On Wed, Oct 04, 2023 at 02:55:47PM +0200, Artem Savkov wrote:
> 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@xxxxxxxxxx> 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.
Turns out it is even more confusing. The tests dereference the context as
struct trace_event_raw_sys_enter so bpf verifier sets max_ctx_offset
based on that, then perf_event_set_bpf_prog() checks this offset against
the one in struct syscall_trace_enter, but what bpf prog really gets is
a pointer to struct syscall_tp_t from kernel/trace/trace_syscalls.c.
I don't know the history behind these decisions, but should the tests
dereference context as struct syscall_trace_enter instead and struct
syscall_tp_t be changed to have syscall_nr as int?
--
Artem
