On Mon, Sep 01, 2025 at 01:05:16PM +0200, Jann Horn wrote:

On Thu, Aug 28, 2025 at 11:01 PM Serge E. Hallyn [off-list ref] wrote:

quoted

On Wed, Aug 27, 2025 at 05:32:02PM -0700, Andy Lutomirski wrote:

quoted

On Wed, Aug 27, 2025 at 5:14 PM Aleksa Sarai [off-list ref] wrote:

quoted

On 2025-08-26, Mickaël Salaün [off-list ref] wrote:

quoted

On Tue, Aug 26, 2025 at 11:07:03AM +0200, Christian Brauner wrote:

quoted

Nothing has changed in that regard and I'm not interested in stuffing
the VFS APIs full of special-purpose behavior to work around the fact
that this is work that needs to be done in userspace. Change the apps,
stop pushing more and more cruft into the VFS that has no business
there.

It would be interesting to know how to patch user space to get the same
guarantees...  Do you think I would propose a kernel patch otherwise?

You could mmap the script file with MAP_PRIVATE. This is the *actual*
protection the kernel uses against overwriting binaries (yes, ETXTBSY is
nice but IIRC there are ways to get around it anyway).

Wait, really?  MAP_PRIVATE prevents writes to the mapping from
affecting the file, but I don't think that writes to the file will
break the MAP_PRIVATE CoW if it's not already broken.

IPython says:

In [1]: import mmap, tempfile

In [2]: f = tempfile.TemporaryFile()

In [3]: f.write(b'initial contents')
Out[3]: 16

In [4]: f.flush()

In [5]: map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE,
prot=mmap.PROT_READ)

In [6]: map[:]
Out[6]: b'initial contents'

In [7]: f.seek(0)
Out[7]: 0

In [8]: f.write(b'changed')
Out[8]: 7

In [9]: f.flush()

In [10]: map[:]
Out[10]: b'changed contents'

That was surprising to me, however, if I split the reader
and writer into different processes, so

Testing this in python is a terrible idea because it obfuscates the
actual syscalls from you.

Hah, I was just trying to fit in :), but of course you're right.
Redoing it in straight c, I'm getting the updates.

-serge

// mmap-w.c (creates an overwrites)
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#define FIRST "Initial contents"
#define SECOND "updated contents"

int main() {
	int fd, rc;
	char c;

	fd = open("/tmp/m", O_CREAT | O_RDWR, 0644);
	if (fd < 0) {
		printf("failed to open /tmp/m: %m\n");
		_exit(1);
	}
	rc = write(fd, FIRST, sizeof(FIRST));
	if (rc < 0) {
		printf("write failed: %m\n");
		_exit(1);
	}
	rc = fsync(fd);
	if (rc < 0) {
		printf("flush failed: %m\n");
		_exit(1);
	}

	read(STDIN_FILENO, &c, 1);

	printf("updating the contents\n");

	rc = lseek(fd, 0, SEEK_SET);
	if (rc < 0) {
		printf("seek failed; %m\n");
		_exit(1);
	}

	rc = write(fd, SECOND, sizeof(SECOND));
	if (fd < 0) {
		printf("write failed: %m\n");
		_exit(1);
	}
	rc = close(fd);
	if (rc < 0) {
		printf("close failed: %m\n");
		_exit(1);
	}
	printf("done\n");
}

// mmap-r.c (checks and re-checks contents)
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>

#define FIRST "Initial contents"
#define SECOND "Updated contents"

int main() {
	int fd, rc;
	char *m;
	char c;

	fd = open("/tmp/m", O_RDONLY);
	if (fd < 0) {
		printf("failed to open /tmp/m: %m\n");
		_exit(1);
	}

	m = mmap(NULL, 40, PROT_READ, MAP_PRIVATE, fd, 0);
	if (m == MAP_FAILED) {
		printf("mmap failed: %m\n");
		_exit(1);
	}

	if (strncmp(m, FIRST, 7) != 0) {
		printf("m is %c%c%c%c%c%c%c\n",
			m[0], m[1], m[2], m[3], m[4], m[5], m[6]);
		_exit(1);
	}

	read(STDIN_FILENO, &c, 1);

	if (strncmp(m, SECOND, 7) != 0) {
		printf("m is %c%c%c%c%c%c%c\n",
			m[0], m[1], m[2], m[3], m[4], m[5], m[6]);
		_exit(1);
	}

	printf("done\n");
}