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

Steven Rostedt
2025-01-06T17:20:40Z

On Thu, 2 Jan 2025 17:43:17 +0000
<shiju.jose@huawei.com> wrote:

> 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>

How is this an issue? This is common for all pseudo files and can be
handled properly from user space.

Here, with this program:

read.c:
-------------------------8<-------------------------
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>

int main (int argc, char **argv)
{
char *buf;
int fd;
off_t size;
int r, s;

if (argc < 2) {
printf("usage: %s file\n", argv[0]);
exit(-1);
}

fd = open(argv[1], O_RDONLY);
if (fd < 0) {
perror(argv[1]);
exit(-1);
}

size = BUFSIZ * 10;

buf = malloc(size);
for (s = 0, r = 1; r > 0; s += r) {
r = read(fd, buf, size);
if (r < 0) {
perror(argv[1]);
exit(-1);
}
printf("Read %d bytes from %s\n", r, argv[1]);
}
free(buf);
close(fd);
return 0;
}
------------------------->8-------------------------

$ read /proc/kallsyms
Read 4091 bytes from /proc/kallsyms
Read 4075 bytes from /proc/kallsyms
Read 4078 bytes from /proc/kallsyms
Read 4083 bytes from /proc/kallsyms
Read 4093 bytes from /proc/kallsyms
Read 4076 bytes from /proc/kallsyms
Read 4080 bytes from /proc/kallsyms
Read 4086 bytes from /proc/kallsyms
Read 4080 bytes from /proc/kallsyms
Read 4064 bytes from /proc/kallsyms
Read 4071 bytes from /proc/kallsyms
Read 4063 bytes from /proc/kallsyms
Read 4069 bytes from /proc/kallsyms
Read 4079 bytes from /proc/kallsyms
Read 4063 bytes from /proc/kallsyms
Read 4072 bytes from /proc/kallsyms
Read 4046 bytes from /proc/kallsyms
Read 4091 bytes from /proc/kallsyms
Read 4090 bytes from /proc/kallsyms
Read 4067 bytes from /proc/kallsyms
Read 4080 bytes from /proc/kallsyms
Read 4066 bytes from /proc/kallsyms
Read 4085 bytes from /proc/kallsyms
Read 4095 bytes from /proc/kallsyms
Read 4076 bytes from /proc/kallsyms
Read 4090 bytes from /proc/kallsyms
Read 4066 bytes from /proc/kallsyms
Read 4073 bytes from /proc/kallsyms
Read 4091 bytes from /proc/kallsyms
Read 4075 bytes from /proc/kallsyms
Read 4076 bytes from /proc/kallsyms
Read 4048 bytes from /proc/kallsyms
Read 4074 bytes from /proc/kallsyms
Read 4058 bytes from /proc/kallsyms
Read 4074 bytes from /proc/kallsyms
[..]
Read 4052 bytes from /proc/kallsyms
Read 4061 bytes from /proc/kallsyms
Read 4061 bytes from /proc/kallsyms
Read 4053 bytes from /proc/kallsyms
Read 4083 bytes from /proc/kallsyms
Read 4066 bytes from /proc/kallsyms
Read 4093 bytes from /proc/kallsyms
Read 4072 bytes from /proc/kallsyms
Read 1982 bytes from /proc/kallsyms
Read 0 bytes from /proc/kallsyms

You see, it requires multiple reads to pull in an entire kernel pseudo
file. None of those reads are greater than PAGE_SIZE. Why should trace
format files be any different?

libtracefs handles this perfectly fine:

https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/tree/src/tracefs-utils.c#n343

Looks like you are trying to change the kernel to fix a user space bug :-/

NAK!

-- Steve