[PATCH v4 05/13] tracing: Have system call events record user array data

Steven Rostedt
2025-10-21T00:52:12Z

For system call events that have a length field, add a "user_arg_size"
parameter to the system call meta data that denotes the index of the args
array that holds the size of arg that the user_mask field has a bit set
for.

The "user_mask" has a bit set that denotes the arg that points to an array
in the user space address space and if a system call event has the
user_mask field set and the user_arg_size set, it will then record the
content of that address into the trace event, up to the size defined by
SYSCALL_FAULT_BUF_SZ - 1.

This allows the output to look like:

sys_write(fd: 0xa, buf: 0x5646978d13c0 (01:00:05:00:00:00:00:00:01:87:55:89:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00), count: 0x20)

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
include/trace/syscall.h | 4 +-
kernel/trace/trace_syscalls.c | 121 ++++++++++++++++++++++++----------
2 files changed, 90 insertions(+), 35 deletions(-)

diff --git a/include/trace/syscall.h b/include/trace/syscall.h
index 85f21ca15a41..9413c139da66 100644
--- a/include/trace/syscall.h
+++ b/include/trace/syscall.h
@@ -16,6 +16,7 @@
* @name: name of the syscall
* @syscall_nr: number of the syscall
* @nb_args: number of parameters it takes
+ * @user_arg_size: holds @arg that has size of the user space to read
* @user_mask: mask of @args that will read user space
* @types: list of types as strings
* @args: list of args as strings (args[i] matches types[i])
@@ -26,7 +27,8 @@
struct syscall_metadata {
const char *name;
int syscall_nr;
- short nb_args;
+ u8 nb_args;
+ s8 user_arg_size;
short user_mask;
const char **types;
const char **args;
diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index bc736839c323..6b9eb5a8e156 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -124,7 +124,7 @@ const char *get_syscall_name(int syscall)
return entry->name;
}

-/* Added to user strings when max limit is reached */
+/* Added to user strings or arrays when max limit is reached */
#define EXTRA "..."

static enum print_line_t
@@ -136,9 +136,8 @@ 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, val;
+ int i, syscall, val, len;
unsigned char *ptr;
- int len;

trace = (typeof(trace))ent;
syscall = trace->nr;
@@ -185,7 +184,23 @@ print_syscall_enter(struct trace_iterator *iter, int flags,
ptr = (void *)ent + (val & 0xffff);
len = val >> 16;

- trace_seq_printf(s, " \"%.*s\"", len, ptr);
+ if (entry->user_arg_size < 0) {
+ trace_seq_printf(s, " \"%.*s\"", len, ptr);
+ continue;
+ }
+
+ val = trace->args[entry->user_arg_size];
+
+ trace_seq_puts(s, " (");
+ for (int x = 0; x < len; x++, ptr++) {
+ if (x)
+ trace_seq_putc(s, ':');
+ trace_seq_printf(s, "%02x", *ptr);
+ }
+ if (len < val)
+ trace_seq_printf(s, ", %s", EXTRA);
+
+ trace_seq_putc(s, ')');
}

trace_seq_putc(s, ')');
@@ -250,8 +265,11 @@ __set_enter_print_fmt(struct syscall_metadata *entry, char *buf, int len)
if (!(BIT(i) & entry->user_mask))
continue;

- /* Add the format for the user space string */
- pos += snprintf(buf + pos, LEN_OR_ZERO, " \\\"%%s\\\"");
+ /* Add the format for the user space string or array */
+ if (entry->user_arg_size < 0)
+ pos += snprintf(buf + pos, LEN_OR_ZERO, " \\\"%%s\\\"");
+ else
+ pos += snprintf(buf + pos, LEN_OR_ZERO, " (%%s)");
}
pos += snprintf(buf + pos, LEN_OR_ZERO, "\"");

@@ -260,9 +278,14 @@ __set_enter_print_fmt(struct syscall_metadata *entry, char *buf, int len)
", ((unsigned long)(REC->%s))", entry->args[i]);
if (!(BIT(i) & entry->user_mask))
continue;
- /* The user space string for arg has name __<arg>_val */
- pos += snprintf(buf + pos, LEN_OR_ZERO, ", __get_str(__%s_val)",
- entry->args[i]);
+ /* The user space data for arg has name __<arg>_val */
+ if (entry->user_arg_size < 0) {
+ pos += snprintf(buf + pos, LEN_OR_ZERO, ", __get_str(__%s_val)",
+ entry->args[i]);
+ } else {
+ pos += snprintf(buf + pos, LEN_OR_ZERO, ", __print_dynamic_array(__%s_val, 1)",
+ entry->args[i]);
+ }
}

#undef LEN_OR_ZERO
@@ -333,9 +356,9 @@ static int __init syscall_enter_define_fields(struct trace_event_call *call)
idx = ffs(mask) - 1;

/*
- * User space strings are faulted into a temporary buffer and then
- * added as a dynamic string to the end of the event.
- * The user space string name for the arg pointer is "__<arg>_val".
+ * User space data is faulted into a temporary buffer and then
+ * added as a dynamic string or array to the end of the event.
+ * The user space data name for the arg pointer is "__<arg>_val".
*/
len = strlen(meta->args[idx]) + sizeof("___val");
arg = kmalloc(len, GFP_KERNEL);
@@ -431,9 +454,11 @@ static char *sys_fault_user(struct syscall_metadata *sys_data,
struct syscall_user_buffer *sbuf,
unsigned long *args, unsigned int *data_size)
{
+ trace_user_buf_copy syscall_copy = syscall_copy_user;
unsigned long size = SYSCALL_FAULT_BUF_SZ - 1;
unsigned long mask = sys_data->user_mask;
int idx = ffs(mask) - 1;
+ bool array = false;
char *ptr;
char *buf;

@@ -441,27 +466,43 @@ static char *sys_fault_user(struct syscall_metadata *sys_data,
ptr = (char *)args[idx];
*data_size = 0;

+ /*
+ * If this system call event has a size argument, use
+ * it to define how much of user space memory to read,
+ * and read it as an array and not a string.
+ */
+ if (sys_data->user_arg_size >= 0) {
+ array = true;
+ size = args[sys_data->user_arg_size];
+ if (size > SYSCALL_FAULT_BUF_SZ - 1)
+ size = SYSCALL_FAULT_BUF_SZ - 1;
+ /* use normal copy_from_user() */
+ syscall_copy = NULL;
+ }
+
buf = trace_user_fault_read(&sbuf->buf, ptr, size,
- syscall_copy_user, &size);
+ syscall_copy, &size);
if (!buf)
return NULL;

- /* Replace any non-printable characters with '.' */
- for (int i = 0; i < size; i++) {
- if (!isprint(buf[i]))
- buf[i] = '.';
- }
+ /* For strings, replace any non-printable characters with '.' */
+ if (!array) {
+ for (int i = 0; i < size; i++) {
+ if (!isprint(buf[i]))
+ buf[i] = '.';
+ }

- /*
- * If the text was truncated due to our max limit, add "..." to
- * the string.
- */
- if (size > SYSCALL_FAULT_BUF_SZ - sizeof(EXTRA)) {
- strscpy(buf + SYSCALL_FAULT_BUF_SZ - sizeof(EXTRA),
- EXTRA, sizeof(EXTRA));
- size = SYSCALL_FAULT_BUF_SZ;
- } else {
- buf[size++] = '\0';
+ /*
+ * If the text was truncated due to our max limit, add "..." to
+ * the string.
+ */
+ if (size > SYSCALL_FAULT_BUF_SZ - sizeof(EXTRA)) {
+ strscpy(buf + SYSCALL_FAULT_BUF_SZ - sizeof(EXTRA),
+ EXTRA, sizeof(EXTRA));
+ size = SYSCALL_FAULT_BUF_SZ;
+ } else {
+ buf[size++] = '\0';
+ }
}

*data_size = size;
@@ -492,7 +533,7 @@ syscall_get_data(struct syscall_metadata *sys_data, unsigned long *args,

static void syscall_put_data(struct syscall_metadata *sys_data,
struct syscall_trace_enter *entry,
- char *buffer, int size)
+ char *buffer, int size, int user_size)
{
void *ptr;
int val;
@@ -510,13 +551,16 @@ static void syscall_put_data(struct syscall_metadata *sys_data,
val = (ptr - (void *)entry) + 4;

/* Store the offset and the size into the meta data */
- *(int *)ptr = val | (size << 16);
+ *(int *)ptr = val | (user_size << 16);
+
+ if (WARN_ON_ONCE((ptr - (void *)entry + user_size) > size))
+ user_size = 0;

/* Nothing to do if the user space was empty or faulted */
- if (size) {
+ if (user_size) {
/* Now store the user space data into the event */
ptr += 4;
- memcpy(ptr, buffer, size);
+ memcpy(ptr, buffer, user_size);
}
}

@@ -580,7 +624,7 @@ static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
memcpy(entry->args, args, sizeof(unsigned long) * sys_data->nb_args);

if (mayfault)
- syscall_put_data(sys_data, entry, user_ptr, user_size);
+ syscall_put_data(sys_data, entry, user_ptr, size, user_size);

trace_event_buffer_commit(&fbuffer);
}
@@ -727,7 +771,16 @@ static void check_faultable_syscall(struct trace_event_call *call, int nr)
if (sys_data->enter_event != call)
return;

+ sys_data->user_arg_size = -1;
+
switch (nr) {
+ /* user arg 1 with size arg at 2 */
+ case __NR_write:
+ case __NR_mq_timedsend:
+ case __NR_pwrite64:
+ sys_data->user_mask = BIT(1);
+ sys_data->user_arg_size = 2;
+ break;
/* user arg at position 0 */
case __NR_access:
case __NR_acct:
@@ -1025,7 +1078,7 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
memcpy(&rec->args, args, sizeof(unsigned long) * sys_data->nb_args);

if (mayfault)
- syscall_put_data(sys_data, rec, user_ptr, user_size);
+ syscall_put_data(sys_data, rec, user_ptr, size, user_size);

if ((valid_prog_array &&
!perf_call_bpf_enter(sys_data->enter_event, fake_regs, sys_data, rec)) ||
--
2.51.0