Re: [PATCH v6 bpf-next 08/11] bpf: introduce BPF_RAW_TRACEPOINT

On 3/27/18 5:44 PM, Mathieu Desnoyers wrote:

----- On Mar 27, 2018, at 8:00 PM, Alexei Starovoitov ast@fb.com wrote:

quoted

On 3/27/18 4:13 PM, Mathieu Desnoyers wrote:

quoted

----- On Mar 27, 2018, at 6:48 PM, Alexei Starovoitov ast@fb.com wrote:

quoted

On 3/27/18 2:04 PM, Steven Rostedt wrote:

quoted

+#ifdef CONFIG_BPF_EVENTS
+#define BPF_RAW_TP() . = ALIGN(8);		\

Given that the section consists of a 16-bytes structure elements
on architectures with 8 bytes pointers, this ". = ALIGN(8)" should
be turned into a STRUCT_ALIGN(), especially given that the compiler
is free to up-align the structure on 32 bytes.

STRUCT_ALIGN fixed the 'off by 8' issue with kasan,
but it fails without kasan too.
For some reason the whole region __start__bpf_raw_tp - __stop__bpf_raw_tp
comes inited with cccc:
[   22.703562] i 1 btp ffffffff8288e530 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.704638] i 2 btp ffffffff8288e540 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.705599] i 3 btp ffffffff8288e550 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.706551] i 4 btp ffffffff8288e560 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.707503] i 5 btp ffffffff8288e570 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.708452] i 6 btp ffffffff8288e580 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.709406] i 7 btp ffffffff8288e590 btp->tp cccccccccccccccc func
cccccccccccccccc
[   22.710368] i 8 btp ffffffff8288e5a0 btp->tp cccccccccccccccc func
cccccccccccccccc

while gdb shows that everything is good inside vmlinux
for exactly these addresses.
Some other linker magic missing?

No, Steven's iteration code is incorrect.

+extern struct bpf_raw_event_map __start__bpf_raw_tp;
+extern struct bpf_raw_event_map __stop__bpf_raw_tp;

That should be:

extern struct bpf_raw_event_map __start__bpf_raw_tp[];
extern struct bpf_raw_event_map __stop__bpf_raw_tp[];


+
+struct bpf_raw_event_map *bpf_find_raw_tracepoint(const char *name)
+{
+        const struct bpf_raw_event_map *btp = &__start__bpf_raw_tp;

const struct bpf_raw_event_map *btp = __start__bpf_raw_tp;

+        int i = 0;
+
+        for (; btp < &__stop__bpf_raw_tp; btp++) {

for (; btp < __stop__bpf_raw_tp; btp++) {

Those start/stop symbols are given their address by the linker
automatically (this is a GNU linker extension). We don't want
pointers to the symbols, but rather the symbols per se to act
as start/stop addresses.

right. that part I fixed first.

Turned out it was in init.data section and got poisoned.
this fixes it:

@@ -258,6 +258,7 @@
         LIKELY_PROFILE()                                                \
         BRANCH_PROFILE()                                                \
         TRACE_PRINTKS()                                                 \
+       BPF_RAW_TP()                                                    \
         TRACEPOINT_STR()

  /*

@@ -585,7 +586,6 @@
         *(.init.rodata)                                                 \
         FTRACE_EVENTS()                                                 \
         TRACE_SYSCALLS()                                                \
-       BPF_RAW_TP()                                                    \
         KPROBE_BLACKLIST()                                              \
         ERROR_INJECT_WHITELIST()                                        \
         MEM_DISCARD(init.rodata)                                        \

and it works :)
I will clean few other nits I found while debugging and respin.