[PATCH bpf-next 2/5] bpf: Adjust kprobe_multi entry_ip for CONFIG_X86_KERNEL_IBT

[Jiri Olsa <jolsa@kernel.org>]

Martynas reported bpf_get_func_ip returning +4 address when
CONFIG_X86_KERNEL_IBT option is enabled.

When CONFIG_X86_KERNEL_IBT is enabled we'll have endbr instruction
at the function entry, which screws return value of bpf_get_func_ip()
helper that should return the function address.

There's short term workaround [1] for kprobe_multi bpf program made
by Alexei [1], but we need this fixup also for bpf_get_attach_cookie,
that returns cookie based on the entry_ip value.

Moving the fixup in the fprobe handler, so both bpf_get_func_ip
and bpf_get_attach_cookie get expected function address when
CONFIG_X86_KERNEL_IBT option is enabled.

Keeping the resolved 'addr' in kallsyms_callback, instead of taking
ftrace_location value, because we depend on symbol address in the
cookie related code. With CONFIG_X86_KERNEL_IBT option the
ftrace_location value differs from symbol address.

Cc: Peter Zijlstra <peterz@infradead.org>
Reported-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/trace/bpf_trace.c                         | 4 ++++
 tools/testing/selftests/bpf/progs/kprobe_multi.c | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 68e5cdd24cef..bcada91b0b3b 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2419,6 +2419,10 @@ kprobe_multi_link_handler(struct fprobe *fp, unsigned long entry_ip,
 {
 	struct bpf_kprobe_multi_link *link;
 
+#ifdef CONFIG_X86_KERNEL_IBT
+	if (is_endbr(*((u32 *) entry_ip - 1)))
+		entry_ip -= ENDBR_INSN_SIZE;
+#endif
 	link = container_of(fp, struct bpf_kprobe_multi_link, fp);
 	kprobe_multi_link_prog_run(link, entry_ip, regs);
 }
diff --git a/tools/testing/selftests/bpf/progs/kprobe_multi.c b/tools/testing/selftests/bpf/progs/kprobe_multi.c
index 08f95a8155d1..f165615d0b78 100644
--- a/tools/testing/selftests/bpf/progs/kprobe_multi.c
+++ b/tools/testing/selftests/bpf/progs/kprobe_multi.c
@@ -44,7 +44,7 @@ static void kprobe_multi_check(void *ctx, bool is_return)
 		return;
 
 	__u64 cookie = test_cookie ? bpf_get_attach_cookie(ctx) : 0;
-	__u64 addr = bpf_get_func_ip(ctx) - (CONFIG_X86_KERNEL_IBT ? 4 : 0);
+	__u64 addr = bpf_get_func_ip(ctx);
 
 #define SET(__var, __addr, __cookie) ({			\
 	if (((const void *) addr == __addr) &&		\
-- 
2.35.3