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Date: Thu, 12 Oct 2023 09:44:44 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Artem Savkov <asavkov@...hat.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
 <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 bpf@...r.kernel.org, netdev@...r.kernel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 linux-rt-users@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [RFC PATCH bpf-next] bpf: change syscall_nr type to int in
 struct syscall_tp_t

On Thu, 12 Oct 2023 13:45:50 +0200
Artem Savkov <asavkov@...hat.com> wrote:

> linux-rt-devel tree contains a patch (b1773eac3f29c ("sched: Add support
> for lazy preemption")) 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:
> 
> 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         }
> 
> What bpf program is actually getting is a pointer to struct
> syscall_tp_t, defined in kernel/trace/trace_syscalls.c. This patch fixes
> the problem by aligning struct syscall_tp_t with with struct
> syscall_trace_(enter|exit) and changing the tests to use these structs
> to dereference context.
> 
> Signed-off-by: Artem Savkov <asavkov@...hat.com>

Thanks for doing a proper fix.

Acked-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve

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