On Wed, Aug 9, 2023 at 4:10 PM Masami Hiramatsu [off-list ref] wrote:

Hi Florent,

On Wed, 9 Aug 2023 12:28:38 +0200
Florent Revest [off-list ref] wrote:

quoted

On Mon, Aug 7, 2023 at 8:48 AM Masami Hiramatsu (Google)
[off-list ref] wrote:

quoted

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

This allows fprobes to be available with CONFIG_DYNAMIC_FTRACE_WITH_ARGS
instead of CONFIG_DYNAMIC_FTRACE_WITH_REGS, then we can enable fprobe
on arm64.

This patch lets fprobe code build on configs WITH_ARGS and !WITH_REGS
but fprobe wouldn't run on these builds because fprobe still registers
to ftrace with FTRACE_OPS_FL_SAVE_REGS, which would fail on
!WITH_REGS. Shouldn't we also let the fprobe_init callers decide
whether they want REGS or not ?

Ah, I think you meant FPROBE_EVENTS? Yes I forgot to add the dependency
on it. But fprobe itself can work because fprobe just pass the ftrace_regs
to the handlers. (Note that exit callback may not work until next patch)

No, I mean that fprobe still registers its ftrace ops with the
FTRACE_OPS_FL_SAVE_REGS flag:

https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/kernel/trace/fprobe.c?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n185

Which means that __register_ftrace_function will return -EINVAL on
builds !WITH_REGS:

https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/kernel/trace/ftrace.c?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n338

As documented here:

https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/include/linux/ftrace.h?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n188

There are two parts to using sparse pt_regs. One is "static": having
WITH_ARGS in the config, the second one is "dynamic": a ftrace ops
needs to specify that it doesn't want to go through the ftrace
trampoline that saves a full pt_regs, by not giving
FTRACE_OPS_FL_SAVE_REGS. If we want fprobe to work on builds
!WITH_REGS then we should both remove Kconfig dependencies to
WITH_REGS (like you've done) but also stop passing this ftrace ops
flag.

quoted

quoted

 static int
 kprobe_multi_link_handler(struct fprobe *fp, unsigned long fentry_ip,
-                         unsigned long ret_ip, struct pt_regs *regs,
+                         unsigned long ret_ip, struct ftrace_regs *fregs,
                          void *data)
 {
        struct bpf_kprobe_multi_link *link;
+       struct pt_regs *regs = ftrace_get_regs(fregs);
+
+       if (!regs)
+               return 0;

(with the above comment addressed) this means that BPF multi_kprobe
would successfully attach on builds WITH_ARGS but the programs would
never actually run because here regs would be 0. This is a confusing
failure mode for the user. I think that if multi_kprobe won't work
(because we don't have a pt_regs conversion path yet), the user should
be notified at attachment time that they won't be getting any events.

Yes, so I changed it will not be compiled in that case.

Ah ok I missed the CONFIG_DYNAMIC_FTRACE_WITH_REGS guard that you keep
between patch 1 and 6 to avoid this case. (after patch 6, it's no
longer an issue) then that's fine.