[PATCH 1/1] tracing: Support reading trace event format file larger than PAGE_SIZE

shiju.jose
2025-01-02T17:43:36Z

From: Shiju Jose <shiju.jose@huawei.com>

When userspace reads a trace event format file, the maximum data size
that can be read is limited to PAGE_SIZE by the seq_read() and
seq_read_iter() functions. This results in userspace receiving partial
data if the format file is larger than PAGE_SIZE, requiring a workaround
to read the complete data from the format file.

Add support for reading trace event format files larger than PAGE_SIZE when
needed by userspace.

Signed-off-by: Shiju Jose <shiju.jose@huawei.com>
---
kernel/trace/trace_events.c | 32 +++++++++++++++++++++++++++++++-
1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 1545cc8b49d0..ef33614e7f22 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1801,6 +1801,36 @@ static int trace_format_open(struct inode *inode, struct file *file)
return 0;
}

+/**
+ * trace_format_read - read() method for format file.
+ * @file: the file to read from
+ * @buf: the buffer to read to
+ * @size: the maximum number of bytes to read
+ * @ppos: the current position in the file
+ *
+ * * Return:
+ * * %0 - No bytes copied (EOF).
+ * * %>0 - Number of bytes copied.
+ * * %<0 - Error code.
+ */
+static ssize_t trace_format_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
+{
+ ssize_t copied = 0;
+ ssize_t ret;
+
+ do {
+ ret = seq_read(file, buf + copied, size - copied, ppos);
+ if (ret < 0)
+ return ret;
+
+ copied += ret;
+ if (copied >= size)
+ break;
+ } while (ret);
+
+ return copied;
+}
+
#ifdef CONFIG_PERF_EVENTS
static ssize_t
event_id_read(struct file *filp, char __user *ubuf, size_t cnt, loff_t *ppos)
@@ -2284,7 +2314,7 @@ static const struct file_operations ftrace_enable_fops = {

static const struct file_operations ftrace_event_format_fops = {
.open = trace_format_open,
- .read = seq_read,
+ .read = trace_format_read,
.llseek = seq_lseek,
.release = seq_release,
};
--
2.43.0