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

[Artem Savkov]

On Tue, Oct 03, 2023 at 03:11:15PM -0700, Andrii Nakryiko wrote:
> On Mon, Oct 2, 2023 at 6:53 AM Artem Savkov <asavkov@xxxxxxxxxx> wrote:
> >
> > linux-rt-devel tree contains a patch that adds an extra member to struct
> 
> can you please point to the patch itself that makes that change?

Of course, some context would be useful. The patch in question is b1773eac3f29c
("sched: Add support for lazy preemption") from rt-devel tree [0]. It came up
a couple of times before: [1] [2] [3] [4].

[0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?id=b1773eac3f29cbdcdfd16e0339f1a164066e9f71
[1] https://lore.kernel.org/linux-rt-users/20200221153541.681468-1-jolsa@xxxxxxxxxx/t/#u
[2] https://github.com/iovisor/bpftrace/commit/a2e3d5dbc03ceb49b776cf5602d31896158844a7
[3] https://lore.kernel.org/bpf/xunyjzy64q9b.fsf@xxxxxxxxxx/t/#u
[4] https://lore.kernel.org/bpf/20230727150647.397626-1-ykaliuta@xxxxxxxxxx/t/#u

> > 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:
> >
> > 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:
> >
> >   10488         if (is_tracepoint || is_syscall_tp) {
> >   10489                 int off = trace_event_get_offsets(event->tp_event);
> >   10490
> >   10491                 if (prog->aux->max_ctx_offset > off)
> >   10492                         return -EACCES;
> >   10493         }
> >
> > This patch changes the type of nr member in syscall_trace_* structs to
> > be long so that "args" offset is equal to that in struct
> > trace_event_raw_sys_enter.
> >
> > Signed-off-by: Artem Savkov <asavkov@xxxxxxxxxx>
> > ---
> >  kernel/trace/trace.h          | 4 ++--
> >  kernel/trace/trace_syscalls.c | 7 ++++---
> >  2 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> > index 77debe53f07cf..cd1d24df85364 100644
> > --- a/kernel/trace/trace.h
> > +++ b/kernel/trace/trace.h
> > @@ -135,13 +135,13 @@ enum trace_type {
> >   */
> >  struct syscall_trace_enter {
> >         struct trace_entry      ent;
> > -       int                     nr;
> > +       long                    nr;
> >         unsigned long           args[];
> >  };
> >
> >  struct syscall_trace_exit {
> >         struct trace_entry      ent;
> > -       int                     nr;
> > +       long                    nr;
> >         long                    ret;
> >  };
> >
> > diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> > index de753403cdafb..c26939119f2e4 100644
> > --- a/kernel/trace/trace_syscalls.c
> > +++ b/kernel/trace/trace_syscalls.c
> > @@ -101,7 +101,7 @@ find_syscall_meta(unsigned long syscall)
> >         return NULL;
> >  }
> >
> > -static struct syscall_metadata *syscall_nr_to_meta(int nr)
> > +static struct syscall_metadata *syscall_nr_to_meta(long nr)
> >  {
> >         if (IS_ENABLED(CONFIG_HAVE_SPARSE_SYSCALL_NR))
> >                 return xa_load(&syscalls_metadata_sparse, (unsigned long)nr);
> > @@ -132,7 +132,8 @@ print_syscall_enter(struct trace_iterator *iter, int flags,
> >         struct trace_entry *ent = iter->ent;
> >         struct syscall_trace_enter *trace;
> >         struct syscall_metadata *entry;
> > -       int i, syscall;
> > +       int i;
> > +       long syscall;
> >
> >         trace = (typeof(trace))ent;
> >         syscall = trace->nr;
> > @@ -177,7 +178,7 @@ print_syscall_exit(struct trace_iterator *iter, int flags,
> >         struct trace_seq *s = &iter->seq;
> >         struct trace_entry *ent = iter->ent;
> >         struct syscall_trace_exit *trace;
> > -       int syscall;
> > +       long syscall;
> >         struct syscall_metadata *entry;
> >
> >         trace = (typeof(trace))ent;
> > --
> > 2.41.0
> >
> >
> 

-- 
 Artem