[PATCH bpf-next v2] bpf: Use fake pt_regs when doing bpf syscall tracepoint tracing

[Yonghong Song <yonghong.song@xxxxxxxxx>]

Salvatore Benedetto reported an issue that when doing syscall tracepoint
tracing the kernel stack is empty. For example, using the following
command line
  bpftrace -e 'tracepoint:syscalls:sys_enter_read { print("Kernel Stack\n"); print(kstack()); }'
  bpftrace -e 'tracepoint:syscalls:sys_exit_read { print("Kernel Stack\n"); print(kstack()); }'
the output for both commands is
===
  Kernel Stack
===

Further analysis shows that pt_regs used for bpf syscall tracepoint
tracing is from the one constructed during user->kernel transition.
The call stack looks like
  perf_syscall_enter+0x88/0x7c0
  trace_sys_enter+0x41/0x80
  syscall_trace_enter+0x100/0x160
  do_syscall_64+0x38/0xf0
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

The ip address stored in pt_regs is from user space hence no kernel
stack is printed.

To fix the issue, kernel address from pt_regs is required.
In kernel repo, there are already a few cases like this. For example,
in kernel/trace/bpf_trace.c, several perf_fetch_caller_regs(fake_regs_ptr)
instances are used to supply ip address or use ip address to construct
call stack.

Instead of allocate fake_regs in the stack which may consume
a lot of bytes, the function perf_trace_buf_alloc() in
perf_syscall_{enter, exit}() is leveraged to create fake_regs,
which will be passed to perf_call_bpf_{enter,exit}().

For the above bpftrace script, I got the following output with this patch:
for tracepoint:syscalls:sys_enter_read
===
  Kernel Stack

        syscall_trace_enter+407
        syscall_trace_enter+407
        do_syscall_64+74
        entry_SYSCALL_64_after_hwframe+75
===
and for tracepoint:syscalls:sys_exit_read
===
Kernel Stack

        syscall_exit_work+185
        syscall_exit_work+185
        syscall_exit_to_user_mode+305
        do_syscall_64+118
        entry_SYSCALL_64_after_hwframe+75
===

Reported-by: Salvatore Benedetto <salvabenedetto@xxxxxxxx>
Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
 kernel/trace/trace_syscalls.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 9c581d6da843..785733245ead 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -564,6 +564,7 @@ static int perf_call_bpf_enter(struct trace_event_call *call, struct pt_regs *re
 	BUILD_BUG_ON(sizeof(param.ent) < sizeof(void *));
 
 	/* bpf prog requires 'regs' to be the first member in the ctx (a.k.a. &param) */
+	perf_fetch_caller_regs(regs);
 	*(struct pt_regs **)&param = regs;
 	param.syscall_nr = rec->nr;
 	for (i = 0; i < sys_data->nb_args; i++)
@@ -575,6 +576,7 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
 {
 	struct syscall_metadata *sys_data;
 	struct syscall_trace_enter *rec;
+	struct pt_regs *fake_regs;
 	struct hlist_head *head;
 	unsigned long args[6];
 	bool valid_prog_array;
@@ -602,7 +604,7 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
 	size = ALIGN(size + sizeof(u32), sizeof(u64));
 	size -= sizeof(u32);
 
-	rec = perf_trace_buf_alloc(size, NULL, &rctx);
+	rec = perf_trace_buf_alloc(size, &fake_regs, &rctx);
 	if (!rec)
 		return;
 
@@ -611,7 +613,7 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
 	memcpy(&rec->args, args, sizeof(unsigned long) * sys_data->nb_args);
 
 	if ((valid_prog_array &&
-	     !perf_call_bpf_enter(sys_data->enter_event, regs, sys_data, rec)) ||
+	     !perf_call_bpf_enter(sys_data->enter_event, fake_regs, sys_data, rec)) ||
 	    hlist_empty(head)) {
 		perf_swevent_put_recursion_context(rctx);
 		return;
@@ -666,6 +668,7 @@ static int perf_call_bpf_exit(struct trace_event_call *call, struct pt_regs *reg
 	} __aligned(8) param;
 
 	/* bpf prog requires 'regs' to be the first member in the ctx (a.k.a. &param) */
+	perf_fetch_caller_regs(regs);
 	*(struct pt_regs **)&param = regs;
 	param.syscall_nr = rec->nr;
 	param.ret = rec->ret;
@@ -676,6 +679,7 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
 {
 	struct syscall_metadata *sys_data;
 	struct syscall_trace_exit *rec;
+	struct pt_regs *fake_regs;
 	struct hlist_head *head;
 	bool valid_prog_array;
 	int syscall_nr;
@@ -701,7 +705,7 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
 	size = ALIGN(sizeof(*rec) + sizeof(u32), sizeof(u64));
 	size -= sizeof(u32);
 
-	rec = perf_trace_buf_alloc(size, NULL, &rctx);
+	rec = perf_trace_buf_alloc(size, &fake_regs, &rctx);
 	if (!rec)
 		return;
 
@@ -709,7 +713,7 @@ static void perf_syscall_exit(void *ignore, struct pt_regs *regs, long ret)
 	rec->ret = syscall_get_return_value(current, regs);
 
 	if ((valid_prog_array &&
-	     !perf_call_bpf_exit(sys_data->exit_event, regs, rec)) ||
+	     !perf_call_bpf_exit(sys_data->exit_event, fake_regs, rec)) ||
 	    hlist_empty(head)) {
 		perf_swevent_put_recursion_context(rctx);
 		return;
-- 
2.43.5