Re: [PATCH v5 11/23] perf trace: Do not set unaugmented BPF program on sys_exit map

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Sep 23, 2026 at 12:13:51AM -0700, Ian Rogers wrote:
> In trace__init_syscalls_bpf_prog_array_maps(), the BPF program array map
> for sys_exit (syscalls_sys_exit) was populated with the result of
> trace__bpf_prog_sys_exit_fd().
> 
> When a syscall had no specific exit augmenter,
> trace__find_syscall_bpf_prog() fell back to unaugmented_prog
> (syscall_unaugmented). However, syscall_unaugmented is a sys_enter
> program that outputs enter arguments to __augmented_syscalls__.
> 
> As a consequence, when an unaugmented syscall exited, sys_exit
> tail-called syscall_unaugmented, which interpreted the exit arguments as
> enter arguments and emitted a duplicate, corrupt sys_enter event into
> __augmented_syscalls__ right as the syscall completed.

Do you have a reproducer of this problem?  I've never seen this before.
IIRC there's no augmentation at exit, then do we need to keep the tail
call?

Thanks,
Namhyung

> 
> Fix this by:
> 1. Returning NULL from trace__find_syscall_bpf_prog() when looking up exit
>    augmenters and none is found.
> 2. Returning -1 from trace__bpf_prog_sys_exit_fd() when no exit program
>    is present.
> 3. Only updating map_exit_fd when prog_fd >= 0.
> 4. Clearing err = 0 when trace__bpf_sys_enter_beauty_map() returns
>    non-zero (indicating the syscall has no augmentable pointer arguments)
>    before continuing the loop, so a trailing run of such syscalls (e.g.
>    'perf trace -e close') does not leave err non-zero on return and abort
>    the session.
> 
> Assisted-by: Antigravity:gemini-3.1-pro
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/builtin-trace.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index aa2d64eb56bd..0ae14ecd9f00 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -4176,7 +4176,12 @@ static struct bpf_program *trace__find_syscall_bpf_prog(struct trace *trace __ma
>  	pr_debug("Couldn't find BPF prog \"%s\" to associate with syscalls:sys_%s_%s, not augmenting it\n",
>  		 prog_name, type, sc->name);
>  out_unaugmented:
> -	return unaugmented_prog;
> +	/*
> +	 * Do not set unaugmented_prog for exit: syscall_unaugmented is a
> +	 * sys_enter program that outputs enter arguments. Exit without a
> +	 * specialized return augmenter returns 1 directly from sys_exit.
> +	 */
> +	return !strcmp(type, "exit") ? NULL : unaugmented_prog;
>  }
>  
>  static void trace__init_syscall_bpf_progs(struct trace *trace, int e_machine, int id)
> @@ -4199,7 +4204,7 @@ static int trace__bpf_prog_sys_enter_fd(struct trace *trace, int e_machine, int
>  static int trace__bpf_prog_sys_exit_fd(struct trace *trace, int e_machine, int id)
>  {
>  	struct syscall *sc = trace__syscall_info(trace, NULL, e_machine, id);
> -	return sc ? bpf_program__fd(sc->bpf_prog.sys_exit) : bpf_program__fd(unaugmented_prog);
> +	return sc && sc->bpf_prog.sys_exit ? bpf_program__fd(sc->bpf_prog.sys_exit) : -1;
>  }
>  
>  static int trace__bpf_sys_enter_beauty_map(struct trace *trace, int e_machine, int key, unsigned int *beauty_array)
> @@ -4454,16 +4459,27 @@ static int trace__init_syscalls_bpf_prog_array_maps(struct trace *trace, int e_m
>  		err = bpf_map_update_elem(map_enter_fd, &key, &prog_fd, BPF_ANY);
>  		if (err)
>  			break;
> +		/* Only update the exit prog array map if an exit augmenter exists */
>  		prog_fd = trace__bpf_prog_sys_exit_fd(trace, e_machine, key);
> -		err = bpf_map_update_elem(map_exit_fd, &key, &prog_fd, BPF_ANY);
> -		if (err)
> -			break;
> +		if (prog_fd >= 0) {
> +			err = bpf_map_update_elem(map_exit_fd, &key, &prog_fd, BPF_ANY);
> +			if (err)
> +				break;
> +		}
>  
>  		/* use beauty_map to tell BPF how many bytes to collect, set beauty_map's value here */
>  		memset(beauty_array, 0, sizeof(beauty_array));
>  		err = trace__bpf_sys_enter_beauty_map(trace, e_machine, key, (unsigned int *)beauty_array);
> -		if (err)
> +		if (err) {
> +			/*
> +			 * Not a failure: the syscall just has no augmentable
> +			 * arguments. Clear err, or a trailing run of such
> +			 * syscalls, e.g. all of them for 'perf trace -e close',
> +			 * would leave it set on return and abort the session.
> +			 */
> +			err = 0;
>  			continue;
> +		}
>  		err = bpf_map_update_elem(beauty_map_fd, &key, beauty_array, BPF_ANY);
>  		if (err)
>  			break;
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
>