BUG: demonstration of uprobe/uretprobe corrupted stack traces

Hi Masami,

We've got reports about partially corrupt stack traces when being captured from
uretprobes. Trying the simplest repro seems to confirm that something is not
quite right here. I'll try to debug it a bit more this week, but I was hoping
for you to take a look as well, if you get a chance.

Simple repro built on top of BPF selftests.

  $ sudo ./test_progs -a uprobe_autoattach -v
  ...
  FN ADDR 0x55fde0 - 0x55fdef
  UPROBE SZ 40 (CNT 5)       URETPROBE SZ 40 (CNT 5)
  UPROBE 0x55fde0           URETPROBE 0x55ffd4
  UPROBE 0x584653           URETPROBE 0x584653
  UPROBE 0x585cc9           URETPROBE 0x585cc9
  UPROBE 0x7fa9a31eaca3     URETPROBE 0x7fa9a31eaca3
  UPROBE 0x5541f689495641d7 URETPROBE 0x5541f689495641d7
  ...
  #203     uprobe_autoattach:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

There seem to be two distinct problems.

1. Last two entries for both uprobe and uretprobe stacks are not user-space
addressed (0x7fa9a31eaca3) and the very last one doesn't even look like a valid
address (0x5541f689495641d7).

2. Looking at first entry for UPROBE vs URETPROBE, you can see that uprobe
one's is correct and points exactly to the beginning of autoattach_trigger_func
(0x55fde0) as expected, but uretprobe entry (0x55ffd4) is way out of
autoattach_trigger_func (which is just 15 bytes long and ends at 0x55fdef).
Using addr2line it shows that it points to:

  0x000000000055ffd4: test_uprobe_autoattach at /data/users/andriin/linux/tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c:33

Which is a valid function and location to which autoattach_trigger_func()
should return (see objdump snippet below), but from uretprobe I'd imagine that
we are going to get address within traced user function (that is 0x55fde0 -
0x55fdef range), not the return address in a parent function.

     55ffc4:       89 83 3c 08 00 00       mov    %eax,0x83c(%rbx)
     55ffca:       8b 45 e8                mov    -0x18(%rbp),%eax
     55ffcd:       89 c7                   mov    %eax,%edi
     55ffcf:       e8 0c fe ff ff          call   55fde0 <autoattach_trigger_func>
-->  55ffd4:       89 45 a8                mov    %eax,-0x58(%rbp)
     55ffd7:       ba ef fd 55 00          mov    $0x55fdef,%edx

Both issues above seem unexpected, can you please see if I have some wrong
assumptions here?

Thanks in advance for taking a look!

Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Riham Selim <rihams@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -23,7 +23,8 @@ BPF_GCC		?= $(shell command -v bpf-gcc;)
 SAN_CFLAGS	?=
 CFLAGS += -g -O0 -rdynamic -Wall -Werror $(GENFLAGS) $(SAN_CFLAGS)	\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
-	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT)
+	  -I$(TOOLSINCDIR) -I$(APIDIR) -I$(OUTPUT) -fno-omit-frame-pointer
+
 LDFLAGS += $(SAN_CFLAGS)
 LDLIBS += -lelf -lz -lrt -lpthread
 

tools/testing/selftests/bpf/prog_tests/uprobe_autoattach.c

@@ -10,13 +10,15 @@ static noinline int autoattach_trigger_func(int arg)
 	asm volatile ("");
 	return arg + 1;
 }
+static noinline int autoattach_trigger_func_post(int arg) { return 0; }
 
 void test_uprobe_autoattach(void)
 {
 	struct test_uprobe_autoattach *skel;
 	int trigger_val = 100, trigger_ret;
 	size_t malloc_sz = 1;
 	char *mem;
+	int i;
 
 	skel = test_uprobe_autoattach__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "skel_open"))
@@ -30,6 +32,18 @@ void test_uprobe_autoattach(void)
 	/* trigger & validate uprobe & uretprobe */
 	trigger_ret = autoattach_trigger_func(trigger_val);
 
+	printf("FN ADDR %p - %p\n", autoattach_trigger_func, autoattach_trigger_func_post);
+	printf("UPROBE SZ %d (CNT %d)      URETPROBE SZ %d (CNT %d)\n",
+		skel->bss->uprobe_stack_sz,
+		skel->bss->uprobe_stack_sz / 8,
+		skel->bss->uretprobe_stack_sz,
+		skel->bss->uretprobe_stack_sz / 8);
+	for (i = 0; i < skel->bss->uprobe_stack_sz / 8; i++) {
+		printf("UPROBE %-18p URETPROBE %-18p\n",
+			(void *)skel->bss->uprobe_stack[i],
+			(void *)skel->bss->uretprobe_stack[i]);
+	}
+
 	skel->bss->test_pid = getpid();
 
 	/* trigger & validate shared library u[ret]probes attached by name */

tools/testing/selftests/bpf/progs/test_uprobe_autoattach.c

@@ -27,11 +27,19 @@ int handle_uprobe_noautoattach(struct pt_regs *ctx)
 	return 0;
 }
 
+__u64 uprobe_stack[128];
+__u64 uretprobe_stack[128];
+int uprobe_stack_sz, uretprobe_stack_sz;
+
 SEC("uprobe//proc/self/exe:autoattach_trigger_func")
 int handle_uprobe_byname(struct pt_regs *ctx)
 {
 	uprobe_byname_parm1 = PT_REGS_PARM1_CORE(ctx);
 	uprobe_byname_ran = 1;
+
+	uprobe_stack_sz = bpf_get_stack(ctx,
+					uprobe_stack, sizeof(uprobe_stack),
+					BPF_F_USER_STACK);
 	return 0;
 }
 
@@ -40,6 +48,9 @@ int handle_uretprobe_byname(struct pt_regs *ctx)
 {
 	uretprobe_byname_rc = PT_REGS_RC_CORE(ctx);
 	uretprobe_byname_ran = 2;
+	uretprobe_stack_sz = bpf_get_stack(ctx,
+					   uretprobe_stack, sizeof(uretprobe_stack),
+					   BPF_F_USER_STACK);
 	return 0;
 }