On Tue, 5 Nov 2024 14:34:05 +0100
Marco Elver <[email protected]> wrote:
> prctl() is a complex syscall which multiplexes its functionality based
> on a large set of PR_* options. Currently we count 64 such options. The
> return value of unknown options is -EINVAL, and doesn't distinguish from
> known options that were passed invalid args that also return -EINVAL.
>
> To understand if programs are attempting to use prctl() options not yet
> available on the running kernel, provide the task_prctl_unknown
> tracepoint.
>
> Note, this tracepoint is in an unlikely cold path, and would therefore
> be suitable for continuous monitoring (e.g. via perf_event_open).
>
> While the above is likely the simplest usecase, additionally this
> tracepoint can help unlock some testing scenarios (where probing
> sys_enter or sys_exit causes undesirable performance overheads):
>
> a. unprivileged triggering of a test module: test modules may register a
> probe to be called back on task_prctl_unknown, and pick a very large
> unknown prctl() option upon which they perform a test function for an
> unprivileged user;
>
> b. unprivileged triggering of an eBPF program function: similar
> as idea (a).
>
> Example trace_pipe output:
>
> <...>-366 [004] ..... 146.439400: task_prctl_unknown: pid=366
> comm=a.out option=1234 arg2=101 arg3=102 arg4=103 arg5=104
^^^ ^^^
>
> Signed-off-by: Marco Elver <[email protected]>
> ---
> include/trace/events/task.h | 43 +++++++++++++++++++++++++++++++++++++
> kernel/sys.c | 3 +++
> 2 files changed, 46 insertions(+)
>
> diff --git a/include/trace/events/task.h b/include/trace/events/task.h
> index 47b527464d1a..ab711e581094 100644
> --- a/include/trace/events/task.h
> +++ b/include/trace/events/task.h
> @@ -56,6 +56,49 @@ TRACE_EVENT(task_rename,
> __entry->newcomm, __entry->oom_score_adj)
> );
>
> +/**
> + * task_prctl_unknown - called on unknown prctl() option
> + * @task: pointer to the current task
> + * @option: option passed
> + * @arg2: arg2 passed
> + * @arg3: arg3 passed
> + * @arg4: arg4 passed
> + * @arg5: arg5 passed
> + *
> + * Called on an unknown prctl() option.
> + */
> +TRACE_EVENT(task_prctl_unknown,
> +
> + TP_PROTO(struct task_struct *task, int option, unsigned long arg2,
> unsigned long arg3,
> + unsigned long arg4, unsigned long arg5),
> +
> + TP_ARGS(task, option, arg2, arg3, arg4, arg5),
> +
> + TP_STRUCT__entry(
> + __field( pid_t, pid )
Why record the pid that is already recorded by the event header?
> + __string( comm, task->comm )
I'm also surprised that the comm didn't show in the trace_pipe. I've
updated the code so that it should usually find it. But saving it here may
not be a big deal.
-- Steve
> + __field( int, option)
> + __field( unsigned long, arg2)
> + __field( unsigned long, arg3)
> + __field( unsigned long, arg4)
> + __field( unsigned long, arg5)
> + ),
> +
> + TP_fast_assign(
> + __entry->pid = task->pid;
> + __assign_str(comm);
> + __entry->option = option;
> + __entry->arg2 = arg2;
> + __entry->arg3 = arg3;
> + __entry->arg4 = arg4;
> + __entry->arg5 = arg5;
> + ),
> +
> + TP_printk("pid=%d comm=%s option=%d arg2=%ld arg3=%ld arg4=%ld
> arg5=%ld",
> + __entry->pid, __get_str(comm), __entry->option,
> + __entry->arg2, __entry->arg3, __entry->arg4, __entry->arg5)
> +);
> +
> #endif
>
> /* This part must be outside protection */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 4da31f28fda8..dd0a71b68558 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -75,6 +75,8 @@
> #include <asm/io.h>
> #include <asm/unistd.h>
>
> +#include <trace/events/task.h>
> +
> #include "uid16.h"
>
> #ifndef SET_UNALIGN_CTL
> @@ -2785,6 +2787,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long,
> arg2, unsigned long, arg3,
> error = RISCV_SET_ICACHE_FLUSH_CTX(arg2, arg3);
> break;
> default:
> + trace_task_prctl_unknown(me, option, arg2, arg3, arg4, arg5);
> error = -EINVAL;
> break;
> }
