Re: [PATCH v3] mm/madvise: unrestrict process_madvise() for current process

On Fri, Sep 27, 2024 at 10:04:25AM GMT, Christian Brauner wrote:

On Thu, Sep 26, 2024 at 08:52:32AM GMT, Shakeel Butt wrote:

quoted

On Thu, Sep 26, 2024 at 04:10:19PM GMT, Lorenzo Stoakes wrote:

quoted

The process_madvise() call was introduced in commit ecb8ac8b1f14
("mm/madvise: introduce process_madvise() syscall: an external memory
hinting API") as a means of performing madvise() operations on another
process.

However, as it provides the means by which to perform multiple madvise()
operations in a batch via an iovec, it is useful to utilise the same
interface for performing operations on the current process rather than a
remote one.

Commit 22af8caff7d1 ("mm/madvise: process_madvise() drop capability check
if same mm") removed the need for a caller invoking process_madvise() on
its own pidfd to possess the CAP_SYS_NICE capability, however this leaves
the restrictions on operation in place.

Resolve this by only applying the restriction on operations when accessing
a remote process.

Moving forward we plan to implement a simpler means of specifying this
condition other than needing to establish a self pidfd, perhaps in the form
of a sentinel pidfd.

Also take the opportunity to refactor the system call implementation
abstracting the vectorised operation.

Signed-off-by: Lorenzo Stoakes <redacted>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

quoted

---
v3:
* Avoid introducing PR_MADV_SELF and defer a non-pidfd version until later.

Seems like a good plan to decouple this patch from PR_MADV_SELF vs
PIDFD_SELF decision. I am hoping to see the follow up patch as well.

PIDFD_SELF should absolutely not be a per-system call thing. It should
be generic across all pidfd based system calls similar to AT_FDCWD.

IOW, that should be in:

include/uapi/linux/pidfd.h

#define PIDFD_SELF -200

Yes this is what I was saying elsewhere in the thread :) this is why it's
important to have this as a separate enterprise.

And indeed this is the intent, I will be working on a separate patch series
to this effect. It also gives us the space to implement it in calls which
use pidfd where it makes sense and to extend testing accordingly.