[PATCH bpf] bpf, arm64: Fix BTI type used for freplace attached functions

From: Alexander Duyck
Date: Thu Jul 13 2023 - 12:49:49 EST


From: Alexander Duyck <alexanderduyck@xxxxxx>

When running an freplace attached bpf program on an arm64 system w were
seeing the following issue:
Unhandled 64-bit el1h sync exception on CPU47, ESR 0x0000000036000003 -- BTI

After a bit of work to track it down I determined that what appeared to be
happening is that the 'bti c' at the start of the program was somehow being
reached after a 'br' instruction. Further digging pointed me toward the
fact that the function was attached via freplace. This in turn led me to
build_plt which I believe is invoking the long jump which is triggering
this error.

To resolve it we can replace the 'bti c' with 'bti jc' and add a comment
explaining why this has to be modified as such.

Fixes: b2ad54e1533e ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
Signed-off-by: Alexander Duyck <alexanderduyck@xxxxxx>
---
arch/arm64/net/bpf_jit_comp.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 145b540ec34f..ec2174838f2a 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -322,7 +322,13 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
*
*/

- emit_bti(A64_BTI_C, ctx);
+ /* bpf function may be invoked by 3 instruction types:
+ * 1. bl, attached via freplace to bpf prog via short jump
+ * 2. br, attached via freplace to bpf prog via long jump
+ * 3. blr, working as a function pointer, used by emit_call.
+ * So BTI_JC should used here to support both br and blr.
+ */
+ emit_bti(A64_BTI_JC, ctx);

emit(A64_MOV(1, A64_R(9), A64_LR), ctx);
emit(A64_NOP, ctx);