Re: [RFC PATCH] tracing: change syscall number type in struct syscall_trace_*

[Artem Savkov <asavkov@redhat.com>]

On Tue, Oct 03, 2023 at 09:38:44PM -0400, Steven Rostedt wrote:
> On Mon,  2 Oct 2023 15:52:42 +0200
> Artem Savkov <asavkov@redhat.com> wrote:
> 
> > linux-rt-devel tree contains a patch that adds an extra member to struct
> > trace_entry. This causes the offset of args field in struct
> > trace_event_raw_sys_enter be different from the one in struct
> > syscall_trace_enter:
> 
> This patch looks like it's fixing the symptom and not the issue. No code
> should rely on the two event structures to be related. That's an unwanted
> coupling, that will likely cause issues down the road (like the RT patch
> you mentioned).

I agree, but I didn't see a better solution and that was my way of
starting conversation, thus the RFC.

> > 
> > struct trace_event_raw_sys_enter {
> >         struct trace_entry         ent;                  /*     0    12 */
> > 
> >         /* XXX last struct has 3 bytes of padding */
> >         /* XXX 4 bytes hole, try to pack */
> > 
> >         long int                   id;                   /*    16     8 */
> >         long unsigned int          args[6];              /*    24    48 */
> >         /* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
> >         char                       __data[];             /*    72     0 */
> > 
> >         /* size: 72, cachelines: 2, members: 4 */
> >         /* sum members: 68, holes: 1, sum holes: 4 */
> >         /* paddings: 1, sum paddings: 3 */
> >         /* last cacheline: 8 bytes */
> > };
> > 
> > struct syscall_trace_enter {
> >         struct trace_entry         ent;                  /*     0    12 */
> > 
> >         /* XXX last struct has 3 bytes of padding */
> > 
> >         int                        nr;                   /*    12     4 */
> >         long unsigned int          args[];               /*    16     0 */
> > 
> >         /* size: 16, cachelines: 1, members: 3 */
> >         /* paddings: 1, sum paddings: 3 */
> >         /* last cacheline: 16 bytes */
> > };
> > 
> > This, in turn, causes perf_event_set_bpf_prog() fail while running bpf
> > test_profiler testcase because max_ctx_offset is calculated based on the
> > former struct, while off on the latter:
> 
> The above appears to be pointing to the real bug. The "is calculated based
> on the former struct while off on the latter" Why are the two being used
> together? They are supposed to be *unrelated*!
> 
> 
> > 
> >   10488         if (is_tracepoint || is_syscall_tp) {
> >   10489                 int off = trace_event_get_offsets(event->tp_event);
> 
> So basically this is clumping together the raw_syscalls with the syscalls
> events as if they are the same. But the are not. They are created
> differently. It's basically like using one structure to get the offsets of
> another structure. That would be a bug anyplace else in the kernel. Sounds
> like it's a bug here too.
> 
> I think the issue is with this code, not the tracing code.
> 
> We could expose the struct syscall_trace_enter and syscall_trace_exit if
> the offsets to those are needed.

I don't think we need syscall_trace_* offsets, looks like
trace_event_get_offsets() should return offset trace_event_raw_sys_enter
instead. I am still trying to figure out how all of this works together.
Maybe Alexei or Andrii have more context here.

-- 
 Artem