On Tue, Oct 03, 2023 at 03:11:15PM -0700, Andrii Nakryiko wrote:

On Mon, Oct 2, 2023 at 6:53 AM Artem Savkov [off-list ref] wrote:

quoted

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@kernel.org/t/#u (local)
[2] https://github.com/iovisor/bpftrace/commit/a2e3d5dbc03ceb49b776cf5602d31896158844a7
[3] https://lore.kernel.org/bpf/xunyjzy64q9b.fsf@redhat.com/t/#u (local)
[4] https://lore.kernel.org/bpf/20230727150647.397626-1-ykaliuta@redhat.com/t/#u (local)

quoted

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 <redacted>
---
 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