Re: [PATCH Linux-next] perf test: Fix test case perf trace BTF general tests

Steven Rostedt
2025-11-26T17:37:18Z

On Wed, 26 Nov 2025 12:12:29 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> Arnaldo,
>
> How can I make perf trace not confused by the extra fields in the system
> call trace events?
>
> Ftrace can now show the contents of the system call user space buffers, but
> it appears that this breaks perf!!!
>
> system: syscalls
> name: sys_enter_write
> ID: 791
> format:
> field:unsigned short common_type; offset:0; size:2; signed:0;
> field:unsigned char common_flags; offset:2; size:1; signed:0;
> field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
> field:int common_pid; offset:4; size:4; signed:1;
>
> field:int __syscall_nr; offset:8; size:4; signed:1;
> field:unsigned int fd; offset:16; size:8; signed:0;
> field:const char * buf; offset:24; size:8; signed:0;
> field:size_t count; offset:32; size:8; signed:0;
> field:__data_loc char[] __buf_val; offset:40; size:4; signed:0;
>
> That new __buf_val appears to confuse perf, but I'm having a hell of a time
> trying to figure out where it reads it!
>

Hmm, it gets less confused (at least it doesn't crash), when I don't have
perf read the extra values.

Thomas, if you add the below patch, does it fix things for you?

-- Steve

diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index e96d0063cbcf..add809d226dc 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -1403,7 +1403,6 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
struct hlist_head *head;
unsigned long args[6];
bool valid_prog_array;
- bool mayfault;
char *user_ptr;
int user_sizes[SYSCALL_FAULT_MAX_CNT] = {};
int buf_size = CONFIG_TRACE_SYSCALL_BUF_SIZE_DEFAULT;
@@ -1431,15 +1430,6 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)

syscall_get_arguments(current, regs, args);

- /* Check if this syscall event faults in user space memory */
- mayfault = sys_data->user_mask != 0;
-
- if (mayfault) {
- if (syscall_get_data(sys_data, args, &user_ptr,
- &size, user_sizes, &uargs, buf_size) < 0)
- return;
- }
-
head = this_cpu_ptr(sys_data->enter_event->perf_events);
valid_prog_array = bpf_prog_array_valid(sys_data->enter_event);
if (!valid_prog_array && hlist_empty(head))
@@ -1457,9 +1447,6 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
rec->nr = syscall_nr;
memcpy(&rec->args, args, sizeof(unsigned long) * sys_data->nb_args);

- if (mayfault)
- syscall_put_data(sys_data, rec, user_ptr, size, user_sizes, uargs);
-
if ((valid_prog_array &&
!perf_call_bpf_enter(sys_data->enter_event, fake_regs, sys_data, rec)) ||
hlist_empty(head)) {