[PATCH v2 2/2] perf: Fix bpf prologue generation

[Sumanth Korikkar <sumanthk@xxxxxxxxxxxxx>]

Issue:
bpf_probe_read is no longer available for architecture which has
overlapping address space. Hence bpf prologue generation fails

Fix:
Use bpf_probe_read_kernel for kernel member access. For user
attribute access in kprobes, use bpf_probe_read_user.

Other:
@user attribute was introduced in commit 1e032f7cfa14 ("perf-probe:
 Add user memory access attribute support")

Test:
1. ulimit -l 128 ; ./perf record -e tests/bpf_sched_setscheduler.c
2. cat tests/bpf_sched_setscheduler.c

static void (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
        (void *) 6;
static int (*bpf_probe_read_user)(void *dst, __u32 size,
                                  const void *unsafe_ptr) = (void *) 112;
static int (*bpf_probe_read_kernel)(void *dst, __u32 size,
        const void *unsafe_ptr) = (void *) 113;

SEC("func=do_sched_setscheduler  pid policy param->sched_priority@user")
int bpf_func__setscheduler(void *ctx, int err, pid_t pid, int policy,
                           int param)
{
        char fmt[] = "prio: %ld";
        bpf_trace_printk(fmt, sizeof(fmt), param);
        return 1;
}

char _license[] SEC("license") = "GPL";
int _version SEC("version") = LINUX_VERSION_CODE;

3. ./perf script
   sched 305669 [000] 1614458.838675: perf_bpf_probe:func: (2904e508)
   pid=261614 policy=2 sched_priority=1

4. cat /sys/kernel/debug/tracing/trace
   <...>-309956 [006] .... 1616098.093957: 0: prio: 1

Signed-off-by: Sumanth Korikkar <sumanthk@xxxxxxxxxxxxx>
Reviewed-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
---
 tools/perf/util/bpf-prologue.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/bpf-prologue.c b/tools/perf/util/bpf-prologue.c
index b020a8678eb9..9887ae09242d 100644
--- a/tools/perf/util/bpf-prologue.c
+++ b/tools/perf/util/bpf-prologue.c
@@ -142,7 +142,8 @@ static int
 gen_read_mem(struct bpf_insn_pos *pos,
 	     int src_base_addr_reg,
 	     int dst_addr_reg,
-	     long offset)
+	     long offset,
+	     int probeid)
 {
 	/* mov arg3, src_base_addr_reg */
 	if (src_base_addr_reg != BPF_REG_ARG3)
@@ -159,7 +160,7 @@ gen_read_mem(struct bpf_insn_pos *pos,
 		ins(BPF_MOV64_REG(BPF_REG_ARG1, dst_addr_reg), pos);
 
 	/* Call probe_read  */
-	ins(BPF_EMIT_CALL(BPF_FUNC_probe_read), pos);
+	ins(BPF_EMIT_CALL(probeid), pos);
 	/*
 	 * Error processing: if read fail, goto error code,
 	 * will be relocated. Target should be the start of
@@ -241,7 +242,7 @@ static int
 gen_prologue_slowpath(struct bpf_insn_pos *pos,
 		      struct probe_trace_arg *args, int nargs)
 {
-	int err, i;
+	int err, i, probeid;
 
 	for (i = 0; i < nargs; i++) {
 		struct probe_trace_arg *arg = &args[i];
@@ -276,11 +277,16 @@ gen_prologue_slowpath(struct bpf_insn_pos *pos,
 				stack_offset), pos);
 
 		ref = arg->ref;
+		probeid = BPF_FUNC_probe_read_kernel;
 		while (ref) {
 			pr_debug("prologue: arg %d: offset %ld\n",
 				 i, ref->offset);
+
+			if (ref->user_access)
+				probeid = BPF_FUNC_probe_read_user;
+
 			err = gen_read_mem(pos, BPF_REG_3, BPF_REG_7,
-					   ref->offset);
+					   ref->offset, probeid);
 			if (err) {
 				pr_err("prologue: failed to generate probe_read function call\n");
 				goto errout;
-- 
2.17.1