On Wed, 18 Dec 2024 07:43:46 +0800
Edward Adam Davis [off-list ref] wrote:

quoted

A proper fix is being discussed here:  

First, my fix is the first one.

Yes I saw that.

Second, the root cause of the problem is an overflow when calculating nr_pages.

quoted

https://lore.kernel.org/linux-trace-kernel/20241216164931.57323-1-aha310510@gmail.com/ (local)

Thank you,

-- Steve
  

The calculation of nr_pages below overflows because the pgoff value is 8,
the nr_subbufs value is 3, and the subbuf_order value is 0.

So basically you are saying that passing in the the mmap with the pgoff is
what's causing it.

quoted

quoted

 	nr_pages = ((nr_subbufs + 1) << subbuf_order) - pgoff; /* + meta-page */
 
 	nr_vma_pages = vma_pages(vma);  

Thanks, I believe I now have a reproducer. And yes, I'll take your patch.
(If Vincent is OK with it).

Here's the reproducer:

------------------------8<-------------------------
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <asm/types.h>
#include <sys/mman.h>

int main(int argc, char **argv)
{
	int page_size = getpagesize();
	int fd;
	void *meta;

	system("echo 1 > /sys/kernel/tracing/buffer_size_kb");
	fd = open("/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw", O_RDONLY);

	meta = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, page_size * 5);
}
------------------------>8-------------------------

Thanks,


-- Steve