel.org
parent 76283171

arch/x86/net/bpf_jit_comp.c

+19 −0
Original line number Diff line number Diff line
@@ -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) {

include/linux/bpf.h

+5 −0
Original line number Diff line number Diff line
@@ -579,6 +579,11 @@ 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

 */